Initial import from vendor-sys branch of openzfs

129 files changed, 169703 insertions, 0 deletions

diff --git a/sys/contrib/openzfs/module/zfs/Makefile.in b/sys/contrib/openzfs/module/zfs/Makefile.in
new file mode 100644
index 000000000000..9ddcd6c339d4
--- /dev/null
+++ b/sys/contrib/openzfs/module/zfs/Makefile.in
@@ -0,0 +1,153 @@
+ifneq ($(KBUILD_EXTMOD),)
+src = @abs_srcdir@
+obj = @abs_builddir@
+mfdir = $(obj)
+else
+mfdir = $(srctree)/$(src)
+endif
+
+MODULE := zfs
+
+obj-$(CONFIG_ZFS) := $(MODULE).o
+
+# Suppress unused-value warnings in sparc64 architecture headers
+ccflags-$(CONFIG_SPARC64) += -Wno-unused-value
+
+$(MODULE)-objs += abd.o
+$(MODULE)-objs += aggsum.o
+$(MODULE)-objs += arc.o
+$(MODULE)-objs += blkptr.o
+$(MODULE)-objs += bplist.o
+$(MODULE)-objs += bpobj.o
+$(MODULE)-objs += bptree.o
+$(MODULE)-objs += btree.o
+$(MODULE)-objs += bqueue.o
+$(MODULE)-objs += dataset_kstats.o
+$(MODULE)-objs += dbuf.o
+$(MODULE)-objs += dbuf_stats.o
+$(MODULE)-objs += ddt.o
+$(MODULE)-objs += ddt_zap.o
+$(MODULE)-objs += dmu.o
+$(MODULE)-objs += dmu_diff.o
+$(MODULE)-objs += dmu_object.o
+$(MODULE)-objs += dmu_objset.o
+$(MODULE)-objs += dmu_recv.o
+$(MODULE)-objs += dmu_redact.o
+$(MODULE)-objs += dmu_send.o
+$(MODULE)-objs += dmu_traverse.o
+$(MODULE)-objs += dmu_tx.o
+$(MODULE)-objs += dmu_zfetch.o
+$(MODULE)-objs += dnode.o
+$(MODULE)-objs += dnode_sync.o
+$(MODULE)-objs += dsl_bookmark.o
+$(MODULE)-objs += dsl_crypt.o
+$(MODULE)-objs += dsl_dataset.o
+$(MODULE)-objs += dsl_deadlist.o
+$(MODULE)-objs += dsl_deleg.o
+$(MODULE)-objs += dsl_destroy.o
+$(MODULE)-objs += dsl_dir.o
+$(MODULE)-objs += dsl_pool.o
+$(MODULE)-objs += dsl_prop.o
+$(MODULE)-objs += dsl_scan.o
+$(MODULE)-objs += dsl_synctask.o
+$(MODULE)-objs += dsl_userhold.o
+$(MODULE)-objs += edonr_zfs.o
+$(MODULE)-objs += fm.o
+$(MODULE)-objs += gzip.o
+$(MODULE)-objs += hkdf.o
+$(MODULE)-objs += lz4.o
+$(MODULE)-objs += lzjb.o
+$(MODULE)-objs += metaslab.o
+$(MODULE)-objs += mmp.o
+$(MODULE)-objs += multilist.o
+$(MODULE)-objs += objlist.o
+$(MODULE)-objs += pathname.o
+$(MODULE)-objs += range_tree.o
+$(MODULE)-objs += refcount.o
+$(MODULE)-objs += rrwlock.o
+$(MODULE)-objs += sa.o
+$(MODULE)-objs += sha256.o
+$(MODULE)-objs += skein_zfs.o
+$(MODULE)-objs += spa.o
+$(MODULE)-objs += spa_boot.o
+$(MODULE)-objs += spa_checkpoint.o
+$(MODULE)-objs += spa_config.o
+$(MODULE)-objs += spa_errlog.o
+$(MODULE)-objs += spa_history.o
+$(MODULE)-objs += spa_log_spacemap.o
+$(MODULE)-objs += spa_misc.o
+$(MODULE)-objs += space_map.o
+$(MODULE)-objs += space_reftree.o
+$(MODULE)-objs += txg.o
+$(MODULE)-objs += uberblock.o
+$(MODULE)-objs += unique.o
+$(MODULE)-objs += vdev.o
+$(MODULE)-objs += vdev_cache.o
+$(MODULE)-objs += vdev_indirect.o
+$(MODULE)-objs += vdev_indirect_births.o
+$(MODULE)-objs += vdev_indirect_mapping.o
+$(MODULE)-objs += vdev_initialize.o
+$(MODULE)-objs += vdev_label.o
+$(MODULE)-objs += vdev_mirror.o
+$(MODULE)-objs += vdev_missing.o
+$(MODULE)-objs += vdev_queue.o
+$(MODULE)-objs += vdev_raidz.o
+$(MODULE)-objs += vdev_raidz_math.o
+$(MODULE)-objs += vdev_raidz_math_scalar.o
+$(MODULE)-objs += vdev_rebuild.o
+$(MODULE)-objs += vdev_removal.o
+$(MODULE)-objs += vdev_root.o
+$(MODULE)-objs += vdev_trim.o
+$(MODULE)-objs += zap.o
+$(MODULE)-objs += zap_leaf.o
+$(MODULE)-objs += zap_micro.o
+$(MODULE)-objs += zcp.o
+$(MODULE)-objs += zcp_get.o
+$(MODULE)-objs += zcp_global.o
+$(MODULE)-objs += zcp_iter.o
+$(MODULE)-objs += zcp_set.o
+$(MODULE)-objs += zcp_synctask.o
+$(MODULE)-objs += zfeature.o
+$(MODULE)-objs += zfs_byteswap.o
+$(MODULE)-objs += zfs_fm.o
+$(MODULE)-objs += zfs_fuid.o
+$(MODULE)-objs += zfs_ioctl.o
+$(MODULE)-objs += zfs_log.o
+$(MODULE)-objs += zfs_onexit.o
+$(MODULE)-objs += zfs_quota.o
+$(MODULE)-objs += zfs_ratelimit.o
+$(MODULE)-objs += zfs_replay.o
+$(MODULE)-objs += zfs_rlock.o
+$(MODULE)-objs += zfs_sa.o
+$(MODULE)-objs += zil.o
+$(MODULE)-objs += zio.o
+$(MODULE)-objs += zio_checksum.o
+$(MODULE)-objs += zio_compress.o
+$(MODULE)-objs += zio_inject.o
+$(MODULE)-objs += zle.o
+$(MODULE)-objs += zrlock.o
+$(MODULE)-objs += zthr.o
+$(MODULE)-objs += zvol.o
+
+# Suppress incorrect warnings from versions of objtool which are not
+# aware of x86 EVEX prefix instructions used for AVX512.
+OBJECT_FILES_NON_STANDARD_vdev_raidz_math_avx512bw.o := y
+OBJECT_FILES_NON_STANDARD_vdev_raidz_math_avx512f.o := y
+
+$(MODULE)-$(CONFIG_X86) += vdev_raidz_math_sse2.o
+$(MODULE)-$(CONFIG_X86) += vdev_raidz_math_ssse3.o
+$(MODULE)-$(CONFIG_X86) += vdev_raidz_math_avx2.o
+$(MODULE)-$(CONFIG_X86) += vdev_raidz_math_avx512f.o
+$(MODULE)-$(CONFIG_X86) += vdev_raidz_math_avx512bw.o
+
+$(MODULE)-$(CONFIG_ARM64) += vdev_raidz_math_aarch64_neon.o
+$(MODULE)-$(CONFIG_ARM64) += vdev_raidz_math_aarch64_neonx2.o
+
+$(MODULE)-$(CONFIG_PPC) += vdev_raidz_math_powerpc_altivec.o
+$(MODULE)-$(CONFIG_PPC64) += vdev_raidz_math_powerpc_altivec.o
+
+ifeq ($(CONFIG_ALTIVEC),y)
+$(obj)/vdev_raidz_math_powerpc_altivec.o: c_flags += -maltivec
+endif
+
+include $(mfdir)/../os/linux/zfs/Makefile
diff --git a/sys/contrib/openzfs/module/zfs/THIRDPARTYLICENSE.cityhash b/sys/contrib/openzfs/module/zfs/THIRDPARTYLICENSE.cityhash
new file mode 100644
index 000000000000..e558b2a50358
--- /dev/null
+++ b/sys/contrib/openzfs/module/zfs/THIRDPARTYLICENSE.cityhash
@@ -0,0 +1,19 @@
+Copyright (c) 2011 Google, Inc.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.
diff --git a/sys/contrib/openzfs/module/zfs/THIRDPARTYLICENSE.cityhash.descrip b/sys/contrib/openzfs/module/zfs/THIRDPARTYLICENSE.cityhash.descrip
new file mode 100644
index 000000000000..f98cb76dfc91
--- /dev/null
+++ b/sys/contrib/openzfs/module/zfs/THIRDPARTYLICENSE.cityhash.descrip
@@ -0,0 +1 @@
+CITYHASH CHECKSUM FUNCTIONALITY IN ZFS
diff --git a/sys/contrib/openzfs/module/zfs/abd.c b/sys/contrib/openzfs/module/zfs/abd.c
new file mode 100644
index 000000000000..6018a42ca0d8
--- /dev/null
+++ b/sys/contrib/openzfs/module/zfs/abd.c
@@ -0,0 +1,1213 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright (c) 2014 by Chunwei Chen. All rights reserved.
+ * Copyright (c) 2019 by Delphix. All rights reserved.
+ */
+
+/*
+ * ARC buffer data (ABD).
+ *
+ * ABDs are an abstract data structure for the ARC which can use two
+ * different ways of storing the underlying data:
+ *
+ * (a) Linear buffer. In this case, all the data in the ABD is stored in one
+ *     contiguous buffer in memory (from a zio_[data_]buf_* kmem cache).
+ *
+ *         +-------------------+
+ *         | ABD (linear)      |
+ *         |   abd_flags = ... |
+ *         |   abd_size = ...  |     +--------------------------------+
+ *         |   abd_buf ------------->| raw buffer of size abd_size    |
+ *         +-------------------+     +--------------------------------+
+ *              no abd_chunks
+ *
+ * (b) Scattered buffer. In this case, the data in the ABD is split into
+ *     equal-sized chunks (from the abd_chunk_cache kmem_cache), with pointers
+ *     to the chunks recorded in an array at the end of the ABD structure.
+ *
+ *         +-------------------+
+ *         | ABD (scattered)   |
+ *         |   abd_flags = ... |
+ *         |   abd_size = ...  |
+ *         |   abd_offset = 0  |                           +-----------+
+ *         |   abd_chunks[0] ----------------------------->| chunk 0   |
+ *         |   abd_chunks[1] ---------------------+        +-----------+
+ *         |   ...             |                  |        +-----------+
+ *         |   abd_chunks[N-1] ---------+         +------->| chunk 1   |
+ *         +-------------------+        |                  +-----------+
+ *                                      |                      ...
+ *                                      |                  +-----------+
+ *                                      +----------------->| chunk N-1 |
+ *                                                         +-----------+
+ *
+ * In addition to directly allocating a linear or scattered ABD, it is also
+ * possible to create an ABD by requesting the "sub-ABD" starting at an offset
+ * within an existing ABD. In linear buffers this is simple (set abd_buf of
+ * the new ABD to the starting point within the original raw buffer), but
+ * scattered ABDs are a little more complex. The new ABD makes a copy of the
+ * relevant abd_chunks pointers (but not the underlying data). However, to
+ * provide arbitrary rather than only chunk-aligned starting offsets, it also
+ * tracks an abd_offset field which represents the starting point of the data
+ * within the first chunk in abd_chunks. For both linear and scattered ABDs,
+ * creating an offset ABD marks the original ABD as the offset's parent, and the
+ * original ABD's abd_children refcount is incremented. This data allows us to
+ * ensure the root ABD isn't deleted before its children.
+ *
+ * Most consumers should never need to know what type of ABD they're using --
+ * the ABD public API ensures that it's possible to transparently switch from
+ * using a linear ABD to a scattered one when doing so would be beneficial.
+ *
+ * If you need to use the data within an ABD directly, if you know it's linear
+ * (because you allocated it) you can use abd_to_buf() to access the underlying
+ * raw buffer. Otherwise, you should use one of the abd_borrow_buf* functions
+ * which will allocate a raw buffer if necessary. Use the abd_return_buf*
+ * functions to return any raw buffers that are no longer necessary when you're
+ * done using them.
+ *
+ * There are a variety of ABD APIs that implement basic buffer operations:
+ * compare, copy, read, write, and fill with zeroes. If you need a custom
+ * function which progressively accesses the whole ABD, use the abd_iterate_*
+ * functions.
+ *
+ * As an additional feature, linear and scatter ABD's can be stitched together
+ * by using the gang ABD type (abd_alloc_gang_abd()). This allows for
+ * multiple ABDs to be viewed as a singular ABD.
+ *
+ * It is possible to make all ABDs linear by setting zfs_abd_scatter_enabled to
+ * B_FALSE.
+ */
+
+#include <sys/abd_impl.h>
+#include <sys/param.h>
+#include <sys/zio.h>
+#include <sys/zfs_context.h>
+#include <sys/zfs_znode.h>
+
+/* see block comment above for description */
+int zfs_abd_scatter_enabled = B_TRUE;
+
+boolean_t
+abd_is_linear(abd_t *abd)
+{
+	return ((abd->abd_flags & ABD_FLAG_LINEAR) != 0 ? B_TRUE : B_FALSE);
+}
+
+boolean_t
+abd_is_linear_page(abd_t *abd)
+{
+	return ((abd->abd_flags & ABD_FLAG_LINEAR_PAGE) != 0 ?
+	    B_TRUE : B_FALSE);
+}
+
+boolean_t
+abd_is_gang(abd_t *abd)
+{
+	return ((abd->abd_flags & ABD_FLAG_GANG) != 0 ? B_TRUE :
+	    B_FALSE);
+}
+
+void
+abd_verify(abd_t *abd)
+{
+	ASSERT3U(abd->abd_size, >, 0);
+	ASSERT3U(abd->abd_size, <=, SPA_MAXBLOCKSIZE);
+	ASSERT3U(abd->abd_flags, ==, abd->abd_flags & (ABD_FLAG_LINEAR |
+	    ABD_FLAG_OWNER | ABD_FLAG_META | ABD_FLAG_MULTI_ZONE |
+	    ABD_FLAG_MULTI_CHUNK | ABD_FLAG_LINEAR_PAGE | ABD_FLAG_GANG |
+	    ABD_FLAG_GANG_FREE | ABD_FLAG_ZEROS));
+	IMPLY(abd->abd_parent != NULL, !(abd->abd_flags & ABD_FLAG_OWNER));
+	IMPLY(abd->abd_flags & ABD_FLAG_META, abd->abd_flags & ABD_FLAG_OWNER);
+	if (abd_is_linear(abd)) {
+		ASSERT3P(ABD_LINEAR_BUF(abd), !=, NULL);
+	} else if (abd_is_gang(abd)) {
+		uint_t child_sizes = 0;
+		for (abd_t *cabd = list_head(&ABD_GANG(abd).abd_gang_chain);
+		    cabd != NULL;
+		    cabd = list_next(&ABD_GANG(abd).abd_gang_chain, cabd)) {
+			ASSERT(list_link_active(&cabd->abd_gang_link));
+			child_sizes += cabd->abd_size;
+			abd_verify(cabd);
+		}
+		ASSERT3U(abd->abd_size, ==, child_sizes);
+	} else {
+		abd_verify_scatter(abd);
+	}
+}
+
+uint_t
+abd_get_size(abd_t *abd)
+{
+	abd_verify(abd);
+	return (abd->abd_size);
+}
+
+/*
+ * Allocate an ABD, along with its own underlying data buffers. Use this if you
+ * don't care whether the ABD is linear or not.
+ */
+abd_t *
+abd_alloc(size_t size, boolean_t is_metadata)
+{
+	if (!zfs_abd_scatter_enabled || abd_size_alloc_linear(size))
+		return (abd_alloc_linear(size, is_metadata));
+
+	VERIFY3U(size, <=, SPA_MAXBLOCKSIZE);
+
+	abd_t *abd = abd_alloc_struct(size);
+	abd->abd_flags = ABD_FLAG_OWNER;
+	abd->abd_u.abd_scatter.abd_offset = 0;
+	abd_alloc_chunks(abd, size);
+
+	if (is_metadata) {
+		abd->abd_flags |= ABD_FLAG_META;
+	}
+	abd->abd_size = size;
+	abd->abd_parent = NULL;
+	zfs_refcount_create(&abd->abd_children);
+
+	abd_update_scatter_stats(abd, ABDSTAT_INCR);
+
+	return (abd);
+}
+
+static void
+abd_free_scatter(abd_t *abd)
+{
+	abd_free_chunks(abd);
+
+	zfs_refcount_destroy(&abd->abd_children);
+	abd_update_scatter_stats(abd, ABDSTAT_DECR);
+	abd_free_struct(abd);
+}
+
+static void
+abd_put_gang_abd(abd_t *abd)
+{
+	ASSERT(abd_is_gang(abd));
+	abd_t *cabd;
+
+	while ((cabd = list_remove_head(&ABD_GANG(abd).abd_gang_chain))
+	    != NULL) {
+		ASSERT0(cabd->abd_flags & ABD_FLAG_GANG_FREE);
+		abd->abd_size -= cabd->abd_size;
+		abd_put(cabd);
+	}
+	ASSERT0(abd->abd_size);
+	list_destroy(&ABD_GANG(abd).abd_gang_chain);
+}
+
+/*
+ * Free an ABD allocated from abd_get_offset() or abd_get_from_buf(). Will not
+ * free the underlying scatterlist or buffer.
+ */
+void
+abd_put(abd_t *abd)
+{
+	if (abd == NULL)
+		return;
+
+	abd_verify(abd);
+	ASSERT(!(abd->abd_flags & ABD_FLAG_OWNER));
+
+	if (abd->abd_parent != NULL) {
+		(void) zfs_refcount_remove_many(&abd->abd_parent->abd_children,
+		    abd->abd_size, abd);
+	}
+
+	if (abd_is_gang(abd))
+		abd_put_gang_abd(abd);
+
+	zfs_refcount_destroy(&abd->abd_children);
+	abd_free_struct(abd);
+}
+
+/*
+ * Allocate an ABD that must be linear, along with its own underlying data
+ * buffer. Only use this when it would be very annoying to write your ABD
+ * consumer with a scattered ABD.
+ */
+abd_t *
+abd_alloc_linear(size_t size, boolean_t is_metadata)
+{
+	abd_t *abd = abd_alloc_struct(0);
+
+	VERIFY3U(size, <=, SPA_MAXBLOCKSIZE);
+
+	abd->abd_flags = ABD_FLAG_LINEAR | ABD_FLAG_OWNER;
+	if (is_metadata) {
+		abd->abd_flags |= ABD_FLAG_META;
+	}
+	abd->abd_size = size;
+	abd->abd_parent = NULL;
+	zfs_refcount_create(&abd->abd_children);
+
+	if (is_metadata) {
+		ABD_LINEAR_BUF(abd) = zio_buf_alloc(size);
+	} else {
+		ABD_LINEAR_BUF(abd) = zio_data_buf_alloc(size);
+	}
+
+	abd_update_linear_stats(abd, ABDSTAT_INCR);
+
+	return (abd);
+}
+
+static void
+abd_free_linear(abd_t *abd)
+{
+	if (abd_is_linear_page(abd)) {
+		abd_free_linear_page(abd);
+		return;
+	}
+	if (abd->abd_flags & ABD_FLAG_META) {
+		zio_buf_free(ABD_LINEAR_BUF(abd), abd->abd_size);
+	} else {
+		zio_data_buf_free(ABD_LINEAR_BUF(abd), abd->abd_size);
+	}
+
+	zfs_refcount_destroy(&abd->abd_children);
+	abd_update_linear_stats(abd, ABDSTAT_DECR);
+
+	abd_free_struct(abd);
+}
+
+static void
+abd_free_gang_abd(abd_t *abd)
+{
+	ASSERT(abd_is_gang(abd));
+	abd_t *cabd = list_head(&ABD_GANG(abd).abd_gang_chain);
+
+	while (cabd != NULL) {
+		/*
+		 * We must acquire the child ABDs mutex to ensure that if it
+		 * is being added to another gang ABD we will set the link
+		 * as inactive when removing it from this gang ABD and before
+		 * adding it to the other gang ABD.
+		 */
+		mutex_enter(&cabd->abd_mtx);
+		ASSERT(list_link_active(&cabd->abd_gang_link));
+		list_remove(&ABD_GANG(abd).abd_gang_chain, cabd);
+		mutex_exit(&cabd->abd_mtx);
+		abd->abd_size -= cabd->abd_size;
+		if (cabd->abd_flags & ABD_FLAG_GANG_FREE) {
+			if (cabd->abd_flags & ABD_FLAG_OWNER)
+				abd_free(cabd);
+			else
+				abd_put(cabd);
+		}
+		cabd = list_head(&ABD_GANG(abd).abd_gang_chain);
+	}
+	ASSERT0(abd->abd_size);
+	list_destroy(&ABD_GANG(abd).abd_gang_chain);
+	zfs_refcount_destroy(&abd->abd_children);
+	abd_free_struct(abd);
+}
+
+/*
+ * Free an ABD. Only use this on ABDs allocated with abd_alloc(),
+ * abd_alloc_linear(), or abd_alloc_gang_abd().
+ */
+void
+abd_free(abd_t *abd)
+{
+	if (abd == NULL)
+		return;
+
+	abd_verify(abd);
+	ASSERT3P(abd->abd_parent, ==, NULL);
+	ASSERT(abd->abd_flags & ABD_FLAG_OWNER);
+	if (abd_is_linear(abd))
+		abd_free_linear(abd);
+	else if (abd_is_gang(abd))
+		abd_free_gang_abd(abd);
+	else
+		abd_free_scatter(abd);
+}
+
+/*
+ * Allocate an ABD of the same format (same metadata flag, same scatterize
+ * setting) as another ABD.
+ */
+abd_t *
+abd_alloc_sametype(abd_t *sabd, size_t size)
+{
+	boolean_t is_metadata = (sabd->abd_flags & ABD_FLAG_META) != 0;
+	if (abd_is_linear(sabd) &&
+	    !abd_is_linear_page(sabd)) {
+		return (abd_alloc_linear(size, is_metadata));
+	} else {
+		return (abd_alloc(size, is_metadata));
+	}
+}
+
+
+/*
+ * Create gang ABD that will be the head of a list of ABD's. This is used
+ * to "chain" scatter/gather lists together when constructing aggregated
+ * IO's. To free this abd, abd_free() must be called.
+ */
+abd_t *
+abd_alloc_gang_abd(void)
+{
+	abd_t *abd;
+
+	abd = abd_alloc_struct(0);
+	abd->abd_flags = ABD_FLAG_GANG | ABD_FLAG_OWNER;
+	abd->abd_size = 0;
+	abd->abd_parent = NULL;
+	list_create(&ABD_GANG(abd).abd_gang_chain,
+	    sizeof (abd_t), offsetof(abd_t, abd_gang_link));
+	zfs_refcount_create(&abd->abd_children);
+	return (abd);
+}
+
+/*
+ * Add a child gang ABD to a parent gang ABDs chained list.
+ */
+static void
+abd_gang_add_gang(abd_t *pabd, abd_t *cabd, boolean_t free_on_free)
+{
+	ASSERT(abd_is_gang(pabd));
+	ASSERT(abd_is_gang(cabd));
+
+	if (free_on_free) {
+		/*
+		 * If the parent is responsible for freeing the child gang
+		 * ABD we will just splice the childs children ABD list to
+		 * the parents list and immediately free the child gang ABD
+		 * struct. The parent gang ABDs children from the child gang
+		 * will retain all the free_on_free settings after being
+		 * added to the parents list.
+		 */
+		pabd->abd_size += cabd->abd_size;
+		list_move_tail(&ABD_GANG(pabd).abd_gang_chain,
+		    &ABD_GANG(cabd).abd_gang_chain);
+		ASSERT(list_is_empty(&ABD_GANG(cabd).abd_gang_chain));
+		abd_verify(pabd);
+		abd_free_struct(cabd);
+	} else {
+		for (abd_t *child = list_head(&ABD_GANG(cabd).abd_gang_chain);
+		    child != NULL;
+		    child = list_next(&ABD_GANG(cabd).abd_gang_chain, child)) {
+			/*
+			 * We always pass B_FALSE for free_on_free as it is the
+			 * original child gang ABDs responsibilty to determine
+			 * if any of its child ABDs should be free'd on the call
+			 * to abd_free().
+			 */
+			abd_gang_add(pabd, child, B_FALSE);
+		}
+		abd_verify(pabd);
+	}
+}
+
+/*
+ * Add a child ABD to a gang ABD's chained list.
+ */
+void
+abd_gang_add(abd_t *pabd, abd_t *cabd, boolean_t free_on_free)
+{
+	ASSERT(abd_is_gang(pabd));
+	abd_t *child_abd = NULL;
+
+	/*
+	 * If the child being added is a gang ABD, we will add the
+	 * childs ABDs to the parent gang ABD. This alllows us to account
+	 * for the offset correctly in the parent gang ABD.
+	 */
+	if (abd_is_gang(cabd)) {
+		ASSERT(!list_link_active(&cabd->abd_gang_link));
+		ASSERT(!list_is_empty(&ABD_GANG(cabd).abd_gang_chain));
+		return (abd_gang_add_gang(pabd, cabd, free_on_free));
+	}
+	ASSERT(!abd_is_gang(cabd));
+
+	/*
+	 * In order to verify that an ABD is not already part of
+	 * another gang ABD, we must lock the child ABD's abd_mtx
+	 * to check its abd_gang_link status. We unlock the abd_mtx
+	 * only after it is has been added to a gang ABD, which
+	 * will update the abd_gang_link's status. See comment below
+	 * for how an ABD can be in multiple gang ABD's simultaneously.
+	 */
+	mutex_enter(&cabd->abd_mtx);
+	if (list_link_active(&cabd->abd_gang_link)) {
+		/*
+		 * If the child ABD is already part of another
+		 * gang ABD then we must allocate a new
+		 * ABD to use a separate link. We mark the newly
+		 * allocated ABD with ABD_FLAG_GANG_FREE, before
+		 * adding it to the gang ABD's list, to make the
+		 * gang ABD aware that it is responsible to call
+		 * abd_put(). We use abd_get_offset() in order
+		 * to just allocate a new ABD but avoid copying the
+		 * data over into the newly allocated ABD.
+		 *
+		 * An ABD may become part of multiple gang ABD's. For
+		 * example, when writing ditto bocks, the same ABD
+		 * is used to write 2 or 3 locations with 2 or 3
+		 * zio_t's. Each of the zio's may be aggregated with
+		 * different adjacent zio's. zio aggregation uses gang
+		 * zio's, so the single ABD can become part of multiple
+		 * gang zio's.
+		 *
+		 * The ASSERT below is to make sure that if
+		 * free_on_free is passed as B_TRUE, the ABD can
+		 * not be in multiple gang ABD's. The gang ABD
+		 * can not be responsible for cleaning up the child
+		 * ABD memory allocation if the ABD can be in
+		 * multiple gang ABD's at one time.
+		 */
+		ASSERT3B(free_on_free, ==, B_FALSE);
+		child_abd = abd_get_offset(cabd, 0);
+		child_abd->abd_flags |= ABD_FLAG_GANG_FREE;
+	} else {
+		child_abd = cabd;
+		if (free_on_free)
+			child_abd->abd_flags |= ABD_FLAG_GANG_FREE;
+	}
+	ASSERT3P(child_abd, !=, NULL);
+
+	list_insert_tail(&ABD_GANG(pabd).abd_gang_chain, child_abd);
+	mutex_exit(&cabd->abd_mtx);
+	pabd->abd_size += child_abd->abd_size;
+}
+
+/*
+ * Locate the ABD for the supplied offset in the gang ABD.
+ * Return a new offset relative to the returned ABD.
+ */
+abd_t *
+abd_gang_get_offset(abd_t *abd, size_t *off)
+{
+	abd_t *cabd;
+
+	ASSERT(abd_is_gang(abd));
+	ASSERT3U(*off, <, abd->abd_size);
+	for (cabd = list_head(&ABD_GANG(abd).abd_gang_chain); cabd != NULL;
+	    cabd = list_next(&ABD_GANG(abd).abd_gang_chain, cabd)) {
+		if (*off >= cabd->abd_size)
+			*off -= cabd->abd_size;
+		else
+			return (cabd);
+	}
+	VERIFY3P(cabd, !=, NULL);
+	return (cabd);
+}
+
+/*
+ * Allocate a new ABD to point to offset off of sabd. It shares the underlying
+ * buffer data with sabd. Use abd_put() to free. sabd must not be freed while
+ * any derived ABDs exist.
+ */
+static abd_t *
+abd_get_offset_impl(abd_t *sabd, size_t off, size_t size)
+{
+	abd_t *abd = NULL;
+
+	abd_verify(sabd);
+	ASSERT3U(off, <=, sabd->abd_size);
+
+	if (abd_is_linear(sabd)) {
+		abd = abd_alloc_struct(0);
+
+		/*
+		 * Even if this buf is filesystem metadata, we only track that
+		 * if we own the underlying data buffer, which is not true in
+		 * this case. Therefore, we don't ever use ABD_FLAG_META here.
+		 */
+		abd->abd_flags = ABD_FLAG_LINEAR;
+
+		ABD_LINEAR_BUF(abd) = (char *)ABD_LINEAR_BUF(sabd) + off;
+	} else if (abd_is_gang(sabd)) {
+		size_t left = size;
+		abd = abd_alloc_gang_abd();
+		abd->abd_flags &= ~ABD_FLAG_OWNER;
+		for (abd_t *cabd = abd_gang_get_offset(sabd, &off);
+		    cabd != NULL && left > 0;
+		    cabd = list_next(&ABD_GANG(sabd).abd_gang_chain, cabd)) {
+			int csize = MIN(left, cabd->abd_size - off);
+
+			abd_t *nabd = abd_get_offset_impl(cabd, off, csize);
+			abd_gang_add(abd, nabd, B_FALSE);
+			left -= csize;
+			off = 0;
+		}
+		ASSERT3U(left, ==, 0);
+	} else {
+		abd = abd_get_offset_scatter(sabd, off);
+	}
+
+	abd->abd_size = size;
+	abd->abd_parent = sabd;
+	zfs_refcount_create(&abd->abd_children);
+	(void) zfs_refcount_add_many(&sabd->abd_children, abd->abd_size, abd);
+	return (abd);
+}
+
+abd_t *
+abd_get_offset(abd_t *sabd, size_t off)
+{
+	size_t size = sabd->abd_size > off ? sabd->abd_size - off : 0;
+	VERIFY3U(size, >, 0);
+	return (abd_get_offset_impl(sabd, off, size));
+}
+
+abd_t *
+abd_get_offset_size(abd_t *sabd, size_t off, size_t size)
+{
+	ASSERT3U(off + size, <=, sabd->abd_size);
+	return (abd_get_offset_impl(sabd, off, size));
+}
+
+/*
+ * Return a size scatter ABD. In order to free the returned
+ * ABD abd_put() must be called.
+ */
+abd_t *
+abd_get_zeros(size_t size)
+{
+	ASSERT3P(abd_zero_scatter, !=, NULL);
+	ASSERT3U(size, <=, SPA_MAXBLOCKSIZE);
+	return (abd_get_offset_size(abd_zero_scatter, 0, size));
+}
+
+/*
+ * Allocate a linear ABD structure for buf. You must free this with abd_put()
+ * since the resulting ABD doesn't own its own buffer.
+ */
+abd_t *
+abd_get_from_buf(void *buf, size_t size)
+{
+	abd_t *abd = abd_alloc_struct(0);
+
+	VERIFY3U(size, <=, SPA_MAXBLOCKSIZE);
+
+	/*
+	 * Even if this buf is filesystem metadata, we only track that if we
+	 * own the underlying data buffer, which is not true in this case.
+	 * Therefore, we don't ever use ABD_FLAG_META here.
+	 */
+	abd->abd_flags = ABD_FLAG_LINEAR;
+	abd->abd_size = size;
+	abd->abd_parent = NULL;
+	zfs_refcount_create(&abd->abd_children);
+
+	ABD_LINEAR_BUF(abd) = buf;
+
+	return (abd);
+}
+
+/*
+ * Get the raw buffer associated with a linear ABD.
+ */
+void *
+abd_to_buf(abd_t *abd)
+{
+	ASSERT(abd_is_linear(abd));
+	abd_verify(abd);
+	return (ABD_LINEAR_BUF(abd));
+}
+
+/*
+ * Borrow a raw buffer from an ABD without copying the contents of the ABD
+ * into the buffer. If the ABD is scattered, this will allocate a raw buffer
+ * whose contents are undefined. To copy over the existing data in the ABD, use
+ * abd_borrow_buf_copy() instead.
+ */
+void *
+abd_borrow_buf(abd_t *abd, size_t n)
+{
+	void *buf;
+	abd_verify(abd);
+	ASSERT3U(abd->abd_size, >=, n);
+	if (abd_is_linear(abd)) {
+		buf = abd_to_buf(abd);
+	} else {
+		buf = zio_buf_alloc(n);
+	}
+	(void) zfs_refcount_add_many(&abd->abd_children, n, buf);
+	return (buf);
+}
+
+void *
+abd_borrow_buf_copy(abd_t *abd, size_t n)
+{
+	void *buf = abd_borrow_buf(abd, n);
+	if (!abd_is_linear(abd)) {
+		abd_copy_to_buf(buf, abd, n);
+	}
+	return (buf);
+}
+
+/*
+ * Return a borrowed raw buffer to an ABD. If the ABD is scattered, this will
+ * not change the contents of the ABD and will ASSERT that you didn't modify
+ * the buffer since it was borrowed. If you want any changes you made to buf to
+ * be copied back to abd, use abd_return_buf_copy() instead.
+ */
+void
+abd_return_buf(abd_t *abd, void *buf, size_t n)
+{
+	abd_verify(abd);
+	ASSERT3U(abd->abd_size, >=, n);
+	if (abd_is_linear(abd)) {
+		ASSERT3P(buf, ==, abd_to_buf(abd));
+	} else {
+		ASSERT0(abd_cmp_buf(abd, buf, n));
+		zio_buf_free(buf, n);
+	}
+	(void) zfs_refcount_remove_many(&abd->abd_children, n, buf);
+}
+
+void
+abd_return_buf_copy(abd_t *abd, void *buf, size_t n)
+{
+	if (!abd_is_linear(abd)) {
+		abd_copy_from_buf(abd, buf, n);
+	}
+	abd_return_buf(abd, buf, n);
+}
+
+void
+abd_release_ownership_of_buf(abd_t *abd)
+{
+	ASSERT(abd_is_linear(abd));
+	ASSERT(abd->abd_flags & ABD_FLAG_OWNER);
+
+	/*
+	 * abd_free() needs to handle LINEAR_PAGE ABD's specially.
+	 * Since that flag does not survive the
+	 * abd_release_ownership_of_buf() -> abd_get_from_buf() ->
+	 * abd_take_ownership_of_buf() sequence, we don't allow releasing
+	 * these "linear but not zio_[data_]buf_alloc()'ed" ABD's.
+	 */
+	ASSERT(!abd_is_linear_page(abd));
+
+	abd_verify(abd);
+
+	abd->abd_flags &= ~ABD_FLAG_OWNER;
+	/* Disable this flag since we no longer own the data buffer */
+	abd->abd_flags &= ~ABD_FLAG_META;
+
+	abd_update_linear_stats(abd, ABDSTAT_DECR);
+}
+
+
+/*
+ * Give this ABD ownership of the buffer that it's storing. Can only be used on
+ * linear ABDs which were allocated via abd_get_from_buf(), or ones allocated
+ * with abd_alloc_linear() which subsequently released ownership of their buf
+ * with abd_release_ownership_of_buf().
+ */
+void
+abd_take_ownership_of_buf(abd_t *abd, boolean_t is_metadata)
+{
+	ASSERT(abd_is_linear(abd));
+	ASSERT(!(abd->abd_flags & ABD_FLAG_OWNER));
+	abd_verify(abd);
+
+	abd->abd_flags |= ABD_FLAG_OWNER;
+	if (is_metadata) {
+		abd->abd_flags |= ABD_FLAG_META;
+	}
+
+	abd_update_linear_stats(abd, ABDSTAT_INCR);
+}
+
+/*
+ * Initializes an abd_iter based on whether the abd is a gang ABD
+ * or just a single ABD.
+ */
+static inline abd_t *
+abd_init_abd_iter(abd_t *abd, struct abd_iter *aiter, size_t off)
+{
+	abd_t *cabd = NULL;
+
+	if (abd_is_gang(abd)) {
+		cabd = abd_gang_get_offset(abd, &off);
+		if (cabd) {
+			abd_iter_init(aiter, cabd);
+			abd_iter_advance(aiter, off);
+		}
+	} else {
+		abd_iter_init(aiter, abd);
+		abd_iter_advance(aiter, off);
+	}
+	return (cabd);
+}
+
+/*
+ * Advances an abd_iter. We have to be careful with gang ABD as
+ * advancing could mean that we are at the end of a particular ABD and
+ * must grab the ABD in the gang ABD's list.
+ */
+static inline abd_t *
+abd_advance_abd_iter(abd_t *abd, abd_t *cabd, struct abd_iter *aiter,
+    size_t len)
+{
+	abd_iter_advance(aiter, len);
+	if (abd_is_gang(abd) && abd_iter_at_end(aiter)) {
+		ASSERT3P(cabd, !=, NULL);
+		cabd = list_next(&ABD_GANG(abd).abd_gang_chain, cabd);
+		if (cabd) {
+			abd_iter_init(aiter, cabd);
+			abd_iter_advance(aiter, 0);
+		}
+	}
+	return (cabd);
+}
+
+int
+abd_iterate_func(abd_t *abd, size_t off, size_t size,
+    abd_iter_func_t *func, void *private)
+{
+	int ret = 0;
+	struct abd_iter aiter;
+	boolean_t abd_multi;
+	abd_t *c_abd;
+
+	abd_verify(abd);
+	ASSERT3U(off + size, <=, abd->abd_size);
+
+	abd_multi = abd_is_gang(abd);
+	c_abd = abd_init_abd_iter(abd, &aiter, off);
+
+	while (size > 0) {
+		/* If we are at the end of the gang ABD we are done */
+		if (abd_multi && !c_abd)
+			break;
+
+		abd_iter_map(&aiter);
+
+		size_t len = MIN(aiter.iter_mapsize, size);
+		ASSERT3U(len, >, 0);
+
+		ret = func(aiter.iter_mapaddr, len, private);
+
+		abd_iter_unmap(&aiter);
+
+		if (ret != 0)
+			break;
+
+		size -= len;
+		c_abd = abd_advance_abd_iter(abd, c_abd, &aiter, len);
+	}
+
+	return (ret);
+}
+
+struct buf_arg {
+	void *arg_buf;
+};
+
+static int
+abd_copy_to_buf_off_cb(void *buf, size_t size, void *private)
+{
+	struct buf_arg *ba_ptr = private;
+
+	(void) memcpy(ba_ptr->arg_buf, buf, size);
+	ba_ptr->arg_buf = (char *)ba_ptr->arg_buf + size;
+
+	return (0);
+}
+
+/*
+ * Copy abd to buf. (off is the offset in abd.)
+ */
+void
+abd_copy_to_buf_off(void *buf, abd_t *abd, size_t off, size_t size)
+{
+	struct buf_arg ba_ptr = { buf };
+
+	(void) abd_iterate_func(abd, off, size, abd_copy_to_buf_off_cb,
+	    &ba_ptr);
+}
+
+static int
+abd_cmp_buf_off_cb(void *buf, size_t size, void *private)
+{
+	int ret;
+	struct buf_arg *ba_ptr = private;
+
+	ret = memcmp(buf, ba_ptr->arg_buf, size);
+	ba_ptr->arg_buf = (char *)ba_ptr->arg_buf + size;
+
+	return (ret);
+}
+
+/*
+ * Compare the contents of abd to buf. (off is the offset in abd.)
+ */
+int
+abd_cmp_buf_off(abd_t *abd, const void *buf, size_t off, size_t size)
+{
+	struct buf_arg ba_ptr = { (void *) buf };
+
+	return (abd_iterate_func(abd, off, size, abd_cmp_buf_off_cb, &ba_ptr));
+}
+
+static int
+abd_copy_from_buf_off_cb(void *buf, size_t size, void *private)
+{
+	struct buf_arg *ba_ptr = private;
+
+	(void) memcpy(buf, ba_ptr->arg_buf, size);
+	ba_ptr->arg_buf = (char *)ba_ptr->arg_buf + size;
+
+	return (0);
+}
+
+/*
+ * Copy from buf to abd. (off is the offset in abd.)
+ */
+void
+abd_copy_from_buf_off(abd_t *abd, const void *buf, size_t off, size_t size)
+{
+	struct buf_arg ba_ptr = { (void *) buf };
+
+	(void) abd_iterate_func(abd, off, size, abd_copy_from_buf_off_cb,
+	    &ba_ptr);
+}
+
+/*ARGSUSED*/
+static int
+abd_zero_off_cb(void *buf, size_t size, void *private)
+{
+	(void) memset(buf, 0, size);
+	return (0);
+}
+
+/*
+ * Zero out the abd from a particular offset to the end.
+ */
+void
+abd_zero_off(abd_t *abd, size_t off, size_t size)
+{
+	(void) abd_iterate_func(abd, off, size, abd_zero_off_cb, NULL);
+}
+
+/*
+ * Iterate over two ABDs and call func incrementally on the two ABDs' data in
+ * equal-sized chunks (passed to func as raw buffers). func could be called many
+ * times during this iteration.
+ */
+int
+abd_iterate_func2(abd_t *dabd, abd_t *sabd, size_t doff, size_t soff,
+    size_t size, abd_iter_func2_t *func, void *private)
+{
+	int ret = 0;
+	struct abd_iter daiter, saiter;
+	boolean_t dabd_is_gang_abd, sabd_is_gang_abd;
+	abd_t *c_dabd, *c_sabd;
+
+	abd_verify(dabd);
+	abd_verify(sabd);
+
+	ASSERT3U(doff + size, <=, dabd->abd_size);
+	ASSERT3U(soff + size, <=, sabd->abd_size);
+
+	dabd_is_gang_abd = abd_is_gang(dabd);
+	sabd_is_gang_abd = abd_is_gang(sabd);
+	c_dabd = abd_init_abd_iter(dabd, &daiter, doff);
+	c_sabd = abd_init_abd_iter(sabd, &saiter, soff);
+
+	while (size > 0) {
+		/* if we are at the end of the gang ABD we are done */
+		if ((dabd_is_gang_abd && !c_dabd) ||
+		    (sabd_is_gang_abd && !c_sabd))
+			break;
+
+		abd_iter_map(&daiter);
+		abd_iter_map(&saiter);
+
+		size_t dlen = MIN(daiter.iter_mapsize, size);
+		size_t slen = MIN(saiter.iter_mapsize, size);
+		size_t len = MIN(dlen, slen);
+		ASSERT(dlen > 0 || slen > 0);
+
+		ret = func(daiter.iter_mapaddr, saiter.iter_mapaddr, len,
+		    private);
+
+		abd_iter_unmap(&saiter);
+		abd_iter_unmap(&daiter);
+
+		if (ret != 0)
+			break;
+
+		size -= len;
+		c_dabd =
+		    abd_advance_abd_iter(dabd, c_dabd, &daiter, len);
+		c_sabd =
+		    abd_advance_abd_iter(sabd, c_sabd, &saiter, len);
+	}
+
+	return (ret);
+}
+
+/*ARGSUSED*/
+static int
+abd_copy_off_cb(void *dbuf, void *sbuf, size_t size, void *private)
+{
+	(void) memcpy(dbuf, sbuf, size);
+	return (0);
+}
+
+/*
+ * Copy from sabd to dabd starting from soff and doff.
+ */
+void
+abd_copy_off(abd_t *dabd, abd_t *sabd, size_t doff, size_t soff, size_t size)
+{
+	(void) abd_iterate_func2(dabd, sabd, doff, soff, size,
+	    abd_copy_off_cb, NULL);
+}
+
+/*ARGSUSED*/
+static int
+abd_cmp_cb(void *bufa, void *bufb, size_t size, void *private)
+{
+	return (memcmp(bufa, bufb, size));
+}
+
+/*
+ * Compares the contents of two ABDs.
+ */
+int
+abd_cmp(abd_t *dabd, abd_t *sabd)
+{
+	ASSERT3U(dabd->abd_size, ==, sabd->abd_size);
+	return (abd_iterate_func2(dabd, sabd, 0, 0, dabd->abd_size,
+	    abd_cmp_cb, NULL));
+}
+
+/*
+ * Iterate over code ABDs and a data ABD and call @func_raidz_gen.
+ *
+ * @cabds          parity ABDs, must have equal size
+ * @dabd           data ABD. Can be NULL (in this case @dsize = 0)
+ * @func_raidz_gen should be implemented so that its behaviour
+ *                 is the same when taking linear and when taking scatter
+ */
+void
+abd_raidz_gen_iterate(abd_t **cabds, abd_t *dabd,
+    ssize_t csize, ssize_t dsize, const unsigned parity,
+    void (*func_raidz_gen)(void **, const void *, size_t, size_t))
+{
+	int i;
+	ssize_t len, dlen;
+	struct abd_iter caiters[3];
+	struct abd_iter daiter = {0};
+	void *caddrs[3];
+	unsigned long flags __maybe_unused = 0;
+	abd_t *c_cabds[3];
+	abd_t *c_dabd = NULL;
+	boolean_t cabds_is_gang_abd[3];
+	boolean_t dabd_is_gang_abd = B_FALSE;
+
+	ASSERT3U(parity, <=, 3);
+
+	for (i = 0; i < parity; i++) {
+		cabds_is_gang_abd[i] = abd_is_gang(cabds[i]);
+		c_cabds[i] = abd_init_abd_iter(cabds[i], &caiters[i], 0);
+	}
+
+	if (dabd) {
+		dabd_is_gang_abd = abd_is_gang(dabd);
+		c_dabd = abd_init_abd_iter(dabd, &daiter, 0);
+	}
+
+	ASSERT3S(dsize, >=, 0);
+
+	abd_enter_critical(flags);
+	while (csize > 0) {
+		/* if we are at the end of the gang ABD we are done */
+		if (dabd_is_gang_abd && !c_dabd)
+			break;
+
+		for (i = 0; i < parity; i++) {
+			/*
+			 * If we are at the end of the gang ABD we are
+			 * done.
+			 */
+			if (cabds_is_gang_abd[i] && !c_cabds[i])
+				break;
+			abd_iter_map(&caiters[i]);
+			caddrs[i] = caiters[i].iter_mapaddr;
+		}
+
+		len = csize;
+
+		if (dabd && dsize > 0)
+			abd_iter_map(&daiter);
+
+		switch (parity) {
+			case 3:
+				len = MIN(caiters[2].iter_mapsize, len);
+				/* falls through */
+			case 2:
+				len = MIN(caiters[1].iter_mapsize, len);
+				/* falls through */
+			case 1:
+				len = MIN(caiters[0].iter_mapsize, len);
+		}
+
+		/* must be progressive */
+		ASSERT3S(len, >, 0);
+
+		if (dabd && dsize > 0) {
+			/* this needs precise iter.length */
+			len = MIN(daiter.iter_mapsize, len);
+			dlen = len;
+		} else
+			dlen = 0;
+
+		/* must be progressive */
+		ASSERT3S(len, >, 0);
+		/*
+		 * The iterated function likely will not do well if each
+		 * segment except the last one is not multiple of 512 (raidz).
+		 */
+		ASSERT3U(((uint64_t)len & 511ULL), ==, 0);
+
+		func_raidz_gen(caddrs, daiter.iter_mapaddr, len, dlen);
+
+		for (i = parity-1; i >= 0; i--) {
+			abd_iter_unmap(&caiters[i]);
+			c_cabds[i] =
+			    abd_advance_abd_iter(cabds[i], c_cabds[i],
+			    &caiters[i], len);
+		}
+
+		if (dabd && dsize > 0) {
+			abd_iter_unmap(&daiter);
+			c_dabd =
+			    abd_advance_abd_iter(dabd, c_dabd, &daiter,
+			    dlen);
+			dsize -= dlen;
+		}
+
+		csize -= len;
+
+		ASSERT3S(dsize, >=, 0);
+		ASSERT3S(csize, >=, 0);
+	}
+	abd_exit_critical(flags);
+}
+
+/*
+ * Iterate over code ABDs and data reconstruction target ABDs and call
+ * @func_raidz_rec. Function maps at most 6 pages atomically.
+ *
+ * @cabds           parity ABDs, must have equal size
+ * @tabds           rec target ABDs, at most 3
+ * @tsize           size of data target columns
+ * @func_raidz_rec  expects syndrome data in target columns. Function
+ *                  reconstructs data and overwrites target columns.
+ */
+void
+abd_raidz_rec_iterate(abd_t **cabds, abd_t **tabds,
+    ssize_t tsize, const unsigned parity,
+    void (*func_raidz_rec)(void **t, const size_t tsize, void **c,
+    const unsigned *mul),
+    const unsigned *mul)
+{
+	int i;
+	ssize_t len;
+	struct abd_iter citers[3];
+	struct abd_iter xiters[3];
+	void *caddrs[3], *xaddrs[3];
+	unsigned long flags __maybe_unused = 0;
+	boolean_t cabds_is_gang_abd[3];
+	boolean_t tabds_is_gang_abd[3];
+	abd_t *c_cabds[3];
+	abd_t *c_tabds[3];
+
+	ASSERT3U(parity, <=, 3);
+
+	for (i = 0; i < parity; i++) {
+		cabds_is_gang_abd[i] = abd_is_gang(cabds[i]);
+		tabds_is_gang_abd[i] = abd_is_gang(tabds[i]);
+		c_cabds[i] =
+		    abd_init_abd_iter(cabds[i], &citers[i], 0);
+		c_tabds[i] =
+		    abd_init_abd_iter(tabds[i], &xiters[i], 0);
+	}
+
+	abd_enter_critical(flags);
+	while (tsize > 0) {
+
+		for (i = 0; i < parity; i++) {
+			/*
+			 * If we are at the end of the gang ABD we
+			 * are done.
+			 */
+			if (cabds_is_gang_abd[i] && !c_cabds[i])
+				break;
+			if (tabds_is_gang_abd[i] && !c_tabds[i])
+				break;
+			abd_iter_map(&citers[i]);
+			abd_iter_map(&xiters[i]);
+			caddrs[i] = citers[i].iter_mapaddr;
+			xaddrs[i] = xiters[i].iter_mapaddr;
+		}
+
+		len = tsize;
+		switch (parity) {
+			case 3:
+				len = MIN(xiters[2].iter_mapsize, len);
+				len = MIN(citers[2].iter_mapsize, len);
+				/* falls through */
+			case 2:
+				len = MIN(xiters[1].iter_mapsize, len);
+				len = MIN(citers[1].iter_mapsize, len);
+				/* falls through */
+			case 1:
+				len = MIN(xiters[0].iter_mapsize, len);
+				len = MIN(citers[0].iter_mapsize, len);
+		}
+		/* must be progressive */
+		ASSERT3S(len, >, 0);
+		/*
+		 * The iterated function likely will not do well if each
+		 * segment except the last one is not multiple of 512 (raidz).
+		 */
+		ASSERT3U(((uint64_t)len & 511ULL), ==, 0);
+
+		func_raidz_rec(xaddrs, len, caddrs, mul);
+
+		for (i = parity-1; i >= 0; i--) {
+			abd_iter_unmap(&xiters[i]);
+			abd_iter_unmap(&citers[i]);
+			c_tabds[i] =
+			    abd_advance_abd_iter(tabds[i], c_tabds[i],
+			    &xiters[i], len);
+			c_cabds[i] =
+			    abd_advance_abd_iter(cabds[i], c_cabds[i],
+			    &citers[i], len);
+		}
+
+		tsize -= len;
+		ASSERT3S(tsize, >=, 0);
+	}
+	abd_exit_critical(flags);
+}
diff --git a/sys/contrib/openzfs/module/zfs/aggsum.c b/sys/contrib/openzfs/module/zfs/aggsum.c
new file mode 100644
index 000000000000..a2fec27744e1
--- /dev/null
+++ b/sys/contrib/openzfs/module/zfs/aggsum.c
@@ -0,0 +1,237 @@
+/*
+ * CDDL HEADER START
+ *
+ * This file and its contents are supplied under the terms of the
+ * Common Development and Distribution License ("CDDL"), version 1.0.
+ * You may only use this file in accordance with the terms of version
+ * 1.0 of the CDDL.
+ *
+ * A full copy of the text of the CDDL should have accompanied this
+ * source.  A copy of the CDDL is also available via the Internet at
+ * http://www.illumos.org/license/CDDL.
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright (c) 2017, 2018 by Delphix. All rights reserved.
+ */
+
+#include <sys/zfs_context.h>
+#include <sys/aggsum.h>
+
+/*
+ * Aggregate-sum counters are a form of fanned-out counter, used when atomic
+ * instructions on a single field cause enough CPU cache line contention to
+ * slow system performance. Due to their increased overhead and the expense
+ * involved with precisely reading from them, they should only be used in cases
+ * where the write rate (increment/decrement) is much higher than the read rate
+ * (get value).
+ *
+ * Aggregate sum counters are comprised of two basic parts, the core and the
+ * buckets. The core counter contains a lock for the entire counter, as well
+ * as the current upper and lower bounds on the value of the counter. The
+ * aggsum_bucket structure contains a per-bucket lock to protect the contents of
+ * the bucket, the current amount that this bucket has changed from the global
+ * counter (called the delta), and the amount of increment and decrement we have
+ * "borrowed" from the core counter.
+ *
+ * The basic operation of an aggsum is simple. Threads that wish to modify the
+ * counter will modify one bucket's counter (determined by their current CPU, to
+ * help minimize lock and cache contention). If the bucket already has
+ * sufficient capacity borrowed from the core structure to handle their request,
+ * they simply modify the delta and return.  If the bucket does not, we clear
+ * the bucket's current state (to prevent the borrowed amounts from getting too
+ * large), and borrow more from the core counter. Borrowing is done by adding to
+ * the upper bound (or subtracting from the lower bound) of the core counter,
+ * and setting the borrow value for the bucket to the amount added (or
+ * subtracted).  Clearing the bucket is the opposite; we add the current delta
+ * to both the lower and upper bounds of the core counter, subtract the borrowed
+ * incremental from the upper bound, and add the borrowed decrement from the
+ * lower bound.  Note that only borrowing and clearing require access to the
+ * core counter; since all other operations access CPU-local resources,
+ * performance can be much higher than a traditional counter.
+ *
+ * Threads that wish to read from the counter have a slightly more challenging
+ * task. It is fast to determine the upper and lower bounds of the aggum; this
+ * does not require grabbing any locks. This suffices for cases where an
+ * approximation of the aggsum's value is acceptable. However, if one needs to
+ * know whether some specific value is above or below the current value in the
+ * aggsum, they invoke aggsum_compare(). This function operates by repeatedly
+ * comparing the target value to the upper and lower bounds of the aggsum, and
+ * then clearing a bucket. This proceeds until the target is outside of the
+ * upper and lower bounds and we return a response, or the last bucket has been
+ * cleared and we know that the target is equal to the aggsum's value. Finally,
+ * the most expensive operation is determining the precise value of the aggsum.
+ * To do this, we clear every bucket and then return the upper bound (which must
+ * be equal to the lower bound). What makes aggsum_compare() and aggsum_value()
+ * expensive is clearing buckets. This involves grabbing the global lock
+ * (serializing against themselves and borrow operations), grabbing a bucket's
+ * lock (preventing threads on those CPUs from modifying their delta), and
+ * zeroing out the borrowed value (forcing that thread to borrow on its next
+ * request, which will also be expensive).  This is what makes aggsums well
+ * suited for write-many read-rarely operations.
+ */
+
+/*
+ * We will borrow aggsum_borrow_multiplier times the current request, so we will
+ * have to get the as_lock approximately every aggsum_borrow_multiplier calls to
+ * aggsum_delta().
+ */
+static uint_t aggsum_borrow_multiplier = 10;
+
+void
+aggsum_init(aggsum_t *as, uint64_t value)
+{
+	bzero(as, sizeof (*as));
+	as->as_lower_bound = as->as_upper_bound = value;
+	mutex_init(&as->as_lock, NULL, MUTEX_DEFAULT, NULL);
+	as->as_numbuckets = boot_ncpus;
+	as->as_buckets = kmem_zalloc(boot_ncpus * sizeof (aggsum_bucket_t),
+	    KM_SLEEP);
+	for (int i = 0; i < as->as_numbuckets; i++) {
+		mutex_init(&as->as_buckets[i].asc_lock,
+		    NULL, MUTEX_DEFAULT, NULL);
+	}
+}
+
+void
+aggsum_fini(aggsum_t *as)
+{
+	for (int i = 0; i < as->as_numbuckets; i++)
+		mutex_destroy(&as->as_buckets[i].asc_lock);
+	kmem_free(as->as_buckets, as->as_numbuckets * sizeof (aggsum_bucket_t));
+	mutex_destroy(&as->as_lock);
+}
+
+int64_t
+aggsum_lower_bound(aggsum_t *as)
+{
+	return (as->as_lower_bound);
+}
+
+int64_t
+aggsum_upper_bound(aggsum_t *as)
+{
+	return (as->as_upper_bound);
+}
+
+static void
+aggsum_flush_bucket(aggsum_t *as, struct aggsum_bucket *asb)
+{
+	ASSERT(MUTEX_HELD(&as->as_lock));
+	ASSERT(MUTEX_HELD(&asb->asc_lock));
+
+	/*
+	 * We use atomic instructions for this because we read the upper and
+	 * lower bounds without the lock, so we need stores to be atomic.
+	 */
+	atomic_add_64((volatile uint64_t *)&as->as_lower_bound,
+	    asb->asc_delta + asb->asc_borrowed);
+	atomic_add_64((volatile uint64_t *)&as->as_upper_bound,
+	    asb->asc_delta - asb->asc_borrowed);
+	asb->asc_delta = 0;
+	asb->asc_borrowed = 0;
+}
+
+uint64_t
+aggsum_value(aggsum_t *as)
+{
+	int64_t rv;
+
+	mutex_enter(&as->as_lock);
+	if (as->as_lower_bound == as->as_upper_bound) {
+		rv = as->as_lower_bound;
+		for (int i = 0; i < as->as_numbuckets; i++) {
+			ASSERT0(as->as_buckets[i].asc_delta);
+			ASSERT0(as->as_buckets[i].asc_borrowed);
+		}
+		mutex_exit(&as->as_lock);
+		return (rv);
+	}
+	for (int i = 0; i < as->as_numbuckets; i++) {
+		struct aggsum_bucket *asb = &as->as_buckets[i];
+		mutex_enter(&asb->asc_lock);
+		aggsum_flush_bucket(as, asb);
+		mutex_exit(&asb->asc_lock);
+	}
+	VERIFY3U(as->as_lower_bound, ==, as->as_upper_bound);
+	rv = as->as_lower_bound;
+	mutex_exit(&as->as_lock);
+
+	return (rv);
+}
+
+void
+aggsum_add(aggsum_t *as, int64_t delta)
+{
+	struct aggsum_bucket *asb;
+	int64_t borrow;
+
+	kpreempt_disable();
+	asb = &as->as_buckets[CPU_SEQID % as->as_numbuckets];
+	kpreempt_enable();
+
+	/* Try fast path if we already borrowed enough before. */
+	mutex_enter(&asb->asc_lock);
+	if (asb->asc_delta + delta <= (int64_t)asb->asc_borrowed &&
+	    asb->asc_delta + delta >= -(int64_t)asb->asc_borrowed) {
+		asb->asc_delta += delta;
+		mutex_exit(&asb->asc_lock);
+		return;
+	}
+	mutex_exit(&asb->asc_lock);
+
+	/*
+	 * We haven't borrowed enough.  Take the global lock and borrow
+	 * considering what is requested now and what we borrowed before.
+	 */
+	borrow = (delta < 0 ? -delta : delta) * aggsum_borrow_multiplier;
+	mutex_enter(&as->as_lock);
+	mutex_enter(&asb->asc_lock);
+	delta += asb->asc_delta;
+	asb->asc_delta = 0;
+	if (borrow >= asb->asc_borrowed)
+		borrow -= asb->asc_borrowed;
+	else
+		borrow = (borrow - (int64_t)asb->asc_borrowed) / 4;
+	asb->asc_borrowed += borrow;
+	atomic_add_64((volatile uint64_t *)&as->as_lower_bound,
+	    delta - borrow);
+	atomic_add_64((volatile uint64_t *)&as->as_upper_bound,
+	    delta + borrow);
+	mutex_exit(&asb->asc_lock);
+	mutex_exit(&as->as_lock);
+}
+
+/*
+ * Compare the aggsum value to target efficiently. Returns -1 if the value
+ * represented by the aggsum is less than target, 1 if it's greater, and 0 if
+ * they are equal.
+ */
+int
+aggsum_compare(aggsum_t *as, uint64_t target)
+{
+	if (as->as_upper_bound < target)
+		return (-1);
+	if (as->as_lower_bound > target)
+		return (1);
+	mutex_enter(&as->as_lock);
+	for (int i = 0; i < as->as_numbuckets; i++) {
+		struct aggsum_bucket *asb = &as->as_buckets[i];
+		mutex_enter(&asb->asc_lock);
+		aggsum_flush_bucket(as, asb);
+		mutex_exit(&asb->asc_lock);
+		if (as->as_upper_bound < target) {
+			mutex_exit(&as->as_lock);
+			return (-1);
+		}
+		if (as->as_lower_bound > target) {
+			mutex_exit(&as->as_lock);
+			return (1);
+		}
+	}
+	VERIFY3U(as->as_lower_bound, ==, as->as_upper_bound);
+	ASSERT3U(as->as_lower_bound, ==, target);
+	mutex_exit(&as->as_lock);
+	return (0);
+}
diff --git a/sys/contrib/openzfs/module/zfs/arc.c b/sys/contrib/openzfs/module/zfs/arc.c
new file mode 100644
index 000000000000..bd1a993dca92
--- /dev/null
+++ b/sys/contrib/openzfs/module/zfs/arc.c
@@ -0,0 +1,10565 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, Joyent, Inc.
+ * Copyright (c) 2011, 2019, Delphix. All rights reserved.
+ * Copyright (c) 2014, Saso Kiselkov. All rights reserved.
+ * Copyright (c) 2017, Nexenta Systems, Inc.  All rights reserved.
+ * Copyright (c) 2019, loli10K <ezomori.nozomu@gmail.com>. All rights reserved.
+ * Copyright (c) 2020, George Amanakis. All rights reserved.
+ * Copyright (c) 2019, Klara Inc.
+ * Copyright (c) 2019, Allan Jude
+ * Copyright (c) 2020, The FreeBSD Foundation [1]
+ *
+ * [1] Portions of this software were developed by Allan Jude
+ *     under sponsorship from the FreeBSD Foundation.
+ */
+
+/*
+ * DVA-based Adjustable Replacement Cache
+ *
+ * While much of the theory of operation used here is
+ * based on the self-tuning, low overhead replacement cache
+ * presented by Megiddo and Modha at FAST 2003, there are some
+ * significant differences:
+ *
+ * 1. The Megiddo and Modha model assumes any page is evictable.
+ * Pages in its cache cannot be "locked" into memory.  This makes
+ * the eviction algorithm simple: evict the last page in the list.
+ * This also make the performance characteristics easy to reason
+ * about.  Our cache is not so simple.  At any given moment, some
+ * subset of the blocks in the cache are un-evictable because we
+ * have handed out a reference to them.  Blocks are only evictable
+ * when there are no external references active.  This makes
+ * eviction far more problematic:  we choose to evict the evictable
+ * blocks that are the "lowest" in the list.
+ *
+ * There are times when it is not possible to evict the requested
+ * space.  In these circumstances we are unable to adjust the cache
+ * size.  To prevent the cache growing unbounded at these times we
+ * implement a "cache throttle" that slows the flow of new data
+ * into the cache until we can make space available.
+ *
+ * 2. The Megiddo and Modha model assumes a fixed cache size.
+ * Pages are evicted when the cache is full and there is a cache
+ * miss.  Our model has a variable sized cache.  It grows with
+ * high use, but also tries to react to memory pressure from the
+ * operating system: decreasing its size when system memory is
+ * tight.
+ *
+ * 3. The Megiddo and Modha model assumes a fixed page size. All
+ * elements of the cache are therefore exactly the same size.  So
+ * when adjusting the cache size following a cache miss, its simply
+ * a matter of choosing a single page to evict.  In our model, we
+ * have variable sized cache blocks (ranging from 512 bytes to
+ * 128K bytes).  We therefore choose a set of blocks to evict to make
+ * space for a cache miss that approximates as closely as possible
+ * the space used by the new block.
+ *
+ * See also:  "ARC: A Self-Tuning, Low Overhead Replacement Cache"
+ * by N. Megiddo & D. Modha, FAST 2003
+ */
+
+/*
+ * The locking model:
+ *
+ * A new reference to a cache buffer can be obtained in two
+ * ways: 1) via a hash table lookup using the DVA as a key,
+ * or 2) via one of the ARC lists.  The arc_read() interface
+ * uses method 1, while the internal ARC algorithms for
+ * adjusting the cache use method 2.  We therefore provide two
+ * types of locks: 1) the hash table lock array, and 2) the
+ * ARC list locks.
+ *
+ * Buffers do not have their own mutexes, rather they rely on the
+ * hash table mutexes for the bulk of their protection (i.e. most
+ * fields in the arc_buf_hdr_t are protected by these mutexes).
+ *
+ * buf_hash_find() returns the appropriate mutex (held) when it
+ * locates the requested buffer in the hash table.  It returns
+ * NULL for the mutex if the buffer was not in the table.
+ *
+ * buf_hash_remove() expects the appropriate hash mutex to be
+ * already held before it is invoked.
+ *
+ * Each ARC state also has a mutex which is used to protect the
+ * buffer list associated with the state.  When attempting to
+ * obtain a hash table lock while holding an ARC list lock you
+ * must use: mutex_tryenter() to avoid deadlock.  Also note that
+ * the active state mutex must be held before the ghost state mutex.
+ *
+ * It as also possible to register a callback which is run when the
+ * arc_meta_limit is reached and no buffers can be safely evicted.  In
+ * this case the arc user should drop a reference on some arc buffers so
+ * they can be reclaimed and the arc_meta_limit honored.  For example,
+ * when using the ZPL each dentry holds a references on a znode.  These
+ * dentries must be pruned before the arc buffer holding the znode can
+ * be safely evicted.
+ *
+ * Note that the majority of the performance stats are manipulated
+ * with atomic operations.
+ *
+ * The L2ARC uses the l2ad_mtx on each vdev for the following:
+ *
+ *	- L2ARC buflist creation
+ *	- L2ARC buflist eviction
+ *	- L2ARC write completion, which walks L2ARC buflists
+ *	- ARC header destruction, as it removes from L2ARC buflists
+ *	- ARC header release, as it removes from L2ARC buflists
+ */
+
+/*
+ * ARC operation:
+ *
+ * Every block that is in the ARC is tracked by an arc_buf_hdr_t structure.
+ * This structure can point either to a block that is still in the cache or to
+ * one that is only accessible in an L2 ARC device, or it can provide
+ * information about a block that was recently evicted. If a block is
+ * only accessible in the L2ARC, then the arc_buf_hdr_t only has enough
+ * information to retrieve it from the L2ARC device. This information is
+ * stored in the l2arc_buf_hdr_t sub-structure of the arc_buf_hdr_t. A block
+ * that is in this state cannot access the data directly.
+ *
+ * Blocks that are actively being referenced or have not been evicted
+ * are cached in the L1ARC. The L1ARC (l1arc_buf_hdr_t) is a structure within
+ * the arc_buf_hdr_t that will point to the data block in memory. A block can
+ * only be read by a consumer if it has an l1arc_buf_hdr_t. The L1ARC
+ * caches data in two ways -- in a list of ARC buffers (arc_buf_t) and
+ * also in the arc_buf_hdr_t's private physical data block pointer (b_pabd).
+ *
+ * The L1ARC's data pointer may or may not be uncompressed. The ARC has the
+ * ability to store the physical data (b_pabd) associated with the DVA of the
+ * arc_buf_hdr_t. Since the b_pabd is a copy of the on-disk physical block,
+ * it will match its on-disk compression characteristics. This behavior can be
+ * disabled by setting 'zfs_compressed_arc_enabled' to B_FALSE. When the
+ * compressed ARC functionality is disabled, the b_pabd will point to an
+ * uncompressed version of the on-disk data.
+ *
+ * Data in the L1ARC is not accessed by consumers of the ARC directly. Each
+ * arc_buf_hdr_t can have multiple ARC buffers (arc_buf_t) which reference it.
+ * Each ARC buffer (arc_buf_t) is being actively accessed by a specific ARC
+ * consumer. The ARC will provide references to this data and will keep it
+ * cached until it is no longer in use. The ARC caches only the L1ARC's physical
+ * data block and will evict any arc_buf_t that is no longer referenced. The
+ * amount of memory consumed by the arc_buf_ts' data buffers can be seen via the
+ * "overhead_size" kstat.
+ *
+ * Depending on the consumer, an arc_buf_t can be requested in uncompressed or
+ * compressed form. The typical case is that consumers will want uncompressed
+ * data, and when that happens a new data buffer is allocated where the data is
+ * decompressed for them to use. Currently the only consumer who wants
+ * compressed arc_buf_t's is "zfs send", when it streams data exactly as it
+ * exists on disk. When this happens, the arc_buf_t's data buffer is shared
+ * with the arc_buf_hdr_t.
+ *
+ * Here is a diagram showing an arc_buf_hdr_t referenced by two arc_buf_t's. The
+ * first one is owned by a compressed send consumer (and therefore references
+ * the same compressed data buffer as the arc_buf_hdr_t) and the second could be
+ * used by any other consumer (and has its own uncompressed copy of the data
+ * buffer).
+ *
+ *   arc_buf_hdr_t
+ *   +-----------+
+ *   | fields    |
+ *   | common to |
+ *   | L1- and   |
+ *   | L2ARC     |
+ *   +-----------+
+ *   | l2arc_buf_hdr_t
+ *   |           |
+ *   +-----------+
+ *   | l1arc_buf_hdr_t
+ *   |           |              arc_buf_t
+ *   | b_buf     +------------>+-----------+      arc_buf_t
+ *   | b_pabd    +-+           |b_next     +---->+-----------+
+ *   +-----------+ |           |-----------|     |b_next     +-->NULL
+ *                 |           |b_comp = T |     +-----------+
+ *                 |           |b_data     +-+   |b_comp = F |
+ *                 |           +-----------+ |   |b_data     +-+
+ *                 +->+------+               |   +-----------+ |
+ *        compressed  |      |               |                 |
+ *           data     |      |<--------------+                 | uncompressed
+ *                    +------+          compressed,            |     data
+ *                                        shared               +-->+------+
+ *                                         data                    |      |
+ *                                                                 |      |
+ *                                                                 +------+
+ *
+ * When a consumer reads a block, the ARC must first look to see if the
+ * arc_buf_hdr_t is cached. If the hdr is cached then the ARC allocates a new
+ * arc_buf_t and either copies uncompressed data into a new data buffer from an
+ * existing uncompressed arc_buf_t, decompresses the hdr's b_pabd buffer into a
+ * new data buffer, or shares the hdr's b_pabd buffer, depending on whether the
+ * hdr is compressed and the desired compression characteristics of the
+ * arc_buf_t consumer. If the arc_buf_t ends up sharing data with the
+ * arc_buf_hdr_t and both of them are uncompressed then the arc_buf_t must be
+ * the last buffer in the hdr's b_buf list, however a shared compressed buf can
+ * be anywhere in the hdr's list.
+ *
+ * The diagram below shows an example of an uncompressed ARC hdr that is
+ * sharing its data with an arc_buf_t (note that the shared uncompressed buf is
+ * the last element in the buf list):
+ *
+ *                arc_buf_hdr_t
+ *                +-----------+
+ *                |           |
+ *                |           |
+ *                |           |
+ *                +-----------+
+ * l2arc_buf_hdr_t|           |
+ *                |           |
+ *                +-----------+
+ * l1arc_buf_hdr_t|           |
+ *                |           |                 arc_buf_t    (shared)
+ *                |    b_buf  +------------>+---------+      arc_buf_t
+ *                |           |             |b_next   +---->+---------+
+ *                |  b_pabd   +-+           |---------|     |b_next   +-->NULL
+ *                +-----------+ |           |         |     +---------+
+ *                              |           |b_data   +-+   |         |
+ *                              |           +---------+ |   |b_data   +-+
+ *                              +->+------+             |   +---------+ |
+ *                                 |      |             |               |
+ *                   uncompressed  |      |             |               |
+ *                        data     +------+             |               |
+ *                                    ^                 +->+------+     |
+ *                                    |       uncompressed |      |     |
+ *                                    |           data     |      |     |
+ *                                    |                    +------+     |
+ *                                    +---------------------------------+
+ *
+ * Writing to the ARC requires that the ARC first discard the hdr's b_pabd
+ * since the physical block is about to be rewritten. The new data contents
+ * will be contained in the arc_buf_t. As the I/O pipeline performs the write,
+ * it may compress the data before writing it to disk. The ARC will be called
+ * with the transformed data and will bcopy the transformed on-disk block into
+ * a newly allocated b_pabd. Writes are always done into buffers which have
+ * either been loaned (and hence are new and don't have other readers) or
+ * buffers which have been released (and hence have their own hdr, if there
+ * were originally other readers of the buf's original hdr). This ensures that
+ * the ARC only needs to update a single buf and its hdr after a write occurs.
+ *
+ * When the L2ARC is in use, it will also take advantage of the b_pabd. The
+ * L2ARC will always write the contents of b_pabd to the L2ARC. This means
+ * that when compressed ARC is enabled that the L2ARC blocks are identical
+ * to the on-disk block in the main data pool. This provides a significant
+ * advantage since the ARC can leverage the bp's checksum when reading from the
+ * L2ARC to determine if the contents are valid. However, if the compressed
+ * ARC is disabled, then the L2ARC's block must be transformed to look
+ * like the physical block in the main data pool before comparing the
+ * checksum and determining its validity.
+ *
+ * The L1ARC has a slightly different system for storing encrypted data.
+ * Raw (encrypted + possibly compressed) data has a few subtle differences from
+ * data that is just compressed. The biggest difference is that it is not
+ * possible to decrypt encrypted data (or vice-versa) if the keys aren't loaded.
+ * The other difference is that encryption cannot be treated as a suggestion.
+ * If a caller would prefer compressed data, but they actually wind up with
+ * uncompressed data the worst thing that could happen is there might be a
+ * performance hit. If the caller requests encrypted data, however, we must be
+ * sure they actually get it or else secret information could be leaked. Raw
+ * data is stored in hdr->b_crypt_hdr.b_rabd. An encrypted header, therefore,
+ * may have both an encrypted version and a decrypted version of its data at
+ * once. When a caller needs a raw arc_buf_t, it is allocated and the data is
+ * copied out of this header. To avoid complications with b_pabd, raw buffers
+ * cannot be shared.
+ */
+
+#include <sys/spa.h>
+#include <sys/zio.h>
+#include <sys/spa_impl.h>
+#include <sys/zio_compress.h>
+#include <sys/zio_checksum.h>
+#include <sys/zfs_context.h>
+#include <sys/arc.h>
+#include <sys/zfs_refcount.h>
+#include <sys/vdev.h>
+#include <sys/vdev_impl.h>
+#include <sys/dsl_pool.h>
+#include <sys/zio_checksum.h>
+#include <sys/multilist.h>
+#include <sys/abd.h>
+#include <sys/zil.h>
+#include <sys/fm/fs/zfs.h>
+#include <sys/callb.h>
+#include <sys/kstat.h>
+#include <sys/zthr.h>
+#include <zfs_fletcher.h>
+#include <sys/arc_impl.h>
+#include <sys/trace_zfs.h>
+#include <sys/aggsum.h>
+#include <cityhash.h>
+#include <sys/vdev_trim.h>
+
+#ifndef _KERNEL
+/* set with ZFS_DEBUG=watch, to enable watchpoints on frozen buffers */
+boolean_t arc_watch = B_FALSE;
+#endif
+
+/*
+ * This thread's job is to keep enough free memory in the system, by
+ * calling arc_kmem_reap_soon() plus arc_reduce_target_size(), which improves
+ * arc_available_memory().
+ */
+static zthr_t *arc_reap_zthr;
+
+/*
+ * This thread's job is to keep arc_size under arc_c, by calling
+ * arc_evict(), which improves arc_is_overflowing().
+ */
+static zthr_t *arc_evict_zthr;
+
+static kmutex_t arc_evict_lock;
+static boolean_t arc_evict_needed = B_FALSE;
+
+/*
+ * Count of bytes evicted since boot.
+ */
+static uint64_t arc_evict_count;
+
+/*
+ * List of arc_evict_waiter_t's, representing threads waiting for the
+ * arc_evict_count to reach specific values.
+ */
+static list_t arc_evict_waiters;
+
+/*
+ * When arc_is_overflowing(), arc_get_data_impl() waits for this percent of
+ * the requested amount of data to be evicted.  For example, by default for
+ * every 2KB that's evicted, 1KB of it may be "reused" by a new allocation.
+ * Since this is above 100%, it ensures that progress is made towards getting
+ * arc_size under arc_c.  Since this is finite, it ensures that allocations
+ * can still happen, even during the potentially long time that arc_size is
+ * more than arc_c.
+ */
+int zfs_arc_eviction_pct = 200;
+
+/*
+ * The number of headers to evict in arc_evict_state_impl() before
+ * dropping the sublist lock and evicting from another sublist. A lower
+ * value means we're more likely to evict the "correct" header (i.e. the
+ * oldest header in the arc state), but comes with higher overhead
+ * (i.e. more invocations of arc_evict_state_impl()).
+ */
+int zfs_arc_evict_batch_limit = 10;
+
+/* number of seconds before growing cache again */
+int arc_grow_retry = 5;
+
+/*
+ * Minimum time between calls to arc_kmem_reap_soon().
+ */
+int arc_kmem_cache_reap_retry_ms = 1000;
+
+/* shift of arc_c for calculating overflow limit in arc_get_data_impl */
+int zfs_arc_overflow_shift = 8;
+
+/* shift of arc_c for calculating both min and max arc_p */
+int arc_p_min_shift = 4;
+
+/* log2(fraction of arc to reclaim) */
+int arc_shrink_shift = 7;
+
+/* percent of pagecache to reclaim arc to */
+#ifdef _KERNEL
+uint_t zfs_arc_pc_percent = 0;
+#endif
+
+/*
+ * log2(fraction of ARC which must be free to allow growing).
+ * I.e. If there is less than arc_c >> arc_no_grow_shift free memory,
+ * when reading a new block into the ARC, we will evict an equal-sized block
+ * from the ARC.
+ *
+ * This must be less than arc_shrink_shift, so that when we shrink the ARC,
+ * we will still not allow it to grow.
+ */
+int			arc_no_grow_shift = 5;
+
+
+/*
+ * minimum lifespan of a prefetch block in clock ticks
+ * (initialized in arc_init())
+ */
+static int		arc_min_prefetch_ms;
+static int		arc_min_prescient_prefetch_ms;
+
+/*
+ * If this percent of memory is free, don't throttle.
+ */
+int arc_lotsfree_percent = 10;
+
+/*
+ * The arc has filled available memory and has now warmed up.
+ */
+boolean_t arc_warm;
+
+/*
+ * These tunables are for performance analysis.
+ */
+unsigned long zfs_arc_max = 0;
+unsigned long zfs_arc_min = 0;
+unsigned long zfs_arc_meta_limit = 0;
+unsigned long zfs_arc_meta_min = 0;
+unsigned long zfs_arc_dnode_limit = 0;
+unsigned long zfs_arc_dnode_reduce_percent = 10;
+int zfs_arc_grow_retry = 0;
+int zfs_arc_shrink_shift = 0;
+int zfs_arc_p_min_shift = 0;
+int zfs_arc_average_blocksize = 8 * 1024; /* 8KB */
+
+/*
+ * ARC dirty data constraints for arc_tempreserve_space() throttle.
+ */
+unsigned long zfs_arc_dirty_limit_percent = 50;	/* total dirty data limit */
+unsigned long zfs_arc_anon_limit_percent = 25;	/* anon block dirty limit */
+unsigned long zfs_arc_pool_dirty_percent = 20;	/* each pool's anon allowance */
+
+/*
+ * Enable or disable compressed arc buffers.
+ */
+int zfs_compressed_arc_enabled = B_TRUE;
+
+/*
+ * ARC will evict meta buffers that exceed arc_meta_limit. This
+ * tunable make arc_meta_limit adjustable for different workloads.
+ */
+unsigned long zfs_arc_meta_limit_percent = 75;
+
+/*
+ * Percentage that can be consumed by dnodes of ARC meta buffers.
+ */
+unsigned long zfs_arc_dnode_limit_percent = 10;
+
+/*
+ * These tunables are Linux specific
+ */
+unsigned long zfs_arc_sys_free = 0;
+int zfs_arc_min_prefetch_ms = 0;
+int zfs_arc_min_prescient_prefetch_ms = 0;
+int zfs_arc_p_dampener_disable = 1;
+int zfs_arc_meta_prune = 10000;
+int zfs_arc_meta_strategy = ARC_STRATEGY_META_BALANCED;
+int zfs_arc_meta_adjust_restarts = 4096;
+int zfs_arc_lotsfree_percent = 10;
+
+/* The 6 states: */
+arc_state_t ARC_anon;
+arc_state_t ARC_mru;
+arc_state_t ARC_mru_ghost;
+arc_state_t ARC_mfu;
+arc_state_t ARC_mfu_ghost;
+arc_state_t ARC_l2c_only;
+
+arc_stats_t arc_stats = {
+	{ "hits",			KSTAT_DATA_UINT64 },
+	{ "misses",			KSTAT_DATA_UINT64 },
+	{ "demand_data_hits",		KSTAT_DATA_UINT64 },
+	{ "demand_data_misses",		KSTAT_DATA_UINT64 },
+	{ "demand_metadata_hits",	KSTAT_DATA_UINT64 },
+	{ "demand_metadata_misses",	KSTAT_DATA_UINT64 },
+	{ "prefetch_data_hits",		KSTAT_DATA_UINT64 },
+	{ "prefetch_data_misses",	KSTAT_DATA_UINT64 },
+	{ "prefetch_metadata_hits",	KSTAT_DATA_UINT64 },
+	{ "prefetch_metadata_misses",	KSTAT_DATA_UINT64 },
+	{ "mru_hits",			KSTAT_DATA_UINT64 },
+	{ "mru_ghost_hits",		KSTAT_DATA_UINT64 },
+	{ "mfu_hits",			KSTAT_DATA_UINT64 },
+	{ "mfu_ghost_hits",		KSTAT_DATA_UINT64 },
+	{ "deleted",			KSTAT_DATA_UINT64 },
+	{ "mutex_miss",			KSTAT_DATA_UINT64 },
+	{ "access_skip",		KSTAT_DATA_UINT64 },
+	{ "evict_skip",			KSTAT_DATA_UINT64 },
+	{ "evict_not_enough",		KSTAT_DATA_UINT64 },
+	{ "evict_l2_cached",		KSTAT_DATA_UINT64 },
+	{ "evict_l2_eligible",		KSTAT_DATA_UINT64 },
+	{ "evict_l2_ineligible",	KSTAT_DATA_UINT64 },
+	{ "evict_l2_skip",		KSTAT_DATA_UINT64 },
+	{ "hash_elements",		KSTAT_DATA_UINT64 },
+	{ "hash_elements_max",		KSTAT_DATA_UINT64 },
+	{ "hash_collisions",		KSTAT_DATA_UINT64 },
+	{ "hash_chains",		KSTAT_DATA_UINT64 },
+	{ "hash_chain_max",		KSTAT_DATA_UINT64 },
+	{ "p",				KSTAT_DATA_UINT64 },
+	{ "c",				KSTAT_DATA_UINT64 },
+	{ "c_min",			KSTAT_DATA_UINT64 },
+	{ "c_max",			KSTAT_DATA_UINT64 },
+	{ "size",			KSTAT_DATA_UINT64 },
+	{ "compressed_size",		KSTAT_DATA_UINT64 },
+	{ "uncompressed_size",		KSTAT_DATA_UINT64 },
+	{ "overhead_size",		KSTAT_DATA_UINT64 },
+	{ "hdr_size",			KSTAT_DATA_UINT64 },
+	{ "data_size",			KSTAT_DATA_UINT64 },
+	{ "metadata_size",		KSTAT_DATA_UINT64 },
+	{ "dbuf_size",			KSTAT_DATA_UINT64 },
+	{ "dnode_size",			KSTAT_DATA_UINT64 },
+	{ "bonus_size",			KSTAT_DATA_UINT64 },
+#if defined(COMPAT_FREEBSD11)
+	{ "other_size",			KSTAT_DATA_UINT64 },
+#endif
+	{ "anon_size",			KSTAT_DATA_UINT64 },
+	{ "anon_evictable_data",	KSTAT_DATA_UINT64 },
+	{ "anon_evictable_metadata",	KSTAT_DATA_UINT64 },
+	{ "mru_size",			KSTAT_DATA_UINT64 },
+	{ "mru_evictable_data",		KSTAT_DATA_UINT64 },
+	{ "mru_evictable_metadata",	KSTAT_DATA_UINT64 },
+	{ "mru_ghost_size",		KSTAT_DATA_UINT64 },
+	{ "mru_ghost_evictable_data",	KSTAT_DATA_UINT64 },
+	{ "mru_ghost_evictable_metadata", KSTAT_DATA_UINT64 },
+	{ "mfu_size",			KSTAT_DATA_UINT64 },
+	{ "mfu_evictable_data",		KSTAT_DATA_UINT64 },
+	{ "mfu_evictable_metadata",	KSTAT_DATA_UINT64 },
+	{ "mfu_ghost_size",		KSTAT_DATA_UINT64 },
+	{ "mfu_ghost_evictable_data",	KSTAT_DATA_UINT64 },
+	{ "mfu_ghost_evictable_metadata", KSTAT_DATA_UINT64 },
+	{ "l2_hits",			KSTAT_DATA_UINT64 },
+	{ "l2_misses",			KSTAT_DATA_UINT64 },
+	{ "l2_feeds",			KSTAT_DATA_UINT64 },
+	{ "l2_rw_clash",		KSTAT_DATA_UINT64 },
+	{ "l2_read_bytes",		KSTAT_DATA_UINT64 },
+	{ "l2_write_bytes",		KSTAT_DATA_UINT64 },
+	{ "l2_writes_sent",		KSTAT_DATA_UINT64 },
+	{ "l2_writes_done",		KSTAT_DATA_UINT64 },
+	{ "l2_writes_error",		KSTAT_DATA_UINT64 },
+	{ "l2_writes_lock_retry",	KSTAT_DATA_UINT64 },
+	{ "l2_evict_lock_retry",	KSTAT_DATA_UINT64 },
+	{ "l2_evict_reading",		KSTAT_DATA_UINT64 },
+	{ "l2_evict_l1cached",		KSTAT_DATA_UINT64 },
+	{ "l2_free_on_write",		KSTAT_DATA_UINT64 },
+	{ "l2_abort_lowmem",		KSTAT_DATA_UINT64 },
+	{ "l2_cksum_bad",		KSTAT_DATA_UINT64 },
+	{ "l2_io_error",		KSTAT_DATA_UINT64 },
+	{ "l2_size",			KSTAT_DATA_UINT64 },
+	{ "l2_asize",			KSTAT_DATA_UINT64 },
+	{ "l2_hdr_size",		KSTAT_DATA_UINT64 },
+	{ "l2_log_blk_writes",		KSTAT_DATA_UINT64 },
+	{ "l2_log_blk_avg_asize",	KSTAT_DATA_UINT64 },
+	{ "l2_log_blk_asize",		KSTAT_DATA_UINT64 },
+	{ "l2_log_blk_count",		KSTAT_DATA_UINT64 },
+	{ "l2_data_to_meta_ratio",	KSTAT_DATA_UINT64 },
+	{ "l2_rebuild_success",		KSTAT_DATA_UINT64 },
+	{ "l2_rebuild_unsupported",	KSTAT_DATA_UINT64 },
+	{ "l2_rebuild_io_errors",	KSTAT_DATA_UINT64 },
+	{ "l2_rebuild_dh_errors",	KSTAT_DATA_UINT64 },
+	{ "l2_rebuild_cksum_lb_errors",	KSTAT_DATA_UINT64 },
+	{ "l2_rebuild_lowmem",		KSTAT_DATA_UINT64 },
+	{ "l2_rebuild_size",		KSTAT_DATA_UINT64 },
+	{ "l2_rebuild_asize",		KSTAT_DATA_UINT64 },
+	{ "l2_rebuild_bufs",		KSTAT_DATA_UINT64 },
+	{ "l2_rebuild_bufs_precached",	KSTAT_DATA_UINT64 },
+	{ "l2_rebuild_log_blks",	KSTAT_DATA_UINT64 },
+	{ "memory_throttle_count",	KSTAT_DATA_UINT64 },
+	{ "memory_direct_count",	KSTAT_DATA_UINT64 },
+	{ "memory_indirect_count",	KSTAT_DATA_UINT64 },
+	{ "memory_all_bytes",		KSTAT_DATA_UINT64 },
+	{ "memory_free_bytes",		KSTAT_DATA_UINT64 },
+	{ "memory_available_bytes",	KSTAT_DATA_INT64 },
+	{ "arc_no_grow",		KSTAT_DATA_UINT64 },
+	{ "arc_tempreserve",		KSTAT_DATA_UINT64 },
+	{ "arc_loaned_bytes",		KSTAT_DATA_UINT64 },
+	{ "arc_prune",			KSTAT_DATA_UINT64 },
+	{ "arc_meta_used",		KSTAT_DATA_UINT64 },
+	{ "arc_meta_limit",		KSTAT_DATA_UINT64 },
+	{ "arc_dnode_limit",		KSTAT_DATA_UINT64 },
+	{ "arc_meta_max",		KSTAT_DATA_UINT64 },
+	{ "arc_meta_min",		KSTAT_DATA_UINT64 },
+	{ "async_upgrade_sync",		KSTAT_DATA_UINT64 },
+	{ "demand_hit_predictive_prefetch", KSTAT_DATA_UINT64 },
+	{ "demand_hit_prescient_prefetch", KSTAT_DATA_UINT64 },
+	{ "arc_need_free",		KSTAT_DATA_UINT64 },
+	{ "arc_sys_free",		KSTAT_DATA_UINT64 },
+	{ "arc_raw_size",		KSTAT_DATA_UINT64 },
+	{ "cached_only_in_progress",	KSTAT_DATA_UINT64 },
+	{ "abd_chunk_waste_size",	KSTAT_DATA_UINT64 },
+};
+
+#define	ARCSTAT_MAX(stat, val) {					\
+	uint64_t m;							\
+	while ((val) > (m = arc_stats.stat.value.ui64) &&		\
+	    (m != atomic_cas_64(&arc_stats.stat.value.ui64, m, (val))))	\
+		continue;						\
+}
+
+#define	ARCSTAT_MAXSTAT(stat) \
+	ARCSTAT_MAX(stat##_max, arc_stats.stat.value.ui64)
+
+/*
+ * We define a macro to allow ARC hits/misses to be easily broken down by
+ * two separate conditions, giving a total of four different subtypes for
+ * each of hits and misses (so eight statistics total).
+ */
+#define	ARCSTAT_CONDSTAT(cond1, stat1, notstat1, cond2, stat2, notstat2, stat) \
+	if (cond1) {							\
+		if (cond2) {						\
+			ARCSTAT_BUMP(arcstat_##stat1##_##stat2##_##stat); \
+		} else {						\
+			ARCSTAT_BUMP(arcstat_##stat1##_##notstat2##_##stat); \
+		}							\
+	} else {							\
+		if (cond2) {						\
+			ARCSTAT_BUMP(arcstat_##notstat1##_##stat2##_##stat); \
+		} else {						\
+			ARCSTAT_BUMP(arcstat_##notstat1##_##notstat2##_##stat);\
+		}							\
+	}
+
+/*
+ * This macro allows us to use kstats as floating averages. Each time we
+ * update this kstat, we first factor it and the update value by
+ * ARCSTAT_AVG_FACTOR to shrink the new value's contribution to the overall
+ * average. This macro assumes that integer loads and stores are atomic, but
+ * is not safe for multiple writers updating the kstat in parallel (only the
+ * last writer's update will remain).
+ */
+#define	ARCSTAT_F_AVG_FACTOR	3
+#define	ARCSTAT_F_AVG(stat, value) \
+	do { \
+		uint64_t x = ARCSTAT(stat); \
+		x = x - x / ARCSTAT_F_AVG_FACTOR + \
+		    (value) / ARCSTAT_F_AVG_FACTOR; \
+		ARCSTAT(stat) = x; \
+		_NOTE(CONSTCOND) \
+	} while (0)
+
+kstat_t			*arc_ksp;
+static arc_state_t	*arc_anon;
+static arc_state_t	*arc_mru_ghost;
+static arc_state_t	*arc_mfu_ghost;
+static arc_state_t	*arc_l2c_only;
+
+arc_state_t	*arc_mru;
+arc_state_t	*arc_mfu;
+
+/*
+ * There are several ARC variables that are critical to export as kstats --
+ * but we don't want to have to grovel around in the kstat whenever we wish to
+ * manipulate them.  For these variables, we therefore define them to be in
+ * terms of the statistic variable.  This assures that we are not introducing
+ * the possibility of inconsistency by having shadow copies of the variables,
+ * while still allowing the code to be readable.
+ */
+#define	arc_tempreserve	ARCSTAT(arcstat_tempreserve)
+#define	arc_loaned_bytes	ARCSTAT(arcstat_loaned_bytes)
+#define	arc_meta_limit	ARCSTAT(arcstat_meta_limit) /* max size for metadata */
+/* max size for dnodes */
+#define	arc_dnode_size_limit	ARCSTAT(arcstat_dnode_limit)
+#define	arc_meta_min	ARCSTAT(arcstat_meta_min) /* min size for metadata */
+#define	arc_meta_max	ARCSTAT(arcstat_meta_max) /* max size of metadata */
+#define	arc_need_free	ARCSTAT(arcstat_need_free) /* waiting to be evicted */
+
+/* size of all b_rabd's in entire arc */
+#define	arc_raw_size	ARCSTAT(arcstat_raw_size)
+/* compressed size of entire arc */
+#define	arc_compressed_size	ARCSTAT(arcstat_compressed_size)
+/* uncompressed size of entire arc */
+#define	arc_uncompressed_size	ARCSTAT(arcstat_uncompressed_size)
+/* number of bytes in the arc from arc_buf_t's */
+#define	arc_overhead_size	ARCSTAT(arcstat_overhead_size)
+
+/*
+ * There are also some ARC variables that we want to export, but that are
+ * updated so often that having the canonical representation be the statistic
+ * variable causes a performance bottleneck. We want to use aggsum_t's for these
+ * instead, but still be able to export the kstat in the same way as before.
+ * The solution is to always use the aggsum version, except in the kstat update
+ * callback.
+ */
+aggsum_t arc_size;
+aggsum_t arc_meta_used;
+aggsum_t astat_data_size;
+aggsum_t astat_metadata_size;
+aggsum_t astat_dbuf_size;
+aggsum_t astat_dnode_size;
+aggsum_t astat_bonus_size;
+aggsum_t astat_hdr_size;
+aggsum_t astat_l2_hdr_size;
+aggsum_t astat_abd_chunk_waste_size;
+
+hrtime_t arc_growtime;
+list_t arc_prune_list;
+kmutex_t arc_prune_mtx;
+taskq_t *arc_prune_taskq;
+
+#define	GHOST_STATE(state)	\
+	((state) == arc_mru_ghost || (state) == arc_mfu_ghost ||	\
+	(state) == arc_l2c_only)
+
+#define	HDR_IN_HASH_TABLE(hdr)	((hdr)->b_flags & ARC_FLAG_IN_HASH_TABLE)
+#define	HDR_IO_IN_PROGRESS(hdr)	((hdr)->b_flags & ARC_FLAG_IO_IN_PROGRESS)
+#define	HDR_IO_ERROR(hdr)	((hdr)->b_flags & ARC_FLAG_IO_ERROR)
+#define	HDR_PREFETCH(hdr)	((hdr)->b_flags & ARC_FLAG_PREFETCH)
+#define	HDR_PRESCIENT_PREFETCH(hdr)	\
+	((hdr)->b_flags & ARC_FLAG_PRESCIENT_PREFETCH)
+#define	HDR_COMPRESSION_ENABLED(hdr)	\
+	((hdr)->b_flags & ARC_FLAG_COMPRESSED_ARC)
+
+#define	HDR_L2CACHE(hdr)	((hdr)->b_flags & ARC_FLAG_L2CACHE)
+#define	HDR_L2_READING(hdr)	\
+	(((hdr)->b_flags & ARC_FLAG_IO_IN_PROGRESS) &&	\
+	((hdr)->b_flags & ARC_FLAG_HAS_L2HDR))
+#define	HDR_L2_WRITING(hdr)	((hdr)->b_flags & ARC_FLAG_L2_WRITING)
+#define	HDR_L2_EVICTED(hdr)	((hdr)->b_flags & ARC_FLAG_L2_EVICTED)
+#define	HDR_L2_WRITE_HEAD(hdr)	((hdr)->b_flags & ARC_FLAG_L2_WRITE_HEAD)
+#define	HDR_PROTECTED(hdr)	((hdr)->b_flags & ARC_FLAG_PROTECTED)
+#define	HDR_NOAUTH(hdr)		((hdr)->b_flags & ARC_FLAG_NOAUTH)
+#define	HDR_SHARED_DATA(hdr)	((hdr)->b_flags & ARC_FLAG_SHARED_DATA)
+
+#define	HDR_ISTYPE_METADATA(hdr)	\
+	((hdr)->b_flags & ARC_FLAG_BUFC_METADATA)
+#define	HDR_ISTYPE_DATA(hdr)	(!HDR_ISTYPE_METADATA(hdr))
+
+#define	HDR_HAS_L1HDR(hdr)	((hdr)->b_flags & ARC_FLAG_HAS_L1HDR)
+#define	HDR_HAS_L2HDR(hdr)	((hdr)->b_flags & ARC_FLAG_HAS_L2HDR)
+#define	HDR_HAS_RABD(hdr)	\
+	(HDR_HAS_L1HDR(hdr) && HDR_PROTECTED(hdr) &&	\
+	(hdr)->b_crypt_hdr.b_rabd != NULL)
+#define	HDR_ENCRYPTED(hdr)	\
+	(HDR_PROTECTED(hdr) && DMU_OT_IS_ENCRYPTED((hdr)->b_crypt_hdr.b_ot))
+#define	HDR_AUTHENTICATED(hdr)	\
+	(HDR_PROTECTED(hdr) && !DMU_OT_IS_ENCRYPTED((hdr)->b_crypt_hdr.b_ot))
+
+/* For storing compression mode in b_flags */
+#define	HDR_COMPRESS_OFFSET	(highbit64(ARC_FLAG_COMPRESS_0) - 1)
+
+#define	HDR_GET_COMPRESS(hdr)	((enum zio_compress)BF32_GET((hdr)->b_flags, \
+	HDR_COMPRESS_OFFSET, SPA_COMPRESSBITS))
+#define	HDR_SET_COMPRESS(hdr, cmp) BF32_SET((hdr)->b_flags, \
+	HDR_COMPRESS_OFFSET, SPA_COMPRESSBITS, (cmp));
+
+#define	ARC_BUF_LAST(buf)	((buf)->b_next == NULL)
+#define	ARC_BUF_SHARED(buf)	((buf)->b_flags & ARC_BUF_FLAG_SHARED)
+#define	ARC_BUF_COMPRESSED(buf)	((buf)->b_flags & ARC_BUF_FLAG_COMPRESSED)
+#define	ARC_BUF_ENCRYPTED(buf)	((buf)->b_flags & ARC_BUF_FLAG_ENCRYPTED)
+
+/*
+ * Other sizes
+ */
+
+#define	HDR_FULL_CRYPT_SIZE ((int64_t)sizeof (arc_buf_hdr_t))
+#define	HDR_FULL_SIZE ((int64_t)offsetof(arc_buf_hdr_t, b_crypt_hdr))
+#define	HDR_L2ONLY_SIZE ((int64_t)offsetof(arc_buf_hdr_t, b_l1hdr))
+
+/*
+ * Hash table routines
+ */
+
+#define	HT_LOCK_ALIGN	64
+#define	HT_LOCK_PAD	(P2NPHASE(sizeof (kmutex_t), (HT_LOCK_ALIGN)))
+
+struct ht_lock {
+	kmutex_t	ht_lock;
+#ifdef _KERNEL
+	unsigned char	pad[HT_LOCK_PAD];
+#endif
+};
+
+#define	BUF_LOCKS 8192
+typedef struct buf_hash_table {
+	uint64_t ht_mask;
+	arc_buf_hdr_t **ht_table;
+	struct ht_lock ht_locks[BUF_LOCKS];
+} buf_hash_table_t;
+
+static buf_hash_table_t buf_hash_table;
+
+#define	BUF_HASH_INDEX(spa, dva, birth) \
+	(buf_hash(spa, dva, birth) & buf_hash_table.ht_mask)
+#define	BUF_HASH_LOCK_NTRY(idx) (buf_hash_table.ht_locks[idx & (BUF_LOCKS-1)])
+#define	BUF_HASH_LOCK(idx)	(&(BUF_HASH_LOCK_NTRY(idx).ht_lock))
+#define	HDR_LOCK(hdr) \
+	(BUF_HASH_LOCK(BUF_HASH_INDEX(hdr->b_spa, &hdr->b_dva, hdr->b_birth)))
+
+uint64_t zfs_crc64_table[256];
+
+/*
+ * Level 2 ARC
+ */
+
+#define	L2ARC_WRITE_SIZE	(8 * 1024 * 1024)	/* initial write max */
+#define	L2ARC_HEADROOM		2			/* num of writes */
+
+/*
+ * If we discover during ARC scan any buffers to be compressed, we boost
+ * our headroom for the next scanning cycle by this percentage multiple.
+ */
+#define	L2ARC_HEADROOM_BOOST	200
+#define	L2ARC_FEED_SECS		1		/* caching interval secs */
+#define	L2ARC_FEED_MIN_MS	200		/* min caching interval ms */
+
+/*
+ * We can feed L2ARC from two states of ARC buffers, mru and mfu,
+ * and each of the state has two types: data and metadata.
+ */
+#define	L2ARC_FEED_TYPES	4
+
+#define	l2arc_writes_sent	ARCSTAT(arcstat_l2_writes_sent)
+#define	l2arc_writes_done	ARCSTAT(arcstat_l2_writes_done)
+
+/* L2ARC Performance Tunables */
+unsigned long l2arc_write_max = L2ARC_WRITE_SIZE;	/* def max write size */
+unsigned long l2arc_write_boost = L2ARC_WRITE_SIZE;	/* extra warmup write */
+unsigned long l2arc_headroom = L2ARC_HEADROOM;		/* # of dev writes */
+unsigned long l2arc_headroom_boost = L2ARC_HEADROOM_BOOST;
+unsigned long l2arc_feed_secs = L2ARC_FEED_SECS;	/* interval seconds */
+unsigned long l2arc_feed_min_ms = L2ARC_FEED_MIN_MS;	/* min interval msecs */
+int l2arc_noprefetch = B_TRUE;			/* don't cache prefetch bufs */
+int l2arc_feed_again = B_TRUE;			/* turbo warmup */
+int l2arc_norw = B_FALSE;			/* no reads during writes */
+
+/*
+ * L2ARC Internals
+ */
+static list_t L2ARC_dev_list;			/* device list */
+static list_t *l2arc_dev_list;			/* device list pointer */
+static kmutex_t l2arc_dev_mtx;			/* device list mutex */
+static l2arc_dev_t *l2arc_dev_last;		/* last device used */
+static list_t L2ARC_free_on_write;		/* free after write buf list */
+static list_t *l2arc_free_on_write;		/* free after write list ptr */
+static kmutex_t l2arc_free_on_write_mtx;	/* mutex for list */
+static uint64_t l2arc_ndev;			/* number of devices */
+
+typedef struct l2arc_read_callback {
+	arc_buf_hdr_t		*l2rcb_hdr;		/* read header */
+	blkptr_t		l2rcb_bp;		/* original blkptr */
+	zbookmark_phys_t	l2rcb_zb;		/* original bookmark */
+	int			l2rcb_flags;		/* original flags */
+	abd_t			*l2rcb_abd;		/* temporary buffer */
+} l2arc_read_callback_t;
+
+typedef struct l2arc_data_free {
+	/* protected by l2arc_free_on_write_mtx */
+	abd_t		*l2df_abd;
+	size_t		l2df_size;
+	arc_buf_contents_t l2df_type;
+	list_node_t	l2df_list_node;
+} l2arc_data_free_t;
+
+typedef enum arc_fill_flags {
+	ARC_FILL_LOCKED		= 1 << 0, /* hdr lock is held */
+	ARC_FILL_COMPRESSED	= 1 << 1, /* fill with compressed data */
+	ARC_FILL_ENCRYPTED	= 1 << 2, /* fill with encrypted data */
+	ARC_FILL_NOAUTH		= 1 << 3, /* don't attempt to authenticate */
+	ARC_FILL_IN_PLACE	= 1 << 4  /* fill in place (special case) */
+} arc_fill_flags_t;
+
+static kmutex_t l2arc_feed_thr_lock;
+static kcondvar_t l2arc_feed_thr_cv;
+static uint8_t l2arc_thread_exit;
+
+static kmutex_t l2arc_rebuild_thr_lock;
+static kcondvar_t l2arc_rebuild_thr_cv;
+
+enum arc_hdr_alloc_flags {
+	ARC_HDR_ALLOC_RDATA = 0x1,
+	ARC_HDR_DO_ADAPT = 0x2,
+};
+
+
+static abd_t *arc_get_data_abd(arc_buf_hdr_t *, uint64_t, void *, boolean_t);
+static void *arc_get_data_buf(arc_buf_hdr_t *, uint64_t, void *);
+static void arc_get_data_impl(arc_buf_hdr_t *, uint64_t, void *, boolean_t);
+static void arc_free_data_abd(arc_buf_hdr_t *, abd_t *, uint64_t, void *);
+static void arc_free_data_buf(arc_buf_hdr_t *, void *, uint64_t, void *);
+static void arc_free_data_impl(arc_buf_hdr_t *hdr, uint64_t size, void *tag);
+static void arc_hdr_free_abd(arc_buf_hdr_t *, boolean_t);
+static void arc_hdr_alloc_abd(arc_buf_hdr_t *, int);
+static void arc_access(arc_buf_hdr_t *, kmutex_t *);
+static void arc_buf_watch(arc_buf_t *);
+
+static arc_buf_contents_t arc_buf_type(arc_buf_hdr_t *);
+static uint32_t arc_bufc_to_flags(arc_buf_contents_t);
+static inline void arc_hdr_set_flags(arc_buf_hdr_t *hdr, arc_flags_t flags);
+static inline void arc_hdr_clear_flags(arc_buf_hdr_t *hdr, arc_flags_t flags);
+
+static boolean_t l2arc_write_eligible(uint64_t, arc_buf_hdr_t *);
+static void l2arc_read_done(zio_t *);
+static void l2arc_do_free_on_write(void);
+
+/*
+ * L2ARC TRIM
+ * l2arc_trim_ahead : A ZFS module parameter that controls how much ahead of
+ * 		the current write size (l2arc_write_max) we should TRIM if we
+ * 		have filled the device. It is defined as a percentage of the
+ * 		write size. If set to 100 we trim twice the space required to
+ * 		accommodate upcoming writes. A minimum of 64MB will be trimmed.
+ * 		It also enables TRIM of the whole L2ARC device upon creation or
+ * 		addition to an existing pool or if the header of the device is
+ * 		invalid upon importing a pool or onlining a cache device. The
+ * 		default is 0, which disables TRIM on L2ARC altogether as it can
+ * 		put significant stress on the underlying storage devices. This
+ * 		will vary depending of how well the specific device handles
+ * 		these commands.
+ */
+unsigned long l2arc_trim_ahead = 0;
+
+/*
+ * Performance tuning of L2ARC persistence:
+ *
+ * l2arc_rebuild_enabled : A ZFS module parameter that controls whether adding
+ * 		an L2ARC device (either at pool import or later) will attempt
+ * 		to rebuild L2ARC buffer contents.
+ * l2arc_rebuild_blocks_min_l2size : A ZFS module parameter that controls
+ * 		whether log blocks are written to the L2ARC device. If the L2ARC
+ * 		device is less than 1GB, the amount of data l2arc_evict()
+ * 		evicts is significant compared to the amount of restored L2ARC
+ * 		data. In this case do not write log blocks in L2ARC in order
+ * 		not to waste space.
+ */
+int l2arc_rebuild_enabled = B_TRUE;
+unsigned long l2arc_rebuild_blocks_min_l2size = 1024 * 1024 * 1024;
+
+/* L2ARC persistence rebuild control routines. */
+void l2arc_rebuild_vdev(vdev_t *vd, boolean_t reopen);
+static void l2arc_dev_rebuild_thread(void *arg);
+static int l2arc_rebuild(l2arc_dev_t *dev);
+
+/* L2ARC persistence read I/O routines. */
+static int l2arc_dev_hdr_read(l2arc_dev_t *dev);
+static int l2arc_log_blk_read(l2arc_dev_t *dev,
+    const l2arc_log_blkptr_t *this_lp, const l2arc_log_blkptr_t *next_lp,
+    l2arc_log_blk_phys_t *this_lb, l2arc_log_blk_phys_t *next_lb,
+    zio_t *this_io, zio_t **next_io);
+static zio_t *l2arc_log_blk_fetch(vdev_t *vd,
+    const l2arc_log_blkptr_t *lp, l2arc_log_blk_phys_t *lb);
+static void l2arc_log_blk_fetch_abort(zio_t *zio);
+
+/* L2ARC persistence block restoration routines. */
+static void l2arc_log_blk_restore(l2arc_dev_t *dev,
+    const l2arc_log_blk_phys_t *lb, uint64_t lb_asize, uint64_t lb_daddr);
+static void l2arc_hdr_restore(const l2arc_log_ent_phys_t *le,
+    l2arc_dev_t *dev);
+
+/* L2ARC persistence write I/O routines. */
+static void l2arc_log_blk_commit(l2arc_dev_t *dev, zio_t *pio,
+    l2arc_write_callback_t *cb);
+
+/* L2ARC persistence auxiliary routines. */
+boolean_t l2arc_log_blkptr_valid(l2arc_dev_t *dev,
+    const l2arc_log_blkptr_t *lbp);
+static boolean_t l2arc_log_blk_insert(l2arc_dev_t *dev,
+    const arc_buf_hdr_t *ab);
+boolean_t l2arc_range_check_overlap(uint64_t bottom,
+    uint64_t top, uint64_t check);
+static void l2arc_blk_fetch_done(zio_t *zio);
+static inline uint64_t
+    l2arc_log_blk_overhead(uint64_t write_sz, l2arc_dev_t *dev);
+
+/*
+ * We use Cityhash for this. It's fast, and has good hash properties without
+ * requiring any large static buffers.
+ */
+static uint64_t
+buf_hash(uint64_t spa, const dva_t *dva, uint64_t birth)
+{
+	return (cityhash4(spa, dva->dva_word[0], dva->dva_word[1], birth));
+}
+
+#define	HDR_EMPTY(hdr)						\
+	((hdr)->b_dva.dva_word[0] == 0 &&			\
+	(hdr)->b_dva.dva_word[1] == 0)
+
+#define	HDR_EMPTY_OR_LOCKED(hdr)				\
+	(HDR_EMPTY(hdr) || MUTEX_HELD(HDR_LOCK(hdr)))
+
+#define	HDR_EQUAL(spa, dva, birth, hdr)				\
+	((hdr)->b_dva.dva_word[0] == (dva)->dva_word[0]) &&	\
+	((hdr)->b_dva.dva_word[1] == (dva)->dva_word[1]) &&	\
+	((hdr)->b_birth == birth) && ((hdr)->b_spa == spa)
+
+static void
+buf_discard_identity(arc_buf_hdr_t *hdr)
+{
+	hdr->b_dva.dva_word[0] = 0;
+	hdr->b_dva.dva_word[1] = 0;
+	hdr->b_birth = 0;
+}
+
+static arc_buf_hdr_t *
+buf_hash_find(uint64_t spa, const blkptr_t *bp, kmutex_t **lockp)
+{
+	const dva_t *dva = BP_IDENTITY(bp);
+	uint64_t birth = BP_PHYSICAL_BIRTH(bp);
+	uint64_t idx = BUF_HASH_INDEX(spa, dva, birth);
+	kmutex_t *hash_lock = BUF_HASH_LOCK(idx);
+	arc_buf_hdr_t *hdr;
+
+	mutex_enter(hash_lock);
+	for (hdr = buf_hash_table.ht_table[idx]; hdr != NULL;
+	    hdr = hdr->b_hash_next) {
+		if (HDR_EQUAL(spa, dva, birth, hdr)) {
+			*lockp = hash_lock;
+			return (hdr);
+		}
+	}
+	mutex_exit(hash_lock);
+	*lockp = NULL;
+	return (NULL);
+}
+
+/*
+ * Insert an entry into the hash table.  If there is already an element
+ * equal to elem in the hash table, then the already existing element
+ * will be returned and the new element will not be inserted.
+ * Otherwise returns NULL.
+ * If lockp == NULL, the caller is assumed to already hold the hash lock.
+ */
+static arc_buf_hdr_t *
+buf_hash_insert(arc_buf_hdr_t *hdr, kmutex_t **lockp)
+{
+	uint64_t idx = BUF_HASH_INDEX(hdr->b_spa, &hdr->b_dva, hdr->b_birth);
+	kmutex_t *hash_lock = BUF_HASH_LOCK(idx);
+	arc_buf_hdr_t *fhdr;
+	uint32_t i;
+
+	ASSERT(!DVA_IS_EMPTY(&hdr->b_dva));
+	ASSERT(hdr->b_birth != 0);
+	ASSERT(!HDR_IN_HASH_TABLE(hdr));
+
+	if (lockp != NULL) {
+		*lockp = hash_lock;
+		mutex_enter(hash_lock);
+	} else {
+		ASSERT(MUTEX_HELD(hash_lock));
+	}
+
+	for (fhdr = buf_hash_table.ht_table[idx], i = 0; fhdr != NULL;
+	    fhdr = fhdr->b_hash_next, i++) {
+		if (HDR_EQUAL(hdr->b_spa, &hdr->b_dva, hdr->b_birth, fhdr))
+			return (fhdr);
+	}
+
+	hdr->b_hash_next = buf_hash_table.ht_table[idx];
+	buf_hash_table.ht_table[idx] = hdr;
+	arc_hdr_set_flags(hdr, ARC_FLAG_IN_HASH_TABLE);
+
+	/* collect some hash table performance data */
+	if (i > 0) {
+		ARCSTAT_BUMP(arcstat_hash_collisions);
+		if (i == 1)
+			ARCSTAT_BUMP(arcstat_hash_chains);
+
+		ARCSTAT_MAX(arcstat_hash_chain_max, i);
+	}
+
+	ARCSTAT_BUMP(arcstat_hash_elements);
+	ARCSTAT_MAXSTAT(arcstat_hash_elements);
+
+	return (NULL);
+}
+
+static void
+buf_hash_remove(arc_buf_hdr_t *hdr)
+{
+	arc_buf_hdr_t *fhdr, **hdrp;
+	uint64_t idx = BUF_HASH_INDEX(hdr->b_spa, &hdr->b_dva, hdr->b_birth);
+
+	ASSERT(MUTEX_HELD(BUF_HASH_LOCK(idx)));
+	ASSERT(HDR_IN_HASH_TABLE(hdr));
+
+	hdrp = &buf_hash_table.ht_table[idx];
+	while ((fhdr = *hdrp) != hdr) {
+		ASSERT3P(fhdr, !=, NULL);
+		hdrp = &fhdr->b_hash_next;
+	}
+	*hdrp = hdr->b_hash_next;
+	hdr->b_hash_next = NULL;
+	arc_hdr_clear_flags(hdr, ARC_FLAG_IN_HASH_TABLE);
+
+	/* collect some hash table performance data */
+	ARCSTAT_BUMPDOWN(arcstat_hash_elements);
+
+	if (buf_hash_table.ht_table[idx] &&
+	    buf_hash_table.ht_table[idx]->b_hash_next == NULL)
+		ARCSTAT_BUMPDOWN(arcstat_hash_chains);
+}
+
+/*
+ * Global data structures and functions for the buf kmem cache.
+ */
+
+static kmem_cache_t *hdr_full_cache;
+static kmem_cache_t *hdr_full_crypt_cache;
+static kmem_cache_t *hdr_l2only_cache;
+static kmem_cache_t *buf_cache;
+
+static void
+buf_fini(void)
+{
+	int i;
+
+#if defined(_KERNEL)
+	/*
+	 * Large allocations which do not require contiguous pages
+	 * should be using vmem_free() in the linux kernel\
+	 */
+	vmem_free(buf_hash_table.ht_table,
+	    (buf_hash_table.ht_mask + 1) * sizeof (void *));
+#else
+	kmem_free(buf_hash_table.ht_table,
+	    (buf_hash_table.ht_mask + 1) * sizeof (void *));
+#endif
+	for (i = 0; i < BUF_LOCKS; i++)
+		mutex_destroy(&buf_hash_table.ht_locks[i].ht_lock);
+	kmem_cache_destroy(hdr_full_cache);
+	kmem_cache_destroy(hdr_full_crypt_cache);
+	kmem_cache_destroy(hdr_l2only_cache);
+	kmem_cache_destroy(buf_cache);
+}
+
+/*
+ * Constructor callback - called when the cache is empty
+ * and a new buf is requested.
+ */
+/* ARGSUSED */
+static int
+hdr_full_cons(void *vbuf, void *unused, int kmflag)
+{
+	arc_buf_hdr_t *hdr = vbuf;
+
+	bzero(hdr, HDR_FULL_SIZE);
+	hdr->b_l1hdr.b_byteswap = DMU_BSWAP_NUMFUNCS;
+	cv_init(&hdr->b_l1hdr.b_cv, NULL, CV_DEFAULT, NULL);
+	zfs_refcount_create(&hdr->b_l1hdr.b_refcnt);
+	mutex_init(&hdr->b_l1hdr.b_freeze_lock, NULL, MUTEX_DEFAULT, NULL);
+	list_link_init(&hdr->b_l1hdr.b_arc_node);
+	list_link_init(&hdr->b_l2hdr.b_l2node);
+	multilist_link_init(&hdr->b_l1hdr.b_arc_node);
+	arc_space_consume(HDR_FULL_SIZE, ARC_SPACE_HDRS);
+
+	return (0);
+}
+
+/* ARGSUSED */
+static int
+hdr_full_crypt_cons(void *vbuf, void *unused, int kmflag)
+{
+	arc_buf_hdr_t *hdr = vbuf;
+
+	hdr_full_cons(vbuf, unused, kmflag);
+	bzero(&hdr->b_crypt_hdr, sizeof (hdr->b_crypt_hdr));
+	arc_space_consume(sizeof (hdr->b_crypt_hdr), ARC_SPACE_HDRS);
+
+	return (0);
+}
+
+/* ARGSUSED */
+static int
+hdr_l2only_cons(void *vbuf, void *unused, int kmflag)
+{
+	arc_buf_hdr_t *hdr = vbuf;
+
+	bzero(hdr, HDR_L2ONLY_SIZE);
+	arc_space_consume(HDR_L2ONLY_SIZE, ARC_SPACE_L2HDRS);
+
+	return (0);
+}
+
+/* ARGSUSED */
+static int
+buf_cons(void *vbuf, void *unused, int kmflag)
+{
+	arc_buf_t *buf = vbuf;
+
+	bzero(buf, sizeof (arc_buf_t));
+	mutex_init(&buf->b_evict_lock, NULL, MUTEX_DEFAULT, NULL);
+	arc_space_consume(sizeof (arc_buf_t), ARC_SPACE_HDRS);
+
+	return (0);
+}
+
+/*
+ * Destructor callback - called when a cached buf is
+ * no longer required.
+ */
+/* ARGSUSED */
+static void
+hdr_full_dest(void *vbuf, void *unused)
+{
+	arc_buf_hdr_t *hdr = vbuf;
+
+	ASSERT(HDR_EMPTY(hdr));
+	cv_destroy(&hdr->b_l1hdr.b_cv);
+	zfs_refcount_destroy(&hdr->b_l1hdr.b_refcnt);
+	mutex_destroy(&hdr->b_l1hdr.b_freeze_lock);
+	ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));
+	arc_space_return(HDR_FULL_SIZE, ARC_SPACE_HDRS);
+}
+
+/* ARGSUSED */
+static void
+hdr_full_crypt_dest(void *vbuf, void *unused)
+{
+	arc_buf_hdr_t *hdr = vbuf;
+
+	hdr_full_dest(vbuf, unused);
+	arc_space_return(sizeof (hdr->b_crypt_hdr), ARC_SPACE_HDRS);
+}
+
+/* ARGSUSED */
+static void
+hdr_l2only_dest(void *vbuf, void *unused)
+{
+	arc_buf_hdr_t *hdr __maybe_unused = vbuf;
+
+	ASSERT(HDR_EMPTY(hdr));
+	arc_space_return(HDR_L2ONLY_SIZE, ARC_SPACE_L2HDRS);
+}
+
+/* ARGSUSED */
+static void
+buf_dest(void *vbuf, void *unused)
+{
+	arc_buf_t *buf = vbuf;
+
+	mutex_destroy(&buf->b_evict_lock);
+	arc_space_return(sizeof (arc_buf_t), ARC_SPACE_HDRS);
+}
+
+static void
+buf_init(void)
+{
+	uint64_t *ct = NULL;
+	uint64_t hsize = 1ULL << 12;
+	int i, j;
+
+	/*
+	 * The hash table is big enough to fill all of physical memory
+	 * with an average block size of zfs_arc_average_blocksize (default 8K).
+	 * By default, the table will take up
+	 * totalmem * sizeof(void*) / 8K (1MB per GB with 8-byte pointers).
+	 */
+	while (hsize * zfs_arc_average_blocksize < arc_all_memory())
+		hsize <<= 1;
+retry:
+	buf_hash_table.ht_mask = hsize - 1;
+#if defined(_KERNEL)
+	/*
+	 * Large allocations which do not require contiguous pages
+	 * should be using vmem_alloc() in the linux kernel
+	 */
+	buf_hash_table.ht_table =
+	    vmem_zalloc(hsize * sizeof (void*), KM_SLEEP);
+#else
+	buf_hash_table.ht_table =
+	    kmem_zalloc(hsize * sizeof (void*), KM_NOSLEEP);
+#endif
+	if (buf_hash_table.ht_table == NULL) {
+		ASSERT(hsize > (1ULL << 8));
+		hsize >>= 1;
+		goto retry;
+	}
+
+	hdr_full_cache = kmem_cache_create("arc_buf_hdr_t_full", HDR_FULL_SIZE,
+	    0, hdr_full_cons, hdr_full_dest, NULL, NULL, NULL, 0);
+	hdr_full_crypt_cache = kmem_cache_create("arc_buf_hdr_t_full_crypt",
+	    HDR_FULL_CRYPT_SIZE, 0, hdr_full_crypt_cons, hdr_full_crypt_dest,
+	    NULL, NULL, NULL, 0);
+	hdr_l2only_cache = kmem_cache_create("arc_buf_hdr_t_l2only",
+	    HDR_L2ONLY_SIZE, 0, hdr_l2only_cons, hdr_l2only_dest, NULL,
+	    NULL, NULL, 0);
+	buf_cache = kmem_cache_create("arc_buf_t", sizeof (arc_buf_t),
+	    0, buf_cons, buf_dest, NULL, NULL, NULL, 0);
+
+	for (i = 0; i < 256; i++)
+		for (ct = zfs_crc64_table + i, *ct = i, j = 8; j > 0; j--)
+			*ct = (*ct >> 1) ^ (-(*ct & 1) & ZFS_CRC64_POLY);
+
+	for (i = 0; i < BUF_LOCKS; i++) {
+		mutex_init(&buf_hash_table.ht_locks[i].ht_lock,
+		    NULL, MUTEX_DEFAULT, NULL);
+	}
+}
+
+#define	ARC_MINTIME	(hz>>4) /* 62 ms */
+
+/*
+ * This is the size that the buf occupies in memory. If the buf is compressed,
+ * it will correspond to the compressed size. You should use this method of
+ * getting the buf size unless you explicitly need the logical size.
+ */
+uint64_t
+arc_buf_size(arc_buf_t *buf)
+{
+	return (ARC_BUF_COMPRESSED(buf) ?
+	    HDR_GET_PSIZE(buf->b_hdr) : HDR_GET_LSIZE(buf->b_hdr));
+}
+
+uint64_t
+arc_buf_lsize(arc_buf_t *buf)
+{
+	return (HDR_GET_LSIZE(buf->b_hdr));
+}
+
+/*
+ * This function will return B_TRUE if the buffer is encrypted in memory.
+ * This buffer can be decrypted by calling arc_untransform().
+ */
+boolean_t
+arc_is_encrypted(arc_buf_t *buf)
+{
+	return (ARC_BUF_ENCRYPTED(buf) != 0);
+}
+
+/*
+ * Returns B_TRUE if the buffer represents data that has not had its MAC
+ * verified yet.
+ */
+boolean_t
+arc_is_unauthenticated(arc_buf_t *buf)
+{
+	return (HDR_NOAUTH(buf->b_hdr) != 0);
+}
+
+void
+arc_get_raw_params(arc_buf_t *buf, boolean_t *byteorder, uint8_t *salt,
+    uint8_t *iv, uint8_t *mac)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+
+	ASSERT(HDR_PROTECTED(hdr));
+
+	bcopy(hdr->b_crypt_hdr.b_salt, salt, ZIO_DATA_SALT_LEN);
+	bcopy(hdr->b_crypt_hdr.b_iv, iv, ZIO_DATA_IV_LEN);
+	bcopy(hdr->b_crypt_hdr.b_mac, mac, ZIO_DATA_MAC_LEN);
+	*byteorder = (hdr->b_l1hdr.b_byteswap == DMU_BSWAP_NUMFUNCS) ?
+	    ZFS_HOST_BYTEORDER : !ZFS_HOST_BYTEORDER;
+}
+
+/*
+ * Indicates how this buffer is compressed in memory. If it is not compressed
+ * the value will be ZIO_COMPRESS_OFF. It can be made normally readable with
+ * arc_untransform() as long as it is also unencrypted.
+ */
+enum zio_compress
+arc_get_compression(arc_buf_t *buf)
+{
+	return (ARC_BUF_COMPRESSED(buf) ?
+	    HDR_GET_COMPRESS(buf->b_hdr) : ZIO_COMPRESS_OFF);
+}
+
+/*
+ * Return the compression algorithm used to store this data in the ARC. If ARC
+ * compression is enabled or this is an encrypted block, this will be the same
+ * as what's used to store it on-disk. Otherwise, this will be ZIO_COMPRESS_OFF.
+ */
+static inline enum zio_compress
+arc_hdr_get_compress(arc_buf_hdr_t *hdr)
+{
+	return (HDR_COMPRESSION_ENABLED(hdr) ?
+	    HDR_GET_COMPRESS(hdr) : ZIO_COMPRESS_OFF);
+}
+
+uint8_t
+arc_get_complevel(arc_buf_t *buf)
+{
+	return (buf->b_hdr->b_complevel);
+}
+
+static inline boolean_t
+arc_buf_is_shared(arc_buf_t *buf)
+{
+	boolean_t shared = (buf->b_data != NULL &&
+	    buf->b_hdr->b_l1hdr.b_pabd != NULL &&
+	    abd_is_linear(buf->b_hdr->b_l1hdr.b_pabd) &&
+	    buf->b_data == abd_to_buf(buf->b_hdr->b_l1hdr.b_pabd));
+	IMPLY(shared, HDR_SHARED_DATA(buf->b_hdr));
+	IMPLY(shared, ARC_BUF_SHARED(buf));
+	IMPLY(shared, ARC_BUF_COMPRESSED(buf) || ARC_BUF_LAST(buf));
+
+	/*
+	 * It would be nice to assert arc_can_share() too, but the "hdr isn't
+	 * already being shared" requirement prevents us from doing that.
+	 */
+
+	return (shared);
+}
+
+/*
+ * Free the checksum associated with this header. If there is no checksum, this
+ * is a no-op.
+ */
+static inline void
+arc_cksum_free(arc_buf_hdr_t *hdr)
+{
+	ASSERT(HDR_HAS_L1HDR(hdr));
+
+	mutex_enter(&hdr->b_l1hdr.b_freeze_lock);
+	if (hdr->b_l1hdr.b_freeze_cksum != NULL) {
+		kmem_free(hdr->b_l1hdr.b_freeze_cksum, sizeof (zio_cksum_t));
+		hdr->b_l1hdr.b_freeze_cksum = NULL;
+	}
+	mutex_exit(&hdr->b_l1hdr.b_freeze_lock);
+}
+
+/*
+ * Return true iff at least one of the bufs on hdr is not compressed.
+ * Encrypted buffers count as compressed.
+ */
+static boolean_t
+arc_hdr_has_uncompressed_buf(arc_buf_hdr_t *hdr)
+{
+	ASSERT(hdr->b_l1hdr.b_state == arc_anon || HDR_EMPTY_OR_LOCKED(hdr));
+
+	for (arc_buf_t *b = hdr->b_l1hdr.b_buf; b != NULL; b = b->b_next) {
+		if (!ARC_BUF_COMPRESSED(b)) {
+			return (B_TRUE);
+		}
+	}
+	return (B_FALSE);
+}
+
+
+/*
+ * If we've turned on the ZFS_DEBUG_MODIFY flag, verify that the buf's data
+ * matches the checksum that is stored in the hdr. If there is no checksum,
+ * or if the buf is compressed, this is a no-op.
+ */
+static void
+arc_cksum_verify(arc_buf_t *buf)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+	zio_cksum_t zc;
+
+	if (!(zfs_flags & ZFS_DEBUG_MODIFY))
+		return;
+
+	if (ARC_BUF_COMPRESSED(buf))
+		return;
+
+	ASSERT(HDR_HAS_L1HDR(hdr));
+
+	mutex_enter(&hdr->b_l1hdr.b_freeze_lock);
+
+	if (hdr->b_l1hdr.b_freeze_cksum == NULL || HDR_IO_ERROR(hdr)) {
+		mutex_exit(&hdr->b_l1hdr.b_freeze_lock);
+		return;
+	}
+
+	fletcher_2_native(buf->b_data, arc_buf_size(buf), NULL, &zc);
+	if (!ZIO_CHECKSUM_EQUAL(*hdr->b_l1hdr.b_freeze_cksum, zc))
+		panic("buffer modified while frozen!");
+	mutex_exit(&hdr->b_l1hdr.b_freeze_lock);
+}
+
+/*
+ * This function makes the assumption that data stored in the L2ARC
+ * will be transformed exactly as it is in the main pool. Because of
+ * this we can verify the checksum against the reading process's bp.
+ */
+static boolean_t
+arc_cksum_is_equal(arc_buf_hdr_t *hdr, zio_t *zio)
+{
+	ASSERT(!BP_IS_EMBEDDED(zio->io_bp));
+	VERIFY3U(BP_GET_PSIZE(zio->io_bp), ==, HDR_GET_PSIZE(hdr));
+
+	/*
+	 * Block pointers always store the checksum for the logical data.
+	 * If the block pointer has the gang bit set, then the checksum
+	 * it represents is for the reconstituted data and not for an
+	 * individual gang member. The zio pipeline, however, must be able to
+	 * determine the checksum of each of the gang constituents so it
+	 * treats the checksum comparison differently than what we need
+	 * for l2arc blocks. This prevents us from using the
+	 * zio_checksum_error() interface directly. Instead we must call the
+	 * zio_checksum_error_impl() so that we can ensure the checksum is
+	 * generated using the correct checksum algorithm and accounts for the
+	 * logical I/O size and not just a gang fragment.
+	 */
+	return (zio_checksum_error_impl(zio->io_spa, zio->io_bp,
+	    BP_GET_CHECKSUM(zio->io_bp), zio->io_abd, zio->io_size,
+	    zio->io_offset, NULL) == 0);
+}
+
+/*
+ * Given a buf full of data, if ZFS_DEBUG_MODIFY is enabled this computes a
+ * checksum and attaches it to the buf's hdr so that we can ensure that the buf
+ * isn't modified later on. If buf is compressed or there is already a checksum
+ * on the hdr, this is a no-op (we only checksum uncompressed bufs).
+ */
+static void
+arc_cksum_compute(arc_buf_t *buf)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+
+	if (!(zfs_flags & ZFS_DEBUG_MODIFY))
+		return;
+
+	ASSERT(HDR_HAS_L1HDR(hdr));
+
+	mutex_enter(&buf->b_hdr->b_l1hdr.b_freeze_lock);
+	if (hdr->b_l1hdr.b_freeze_cksum != NULL || ARC_BUF_COMPRESSED(buf)) {
+		mutex_exit(&hdr->b_l1hdr.b_freeze_lock);
+		return;
+	}
+
+	ASSERT(!ARC_BUF_ENCRYPTED(buf));
+	ASSERT(!ARC_BUF_COMPRESSED(buf));
+	hdr->b_l1hdr.b_freeze_cksum = kmem_alloc(sizeof (zio_cksum_t),
+	    KM_SLEEP);
+	fletcher_2_native(buf->b_data, arc_buf_size(buf), NULL,
+	    hdr->b_l1hdr.b_freeze_cksum);
+	mutex_exit(&hdr->b_l1hdr.b_freeze_lock);
+	arc_buf_watch(buf);
+}
+
+#ifndef _KERNEL
+void
+arc_buf_sigsegv(int sig, siginfo_t *si, void *unused)
+{
+	panic("Got SIGSEGV at address: 0x%lx\n", (long)si->si_addr);
+}
+#endif
+
+/* ARGSUSED */
+static void
+arc_buf_unwatch(arc_buf_t *buf)
+{
+#ifndef _KERNEL
+	if (arc_watch) {
+		ASSERT0(mprotect(buf->b_data, arc_buf_size(buf),
+		    PROT_READ | PROT_WRITE));
+	}
+#endif
+}
+
+/* ARGSUSED */
+static void
+arc_buf_watch(arc_buf_t *buf)
+{
+#ifndef _KERNEL
+	if (arc_watch)
+		ASSERT0(mprotect(buf->b_data, arc_buf_size(buf),
+		    PROT_READ));
+#endif
+}
+
+static arc_buf_contents_t
+arc_buf_type(arc_buf_hdr_t *hdr)
+{
+	arc_buf_contents_t type;
+	if (HDR_ISTYPE_METADATA(hdr)) {
+		type = ARC_BUFC_METADATA;
+	} else {
+		type = ARC_BUFC_DATA;
+	}
+	VERIFY3U(hdr->b_type, ==, type);
+	return (type);
+}
+
+boolean_t
+arc_is_metadata(arc_buf_t *buf)
+{
+	return (HDR_ISTYPE_METADATA(buf->b_hdr) != 0);
+}
+
+static uint32_t
+arc_bufc_to_flags(arc_buf_contents_t type)
+{
+	switch (type) {
+	case ARC_BUFC_DATA:
+		/* metadata field is 0 if buffer contains normal data */
+		return (0);
+	case ARC_BUFC_METADATA:
+		return (ARC_FLAG_BUFC_METADATA);
+	default:
+		break;
+	}
+	panic("undefined ARC buffer type!");
+	return ((uint32_t)-1);
+}
+
+void
+arc_buf_thaw(arc_buf_t *buf)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+
+	ASSERT3P(hdr->b_l1hdr.b_state, ==, arc_anon);
+	ASSERT(!HDR_IO_IN_PROGRESS(hdr));
+
+	arc_cksum_verify(buf);
+
+	/*
+	 * Compressed buffers do not manipulate the b_freeze_cksum.
+	 */
+	if (ARC_BUF_COMPRESSED(buf))
+		return;
+
+	ASSERT(HDR_HAS_L1HDR(hdr));
+	arc_cksum_free(hdr);
+	arc_buf_unwatch(buf);
+}
+
+void
+arc_buf_freeze(arc_buf_t *buf)
+{
+	if (!(zfs_flags & ZFS_DEBUG_MODIFY))
+		return;
+
+	if (ARC_BUF_COMPRESSED(buf))
+		return;
+
+	ASSERT(HDR_HAS_L1HDR(buf->b_hdr));
+	arc_cksum_compute(buf);
+}
+
+/*
+ * The arc_buf_hdr_t's b_flags should never be modified directly. Instead,
+ * the following functions should be used to ensure that the flags are
+ * updated in a thread-safe way. When manipulating the flags either
+ * the hash_lock must be held or the hdr must be undiscoverable. This
+ * ensures that we're not racing with any other threads when updating
+ * the flags.
+ */
+static inline void
+arc_hdr_set_flags(arc_buf_hdr_t *hdr, arc_flags_t flags)
+{
+	ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
+	hdr->b_flags |= flags;
+}
+
+static inline void
+arc_hdr_clear_flags(arc_buf_hdr_t *hdr, arc_flags_t flags)
+{
+	ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
+	hdr->b_flags &= ~flags;
+}
+
+/*
+ * Setting the compression bits in the arc_buf_hdr_t's b_flags is
+ * done in a special way since we have to clear and set bits
+ * at the same time. Consumers that wish to set the compression bits
+ * must use this function to ensure that the flags are updated in
+ * thread-safe manner.
+ */
+static void
+arc_hdr_set_compress(arc_buf_hdr_t *hdr, enum zio_compress cmp)
+{
+	ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
+
+	/*
+	 * Holes and embedded blocks will always have a psize = 0 so
+	 * we ignore the compression of the blkptr and set the
+	 * want to uncompress them. Mark them as uncompressed.
+	 */
+	if (!zfs_compressed_arc_enabled || HDR_GET_PSIZE(hdr) == 0) {
+		arc_hdr_clear_flags(hdr, ARC_FLAG_COMPRESSED_ARC);
+		ASSERT(!HDR_COMPRESSION_ENABLED(hdr));
+	} else {
+		arc_hdr_set_flags(hdr, ARC_FLAG_COMPRESSED_ARC);
+		ASSERT(HDR_COMPRESSION_ENABLED(hdr));
+	}
+
+	HDR_SET_COMPRESS(hdr, cmp);
+	ASSERT3U(HDR_GET_COMPRESS(hdr), ==, cmp);
+}
+
+/*
+ * Looks for another buf on the same hdr which has the data decompressed, copies
+ * from it, and returns true. If no such buf exists, returns false.
+ */
+static boolean_t
+arc_buf_try_copy_decompressed_data(arc_buf_t *buf)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+	boolean_t copied = B_FALSE;
+
+	ASSERT(HDR_HAS_L1HDR(hdr));
+	ASSERT3P(buf->b_data, !=, NULL);
+	ASSERT(!ARC_BUF_COMPRESSED(buf));
+
+	for (arc_buf_t *from = hdr->b_l1hdr.b_buf; from != NULL;
+	    from = from->b_next) {
+		/* can't use our own data buffer */
+		if (from == buf) {
+			continue;
+		}
+
+		if (!ARC_BUF_COMPRESSED(from)) {
+			bcopy(from->b_data, buf->b_data, arc_buf_size(buf));
+			copied = B_TRUE;
+			break;
+		}
+	}
+
+	/*
+	 * There were no decompressed bufs, so there should not be a
+	 * checksum on the hdr either.
+	 */
+	if (zfs_flags & ZFS_DEBUG_MODIFY)
+		EQUIV(!copied, hdr->b_l1hdr.b_freeze_cksum == NULL);
+
+	return (copied);
+}
+
+/*
+ * Allocates an ARC buf header that's in an evicted & L2-cached state.
+ * This is used during l2arc reconstruction to make empty ARC buffers
+ * which circumvent the regular disk->arc->l2arc path and instead come
+ * into being in the reverse order, i.e. l2arc->arc.
+ */
+static arc_buf_hdr_t *
+arc_buf_alloc_l2only(size_t size, arc_buf_contents_t type, l2arc_dev_t *dev,
+    dva_t dva, uint64_t daddr, int32_t psize, uint64_t birth,
+    enum zio_compress compress, uint8_t complevel, boolean_t protected,
+    boolean_t prefetch)
+{
+	arc_buf_hdr_t	*hdr;
+
+	ASSERT(size != 0);
+	hdr = kmem_cache_alloc(hdr_l2only_cache, KM_SLEEP);
+	hdr->b_birth = birth;
+	hdr->b_type = type;
+	hdr->b_flags = 0;
+	arc_hdr_set_flags(hdr, arc_bufc_to_flags(type) | ARC_FLAG_HAS_L2HDR);
+	HDR_SET_LSIZE(hdr, size);
+	HDR_SET_PSIZE(hdr, psize);
+	arc_hdr_set_compress(hdr, compress);
+	hdr->b_complevel = complevel;
+	if (protected)
+		arc_hdr_set_flags(hdr, ARC_FLAG_PROTECTED);
+	if (prefetch)
+		arc_hdr_set_flags(hdr, ARC_FLAG_PREFETCH);
+	hdr->b_spa = spa_load_guid(dev->l2ad_vdev->vdev_spa);
+
+	hdr->b_dva = dva;
+
+	hdr->b_l2hdr.b_dev = dev;
+	hdr->b_l2hdr.b_daddr = daddr;
+
+	return (hdr);
+}
+
+/*
+ * Return the size of the block, b_pabd, that is stored in the arc_buf_hdr_t.
+ */
+static uint64_t
+arc_hdr_size(arc_buf_hdr_t *hdr)
+{
+	uint64_t size;
+
+	if (arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF &&
+	    HDR_GET_PSIZE(hdr) > 0) {
+		size = HDR_GET_PSIZE(hdr);
+	} else {
+		ASSERT3U(HDR_GET_LSIZE(hdr), !=, 0);
+		size = HDR_GET_LSIZE(hdr);
+	}
+	return (size);
+}
+
+static int
+arc_hdr_authenticate(arc_buf_hdr_t *hdr, spa_t *spa, uint64_t dsobj)
+{
+	int ret;
+	uint64_t csize;
+	uint64_t lsize = HDR_GET_LSIZE(hdr);
+	uint64_t psize = HDR_GET_PSIZE(hdr);
+	void *tmpbuf = NULL;
+	abd_t *abd = hdr->b_l1hdr.b_pabd;
+
+	ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
+	ASSERT(HDR_AUTHENTICATED(hdr));
+	ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
+
+	/*
+	 * The MAC is calculated on the compressed data that is stored on disk.
+	 * However, if compressed arc is disabled we will only have the
+	 * decompressed data available to us now. Compress it into a temporary
+	 * abd so we can verify the MAC. The performance overhead of this will
+	 * be relatively low, since most objects in an encrypted objset will
+	 * be encrypted (instead of authenticated) anyway.
+	 */
+	if (HDR_GET_COMPRESS(hdr) != ZIO_COMPRESS_OFF &&
+	    !HDR_COMPRESSION_ENABLED(hdr)) {
+		tmpbuf = zio_buf_alloc(lsize);
+		abd = abd_get_from_buf(tmpbuf, lsize);
+		abd_take_ownership_of_buf(abd, B_TRUE);
+		csize = zio_compress_data(HDR_GET_COMPRESS(hdr),
+		    hdr->b_l1hdr.b_pabd, tmpbuf, lsize, hdr->b_complevel);
+		ASSERT3U(csize, <=, psize);
+		abd_zero_off(abd, csize, psize - csize);
+	}
+
+	/*
+	 * Authentication is best effort. We authenticate whenever the key is
+	 * available. If we succeed we clear ARC_FLAG_NOAUTH.
+	 */
+	if (hdr->b_crypt_hdr.b_ot == DMU_OT_OBJSET) {
+		ASSERT3U(HDR_GET_COMPRESS(hdr), ==, ZIO_COMPRESS_OFF);
+		ASSERT3U(lsize, ==, psize);
+		ret = spa_do_crypt_objset_mac_abd(B_FALSE, spa, dsobj, abd,
+		    psize, hdr->b_l1hdr.b_byteswap != DMU_BSWAP_NUMFUNCS);
+	} else {
+		ret = spa_do_crypt_mac_abd(B_FALSE, spa, dsobj, abd, psize,
+		    hdr->b_crypt_hdr.b_mac);
+	}
+
+	if (ret == 0)
+		arc_hdr_clear_flags(hdr, ARC_FLAG_NOAUTH);
+	else if (ret != ENOENT)
+		goto error;
+
+	if (tmpbuf != NULL)
+		abd_free(abd);
+
+	return (0);
+
+error:
+	if (tmpbuf != NULL)
+		abd_free(abd);
+
+	return (ret);
+}
+
+/*
+ * This function will take a header that only has raw encrypted data in
+ * b_crypt_hdr.b_rabd and decrypt it into a new buffer which is stored in
+ * b_l1hdr.b_pabd. If designated in the header flags, this function will
+ * also decompress the data.
+ */
+static int
+arc_hdr_decrypt(arc_buf_hdr_t *hdr, spa_t *spa, const zbookmark_phys_t *zb)
+{
+	int ret;
+	abd_t *cabd = NULL;
+	void *tmp = NULL;
+	boolean_t no_crypt = B_FALSE;
+	boolean_t bswap = (hdr->b_l1hdr.b_byteswap != DMU_BSWAP_NUMFUNCS);
+
+	ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
+	ASSERT(HDR_ENCRYPTED(hdr));
+
+	arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT);
+
+	ret = spa_do_crypt_abd(B_FALSE, spa, zb, hdr->b_crypt_hdr.b_ot,
+	    B_FALSE, bswap, hdr->b_crypt_hdr.b_salt, hdr->b_crypt_hdr.b_iv,
+	    hdr->b_crypt_hdr.b_mac, HDR_GET_PSIZE(hdr), hdr->b_l1hdr.b_pabd,
+	    hdr->b_crypt_hdr.b_rabd, &no_crypt);
+	if (ret != 0)
+		goto error;
+
+	if (no_crypt) {
+		abd_copy(hdr->b_l1hdr.b_pabd, hdr->b_crypt_hdr.b_rabd,
+		    HDR_GET_PSIZE(hdr));
+	}
+
+	/*
+	 * If this header has disabled arc compression but the b_pabd is
+	 * compressed after decrypting it, we need to decompress the newly
+	 * decrypted data.
+	 */
+	if (HDR_GET_COMPRESS(hdr) != ZIO_COMPRESS_OFF &&
+	    !HDR_COMPRESSION_ENABLED(hdr)) {
+		/*
+		 * We want to make sure that we are correctly honoring the
+		 * zfs_abd_scatter_enabled setting, so we allocate an abd here
+		 * and then loan a buffer from it, rather than allocating a
+		 * linear buffer and wrapping it in an abd later.
+		 */
+		cabd = arc_get_data_abd(hdr, arc_hdr_size(hdr), hdr, B_TRUE);
+		tmp = abd_borrow_buf(cabd, arc_hdr_size(hdr));
+
+		ret = zio_decompress_data(HDR_GET_COMPRESS(hdr),
+		    hdr->b_l1hdr.b_pabd, tmp, HDR_GET_PSIZE(hdr),
+		    HDR_GET_LSIZE(hdr), &hdr->b_complevel);
+		if (ret != 0) {
+			abd_return_buf(cabd, tmp, arc_hdr_size(hdr));
+			goto error;
+		}
+
+		abd_return_buf_copy(cabd, tmp, arc_hdr_size(hdr));
+		arc_free_data_abd(hdr, hdr->b_l1hdr.b_pabd,
+		    arc_hdr_size(hdr), hdr);
+		hdr->b_l1hdr.b_pabd = cabd;
+	}
+
+	return (0);
+
+error:
+	arc_hdr_free_abd(hdr, B_FALSE);
+	if (cabd != NULL)
+		arc_free_data_buf(hdr, cabd, arc_hdr_size(hdr), hdr);
+
+	return (ret);
+}
+
+/*
+ * This function is called during arc_buf_fill() to prepare the header's
+ * abd plaintext pointer for use. This involves authenticated protected
+ * data and decrypting encrypted data into the plaintext abd.
+ */
+static int
+arc_fill_hdr_crypt(arc_buf_hdr_t *hdr, kmutex_t *hash_lock, spa_t *spa,
+    const zbookmark_phys_t *zb, boolean_t noauth)
+{
+	int ret;
+
+	ASSERT(HDR_PROTECTED(hdr));
+
+	if (hash_lock != NULL)
+		mutex_enter(hash_lock);
+
+	if (HDR_NOAUTH(hdr) && !noauth) {
+		/*
+		 * The caller requested authenticated data but our data has
+		 * not been authenticated yet. Verify the MAC now if we can.
+		 */
+		ret = arc_hdr_authenticate(hdr, spa, zb->zb_objset);
+		if (ret != 0)
+			goto error;
+	} else if (HDR_HAS_RABD(hdr) && hdr->b_l1hdr.b_pabd == NULL) {
+		/*
+		 * If we only have the encrypted version of the data, but the
+		 * unencrypted version was requested we take this opportunity
+		 * to store the decrypted version in the header for future use.
+		 */
+		ret = arc_hdr_decrypt(hdr, spa, zb);
+		if (ret != 0)
+			goto error;
+	}
+
+	ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
+
+	if (hash_lock != NULL)
+		mutex_exit(hash_lock);
+
+	return (0);
+
+error:
+	if (hash_lock != NULL)
+		mutex_exit(hash_lock);
+
+	return (ret);
+}
+
+/*
+ * This function is used by the dbuf code to decrypt bonus buffers in place.
+ * The dbuf code itself doesn't have any locking for decrypting a shared dnode
+ * block, so we use the hash lock here to protect against concurrent calls to
+ * arc_buf_fill().
+ */
+static void
+arc_buf_untransform_in_place(arc_buf_t *buf, kmutex_t *hash_lock)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+
+	ASSERT(HDR_ENCRYPTED(hdr));
+	ASSERT3U(hdr->b_crypt_hdr.b_ot, ==, DMU_OT_DNODE);
+	ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
+	ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
+
+	zio_crypt_copy_dnode_bonus(hdr->b_l1hdr.b_pabd, buf->b_data,
+	    arc_buf_size(buf));
+	buf->b_flags &= ~ARC_BUF_FLAG_ENCRYPTED;
+	buf->b_flags &= ~ARC_BUF_FLAG_COMPRESSED;
+	hdr->b_crypt_hdr.b_ebufcnt -= 1;
+}
+
+/*
+ * Given a buf that has a data buffer attached to it, this function will
+ * efficiently fill the buf with data of the specified compression setting from
+ * the hdr and update the hdr's b_freeze_cksum if necessary. If the buf and hdr
+ * are already sharing a data buf, no copy is performed.
+ *
+ * If the buf is marked as compressed but uncompressed data was requested, this
+ * will allocate a new data buffer for the buf, remove that flag, and fill the
+ * buf with uncompressed data. You can't request a compressed buf on a hdr with
+ * uncompressed data, and (since we haven't added support for it yet) if you
+ * want compressed data your buf must already be marked as compressed and have
+ * the correct-sized data buffer.
+ */
+static int
+arc_buf_fill(arc_buf_t *buf, spa_t *spa, const zbookmark_phys_t *zb,
+    arc_fill_flags_t flags)
+{
+	int error = 0;
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+	boolean_t hdr_compressed =
+	    (arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF);
+	boolean_t compressed = (flags & ARC_FILL_COMPRESSED) != 0;
+	boolean_t encrypted = (flags & ARC_FILL_ENCRYPTED) != 0;
+	dmu_object_byteswap_t bswap = hdr->b_l1hdr.b_byteswap;
+	kmutex_t *hash_lock = (flags & ARC_FILL_LOCKED) ? NULL : HDR_LOCK(hdr);
+
+	ASSERT3P(buf->b_data, !=, NULL);
+	IMPLY(compressed, hdr_compressed || ARC_BUF_ENCRYPTED(buf));
+	IMPLY(compressed, ARC_BUF_COMPRESSED(buf));
+	IMPLY(encrypted, HDR_ENCRYPTED(hdr));
+	IMPLY(encrypted, ARC_BUF_ENCRYPTED(buf));
+	IMPLY(encrypted, ARC_BUF_COMPRESSED(buf));
+	IMPLY(encrypted, !ARC_BUF_SHARED(buf));
+
+	/*
+	 * If the caller wanted encrypted data we just need to copy it from
+	 * b_rabd and potentially byteswap it. We won't be able to do any
+	 * further transforms on it.
+	 */
+	if (encrypted) {
+		ASSERT(HDR_HAS_RABD(hdr));
+		abd_copy_to_buf(buf->b_data, hdr->b_crypt_hdr.b_rabd,
+		    HDR_GET_PSIZE(hdr));
+		goto byteswap;
+	}
+
+	/*
+	 * Adjust encrypted and authenticated headers to accommodate
+	 * the request if needed. Dnode blocks (ARC_FILL_IN_PLACE) are
+	 * allowed to fail decryption due to keys not being loaded
+	 * without being marked as an IO error.
+	 */
+	if (HDR_PROTECTED(hdr)) {
+		error = arc_fill_hdr_crypt(hdr, hash_lock, spa,
+		    zb, !!(flags & ARC_FILL_NOAUTH));
+		if (error == EACCES && (flags & ARC_FILL_IN_PLACE) != 0) {
+			return (error);
+		} else if (error != 0) {
+			if (hash_lock != NULL)
+				mutex_enter(hash_lock);
+			arc_hdr_set_flags(hdr, ARC_FLAG_IO_ERROR);
+			if (hash_lock != NULL)
+				mutex_exit(hash_lock);
+			return (error);
+		}
+	}
+
+	/*
+	 * There is a special case here for dnode blocks which are
+	 * decrypting their bonus buffers. These blocks may request to
+	 * be decrypted in-place. This is necessary because there may
+	 * be many dnodes pointing into this buffer and there is
+	 * currently no method to synchronize replacing the backing
+	 * b_data buffer and updating all of the pointers. Here we use
+	 * the hash lock to ensure there are no races. If the need
+	 * arises for other types to be decrypted in-place, they must
+	 * add handling here as well.
+	 */
+	if ((flags & ARC_FILL_IN_PLACE) != 0) {
+		ASSERT(!hdr_compressed);
+		ASSERT(!compressed);
+		ASSERT(!encrypted);
+
+		if (HDR_ENCRYPTED(hdr) && ARC_BUF_ENCRYPTED(buf)) {
+			ASSERT3U(hdr->b_crypt_hdr.b_ot, ==, DMU_OT_DNODE);
+
+			if (hash_lock != NULL)
+				mutex_enter(hash_lock);
+			arc_buf_untransform_in_place(buf, hash_lock);
+			if (hash_lock != NULL)
+				mutex_exit(hash_lock);
+
+			/* Compute the hdr's checksum if necessary */
+			arc_cksum_compute(buf);
+		}
+
+		return (0);
+	}
+
+	if (hdr_compressed == compressed) {
+		if (!arc_buf_is_shared(buf)) {
+			abd_copy_to_buf(buf->b_data, hdr->b_l1hdr.b_pabd,
+			    arc_buf_size(buf));
+		}
+	} else {
+		ASSERT(hdr_compressed);
+		ASSERT(!compressed);
+		ASSERT3U(HDR_GET_LSIZE(hdr), !=, HDR_GET_PSIZE(hdr));
+
+		/*
+		 * If the buf is sharing its data with the hdr, unlink it and
+		 * allocate a new data buffer for the buf.
+		 */
+		if (arc_buf_is_shared(buf)) {
+			ASSERT(ARC_BUF_COMPRESSED(buf));
+
+			/* We need to give the buf its own b_data */
+			buf->b_flags &= ~ARC_BUF_FLAG_SHARED;
+			buf->b_data =
+			    arc_get_data_buf(hdr, HDR_GET_LSIZE(hdr), buf);
+			arc_hdr_clear_flags(hdr, ARC_FLAG_SHARED_DATA);
+
+			/* Previously overhead was 0; just add new overhead */
+			ARCSTAT_INCR(arcstat_overhead_size, HDR_GET_LSIZE(hdr));
+		} else if (ARC_BUF_COMPRESSED(buf)) {
+			/* We need to reallocate the buf's b_data */
+			arc_free_data_buf(hdr, buf->b_data, HDR_GET_PSIZE(hdr),
+			    buf);
+			buf->b_data =
+			    arc_get_data_buf(hdr, HDR_GET_LSIZE(hdr), buf);
+
+			/* We increased the size of b_data; update overhead */
+			ARCSTAT_INCR(arcstat_overhead_size,
+			    HDR_GET_LSIZE(hdr) - HDR_GET_PSIZE(hdr));
+		}
+
+		/*
+		 * Regardless of the buf's previous compression settings, it
+		 * should not be compressed at the end of this function.
+		 */
+		buf->b_flags &= ~ARC_BUF_FLAG_COMPRESSED;
+
+		/*
+		 * Try copying the data from another buf which already has a
+		 * decompressed version. If that's not possible, it's time to
+		 * bite the bullet and decompress the data from the hdr.
+		 */
+		if (arc_buf_try_copy_decompressed_data(buf)) {
+			/* Skip byteswapping and checksumming (already done) */
+			return (0);
+		} else {
+			error = zio_decompress_data(HDR_GET_COMPRESS(hdr),
+			    hdr->b_l1hdr.b_pabd, buf->b_data,
+			    HDR_GET_PSIZE(hdr), HDR_GET_LSIZE(hdr),
+			    &hdr->b_complevel);
+
+			/*
+			 * Absent hardware errors or software bugs, this should
+			 * be impossible, but log it anyway so we can debug it.
+			 */
+			if (error != 0) {
+				zfs_dbgmsg(
+				    "hdr %px, compress %d, psize %d, lsize %d",
+				    hdr, arc_hdr_get_compress(hdr),
+				    HDR_GET_PSIZE(hdr), HDR_GET_LSIZE(hdr));
+				if (hash_lock != NULL)
+					mutex_enter(hash_lock);
+				arc_hdr_set_flags(hdr, ARC_FLAG_IO_ERROR);
+				if (hash_lock != NULL)
+					mutex_exit(hash_lock);
+				return (SET_ERROR(EIO));
+			}
+		}
+	}
+
+byteswap:
+	/* Byteswap the buf's data if necessary */
+	if (bswap != DMU_BSWAP_NUMFUNCS) {
+		ASSERT(!HDR_SHARED_DATA(hdr));
+		ASSERT3U(bswap, <, DMU_BSWAP_NUMFUNCS);
+		dmu_ot_byteswap[bswap].ob_func(buf->b_data, HDR_GET_LSIZE(hdr));
+	}
+
+	/* Compute the hdr's checksum if necessary */
+	arc_cksum_compute(buf);
+
+	return (0);
+}
+
+/*
+ * If this function is being called to decrypt an encrypted buffer or verify an
+ * authenticated one, the key must be loaded and a mapping must be made
+ * available in the keystore via spa_keystore_create_mapping() or one of its
+ * callers.
+ */
+int
+arc_untransform(arc_buf_t *buf, spa_t *spa, const zbookmark_phys_t *zb,
+    boolean_t in_place)
+{
+	int ret;
+	arc_fill_flags_t flags = 0;
+
+	if (in_place)
+		flags |= ARC_FILL_IN_PLACE;
+
+	ret = arc_buf_fill(buf, spa, zb, flags);
+	if (ret == ECKSUM) {
+		/*
+		 * Convert authentication and decryption errors to EIO
+		 * (and generate an ereport) before leaving the ARC.
+		 */
+		ret = SET_ERROR(EIO);
+		spa_log_error(spa, zb);
+		zfs_ereport_post(FM_EREPORT_ZFS_AUTHENTICATION,
+		    spa, NULL, zb, NULL, 0, 0);
+	}
+
+	return (ret);
+}
+
+/*
+ * Increment the amount of evictable space in the arc_state_t's refcount.
+ * We account for the space used by the hdr and the arc buf individually
+ * so that we can add and remove them from the refcount individually.
+ */
+static void
+arc_evictable_space_increment(arc_buf_hdr_t *hdr, arc_state_t *state)
+{
+	arc_buf_contents_t type = arc_buf_type(hdr);
+
+	ASSERT(HDR_HAS_L1HDR(hdr));
+
+	if (GHOST_STATE(state)) {
+		ASSERT0(hdr->b_l1hdr.b_bufcnt);
+		ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
+		ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
+		ASSERT(!HDR_HAS_RABD(hdr));
+		(void) zfs_refcount_add_many(&state->arcs_esize[type],
+		    HDR_GET_LSIZE(hdr), hdr);
+		return;
+	}
+
+	ASSERT(!GHOST_STATE(state));
+	if (hdr->b_l1hdr.b_pabd != NULL) {
+		(void) zfs_refcount_add_many(&state->arcs_esize[type],
+		    arc_hdr_size(hdr), hdr);
+	}
+	if (HDR_HAS_RABD(hdr)) {
+		(void) zfs_refcount_add_many(&state->arcs_esize[type],
+		    HDR_GET_PSIZE(hdr), hdr);
+	}
+
+	for (arc_buf_t *buf = hdr->b_l1hdr.b_buf; buf != NULL;
+	    buf = buf->b_next) {
+		if (arc_buf_is_shared(buf))
+			continue;
+		(void) zfs_refcount_add_many(&state->arcs_esize[type],
+		    arc_buf_size(buf), buf);
+	}
+}
+
+/*
+ * Decrement the amount of evictable space in the arc_state_t's refcount.
+ * We account for the space used by the hdr and the arc buf individually
+ * so that we can add and remove them from the refcount individually.
+ */
+static void
+arc_evictable_space_decrement(arc_buf_hdr_t *hdr, arc_state_t *state)
+{
+	arc_buf_contents_t type = arc_buf_type(hdr);
+
+	ASSERT(HDR_HAS_L1HDR(hdr));
+
+	if (GHOST_STATE(state)) {
+		ASSERT0(hdr->b_l1hdr.b_bufcnt);
+		ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
+		ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
+		ASSERT(!HDR_HAS_RABD(hdr));
+		(void) zfs_refcount_remove_many(&state->arcs_esize[type],
+		    HDR_GET_LSIZE(hdr), hdr);
+		return;
+	}
+
+	ASSERT(!GHOST_STATE(state));
+	if (hdr->b_l1hdr.b_pabd != NULL) {
+		(void) zfs_refcount_remove_many(&state->arcs_esize[type],
+		    arc_hdr_size(hdr), hdr);
+	}
+	if (HDR_HAS_RABD(hdr)) {
+		(void) zfs_refcount_remove_many(&state->arcs_esize[type],
+		    HDR_GET_PSIZE(hdr), hdr);
+	}
+
+	for (arc_buf_t *buf = hdr->b_l1hdr.b_buf; buf != NULL;
+	    buf = buf->b_next) {
+		if (arc_buf_is_shared(buf))
+			continue;
+		(void) zfs_refcount_remove_many(&state->arcs_esize[type],
+		    arc_buf_size(buf), buf);
+	}
+}
+
+/*
+ * Add a reference to this hdr indicating that someone is actively
+ * referencing that memory. When the refcount transitions from 0 to 1,
+ * we remove it from the respective arc_state_t list to indicate that
+ * it is not evictable.
+ */
+static void
+add_reference(arc_buf_hdr_t *hdr, void *tag)
+{
+	arc_state_t *state;
+
+	ASSERT(HDR_HAS_L1HDR(hdr));
+	if (!HDR_EMPTY(hdr) && !MUTEX_HELD(HDR_LOCK(hdr))) {
+		ASSERT(hdr->b_l1hdr.b_state == arc_anon);
+		ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
+		ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
+	}
+
+	state = hdr->b_l1hdr.b_state;
+
+	if ((zfs_refcount_add(&hdr->b_l1hdr.b_refcnt, tag) == 1) &&
+	    (state != arc_anon)) {
+		/* We don't use the L2-only state list. */
+		if (state != arc_l2c_only) {
+			multilist_remove(state->arcs_list[arc_buf_type(hdr)],
+			    hdr);
+			arc_evictable_space_decrement(hdr, state);
+		}
+		/* remove the prefetch flag if we get a reference */
+		arc_hdr_clear_flags(hdr, ARC_FLAG_PREFETCH);
+	}
+}
+
+/*
+ * Remove a reference from this hdr. When the reference transitions from
+ * 1 to 0 and we're not anonymous, then we add this hdr to the arc_state_t's
+ * list making it eligible for eviction.
+ */
+static int
+remove_reference(arc_buf_hdr_t *hdr, kmutex_t *hash_lock, void *tag)
+{
+	int cnt;
+	arc_state_t *state = hdr->b_l1hdr.b_state;
+
+	ASSERT(HDR_HAS_L1HDR(hdr));
+	ASSERT(state == arc_anon || MUTEX_HELD(hash_lock));
+	ASSERT(!GHOST_STATE(state));
+
+	/*
+	 * arc_l2c_only counts as a ghost state so we don't need to explicitly
+	 * check to prevent usage of the arc_l2c_only list.
+	 */
+	if (((cnt = zfs_refcount_remove(&hdr->b_l1hdr.b_refcnt, tag)) == 0) &&
+	    (state != arc_anon)) {
+		multilist_insert(state->arcs_list[arc_buf_type(hdr)], hdr);
+		ASSERT3U(hdr->b_l1hdr.b_bufcnt, >, 0);
+		arc_evictable_space_increment(hdr, state);
+	}
+	return (cnt);
+}
+
+/*
+ * Returns detailed information about a specific arc buffer.  When the
+ * state_index argument is set the function will calculate the arc header
+ * list position for its arc state.  Since this requires a linear traversal
+ * callers are strongly encourage not to do this.  However, it can be helpful
+ * for targeted analysis so the functionality is provided.
+ */
+void
+arc_buf_info(arc_buf_t *ab, arc_buf_info_t *abi, int state_index)
+{
+	arc_buf_hdr_t *hdr = ab->b_hdr;
+	l1arc_buf_hdr_t *l1hdr = NULL;
+	l2arc_buf_hdr_t *l2hdr = NULL;
+	arc_state_t *state = NULL;
+
+	memset(abi, 0, sizeof (arc_buf_info_t));
+
+	if (hdr == NULL)
+		return;
+
+	abi->abi_flags = hdr->b_flags;
+
+	if (HDR_HAS_L1HDR(hdr)) {
+		l1hdr = &hdr->b_l1hdr;
+		state = l1hdr->b_state;
+	}
+	if (HDR_HAS_L2HDR(hdr))
+		l2hdr = &hdr->b_l2hdr;
+
+	if (l1hdr) {
+		abi->abi_bufcnt = l1hdr->b_bufcnt;
+		abi->abi_access = l1hdr->b_arc_access;
+		abi->abi_mru_hits = l1hdr->b_mru_hits;
+		abi->abi_mru_ghost_hits = l1hdr->b_mru_ghost_hits;
+		abi->abi_mfu_hits = l1hdr->b_mfu_hits;
+		abi->abi_mfu_ghost_hits = l1hdr->b_mfu_ghost_hits;
+		abi->abi_holds = zfs_refcount_count(&l1hdr->b_refcnt);
+	}
+
+	if (l2hdr) {
+		abi->abi_l2arc_dattr = l2hdr->b_daddr;
+		abi->abi_l2arc_hits = l2hdr->b_hits;
+	}
+
+	abi->abi_state_type = state ? state->arcs_state : ARC_STATE_ANON;
+	abi->abi_state_contents = arc_buf_type(hdr);
+	abi->abi_size = arc_hdr_size(hdr);
+}
+
+/*
+ * Move the supplied buffer to the indicated state. The hash lock
+ * for the buffer must be held by the caller.
+ */
+static void
+arc_change_state(arc_state_t *new_state, arc_buf_hdr_t *hdr,
+    kmutex_t *hash_lock)
+{
+	arc_state_t *old_state;
+	int64_t refcnt;
+	uint32_t bufcnt;
+	boolean_t update_old, update_new;
+	arc_buf_contents_t buftype = arc_buf_type(hdr);
+
+	/*
+	 * We almost always have an L1 hdr here, since we call arc_hdr_realloc()
+	 * in arc_read() when bringing a buffer out of the L2ARC.  However, the
+	 * L1 hdr doesn't always exist when we change state to arc_anon before
+	 * destroying a header, in which case reallocating to add the L1 hdr is
+	 * pointless.
+	 */
+	if (HDR_HAS_L1HDR(hdr)) {
+		old_state = hdr->b_l1hdr.b_state;
+		refcnt = zfs_refcount_count(&hdr->b_l1hdr.b_refcnt);
+		bufcnt = hdr->b_l1hdr.b_bufcnt;
+		update_old = (bufcnt > 0 || hdr->b_l1hdr.b_pabd != NULL ||
+		    HDR_HAS_RABD(hdr));
+	} else {
+		old_state = arc_l2c_only;
+		refcnt = 0;
+		bufcnt = 0;
+		update_old = B_FALSE;
+	}
+	update_new = update_old;
+
+	ASSERT(MUTEX_HELD(hash_lock));
+	ASSERT3P(new_state, !=, old_state);
+	ASSERT(!GHOST_STATE(new_state) || bufcnt == 0);
+	ASSERT(old_state != arc_anon || bufcnt <= 1);
+
+	/*
+	 * If this buffer is evictable, transfer it from the
+	 * old state list to the new state list.
+	 */
+	if (refcnt == 0) {
+		if (old_state != arc_anon && old_state != arc_l2c_only) {
+			ASSERT(HDR_HAS_L1HDR(hdr));
+			multilist_remove(old_state->arcs_list[buftype], hdr);
+
+			if (GHOST_STATE(old_state)) {
+				ASSERT0(bufcnt);
+				ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
+				update_old = B_TRUE;
+			}
+			arc_evictable_space_decrement(hdr, old_state);
+		}
+		if (new_state != arc_anon && new_state != arc_l2c_only) {
+			/*
+			 * An L1 header always exists here, since if we're
+			 * moving to some L1-cached state (i.e. not l2c_only or
+			 * anonymous), we realloc the header to add an L1hdr
+			 * beforehand.
+			 */
+			ASSERT(HDR_HAS_L1HDR(hdr));
+			multilist_insert(new_state->arcs_list[buftype], hdr);
+
+			if (GHOST_STATE(new_state)) {
+				ASSERT0(bufcnt);
+				ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
+				update_new = B_TRUE;
+			}
+			arc_evictable_space_increment(hdr, new_state);
+		}
+	}
+
+	ASSERT(!HDR_EMPTY(hdr));
+	if (new_state == arc_anon && HDR_IN_HASH_TABLE(hdr))
+		buf_hash_remove(hdr);
+
+	/* adjust state sizes (ignore arc_l2c_only) */
+
+	if (update_new && new_state != arc_l2c_only) {
+		ASSERT(HDR_HAS_L1HDR(hdr));
+		if (GHOST_STATE(new_state)) {
+			ASSERT0(bufcnt);
+
+			/*
+			 * When moving a header to a ghost state, we first
+			 * remove all arc buffers. Thus, we'll have a
+			 * bufcnt of zero, and no arc buffer to use for
+			 * the reference. As a result, we use the arc
+			 * header pointer for the reference.
+			 */
+			(void) zfs_refcount_add_many(&new_state->arcs_size,
+			    HDR_GET_LSIZE(hdr), hdr);
+			ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
+			ASSERT(!HDR_HAS_RABD(hdr));
+		} else {
+			uint32_t buffers = 0;
+
+			/*
+			 * Each individual buffer holds a unique reference,
+			 * thus we must remove each of these references one
+			 * at a time.
+			 */
+			for (arc_buf_t *buf = hdr->b_l1hdr.b_buf; buf != NULL;
+			    buf = buf->b_next) {
+				ASSERT3U(bufcnt, !=, 0);
+				buffers++;
+
+				/*
+				 * When the arc_buf_t is sharing the data
+				 * block with the hdr, the owner of the
+				 * reference belongs to the hdr. Only
+				 * add to the refcount if the arc_buf_t is
+				 * not shared.
+				 */
+				if (arc_buf_is_shared(buf))
+					continue;
+
+				(void) zfs_refcount_add_many(
+				    &new_state->arcs_size,
+				    arc_buf_size(buf), buf);
+			}
+			ASSERT3U(bufcnt, ==, buffers);
+
+			if (hdr->b_l1hdr.b_pabd != NULL) {
+				(void) zfs_refcount_add_many(
+				    &new_state->arcs_size,
+				    arc_hdr_size(hdr), hdr);
+			}
+
+			if (HDR_HAS_RABD(hdr)) {
+				(void) zfs_refcount_add_many(
+				    &new_state->arcs_size,
+				    HDR_GET_PSIZE(hdr), hdr);
+			}
+		}
+	}
+
+	if (update_old && old_state != arc_l2c_only) {
+		ASSERT(HDR_HAS_L1HDR(hdr));
+		if (GHOST_STATE(old_state)) {
+			ASSERT0(bufcnt);
+			ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
+			ASSERT(!HDR_HAS_RABD(hdr));
+
+			/*
+			 * When moving a header off of a ghost state,
+			 * the header will not contain any arc buffers.
+			 * We use the arc header pointer for the reference
+			 * which is exactly what we did when we put the
+			 * header on the ghost state.
+			 */
+
+			(void) zfs_refcount_remove_many(&old_state->arcs_size,
+			    HDR_GET_LSIZE(hdr), hdr);
+		} else {
+			uint32_t buffers = 0;
+
+			/*
+			 * Each individual buffer holds a unique reference,
+			 * thus we must remove each of these references one
+			 * at a time.
+			 */
+			for (arc_buf_t *buf = hdr->b_l1hdr.b_buf; buf != NULL;
+			    buf = buf->b_next) {
+				ASSERT3U(bufcnt, !=, 0);
+				buffers++;
+
+				/*
+				 * When the arc_buf_t is sharing the data
+				 * block with the hdr, the owner of the
+				 * reference belongs to the hdr. Only
+				 * add to the refcount if the arc_buf_t is
+				 * not shared.
+				 */
+				if (arc_buf_is_shared(buf))
+					continue;
+
+				(void) zfs_refcount_remove_many(
+				    &old_state->arcs_size, arc_buf_size(buf),
+				    buf);
+			}
+			ASSERT3U(bufcnt, ==, buffers);
+			ASSERT(hdr->b_l1hdr.b_pabd != NULL ||
+			    HDR_HAS_RABD(hdr));
+
+			if (hdr->b_l1hdr.b_pabd != NULL) {
+				(void) zfs_refcount_remove_many(
+				    &old_state->arcs_size, arc_hdr_size(hdr),
+				    hdr);
+			}
+
+			if (HDR_HAS_RABD(hdr)) {
+				(void) zfs_refcount_remove_many(
+				    &old_state->arcs_size, HDR_GET_PSIZE(hdr),
+				    hdr);
+			}
+		}
+	}
+
+	if (HDR_HAS_L1HDR(hdr))
+		hdr->b_l1hdr.b_state = new_state;
+
+	/*
+	 * L2 headers should never be on the L2 state list since they don't
+	 * have L1 headers allocated.
+	 */
+	ASSERT(multilist_is_empty(arc_l2c_only->arcs_list[ARC_BUFC_DATA]) &&
+	    multilist_is_empty(arc_l2c_only->arcs_list[ARC_BUFC_METADATA]));
+}
+
+void
+arc_space_consume(uint64_t space, arc_space_type_t type)
+{
+	ASSERT(type >= 0 && type < ARC_SPACE_NUMTYPES);
+
+	switch (type) {
+	default:
+		break;
+	case ARC_SPACE_DATA:
+		aggsum_add(&astat_data_size, space);
+		break;
+	case ARC_SPACE_META:
+		aggsum_add(&astat_metadata_size, space);
+		break;
+	case ARC_SPACE_BONUS:
+		aggsum_add(&astat_bonus_size, space);
+		break;
+	case ARC_SPACE_DNODE:
+		aggsum_add(&astat_dnode_size, space);
+		break;
+	case ARC_SPACE_DBUF:
+		aggsum_add(&astat_dbuf_size, space);
+		break;
+	case ARC_SPACE_HDRS:
+		aggsum_add(&astat_hdr_size, space);
+		break;
+	case ARC_SPACE_L2HDRS:
+		aggsum_add(&astat_l2_hdr_size, space);
+		break;
+	case ARC_SPACE_ABD_CHUNK_WASTE:
+		/*
+		 * Note: this includes space wasted by all scatter ABD's, not
+		 * just those allocated by the ARC.  But the vast majority of
+		 * scatter ABD's come from the ARC, because other users are
+		 * very short-lived.
+		 */
+		aggsum_add(&astat_abd_chunk_waste_size, space);
+		break;
+	}
+
+	if (type != ARC_SPACE_DATA && type != ARC_SPACE_ABD_CHUNK_WASTE)
+		aggsum_add(&arc_meta_used, space);
+
+	aggsum_add(&arc_size, space);
+}
+
+void
+arc_space_return(uint64_t space, arc_space_type_t type)
+{
+	ASSERT(type >= 0 && type < ARC_SPACE_NUMTYPES);
+
+	switch (type) {
+	default:
+		break;
+	case ARC_SPACE_DATA:
+		aggsum_add(&astat_data_size, -space);
+		break;
+	case ARC_SPACE_META:
+		aggsum_add(&astat_metadata_size, -space);
+		break;
+	case ARC_SPACE_BONUS:
+		aggsum_add(&astat_bonus_size, -space);
+		break;
+	case ARC_SPACE_DNODE:
+		aggsum_add(&astat_dnode_size, -space);
+		break;
+	case ARC_SPACE_DBUF:
+		aggsum_add(&astat_dbuf_size, -space);
+		break;
+	case ARC_SPACE_HDRS:
+		aggsum_add(&astat_hdr_size, -space);
+		break;
+	case ARC_SPACE_L2HDRS:
+		aggsum_add(&astat_l2_hdr_size, -space);
+		break;
+	case ARC_SPACE_ABD_CHUNK_WASTE:
+		aggsum_add(&astat_abd_chunk_waste_size, -space);
+		break;
+	}
+
+	if (type != ARC_SPACE_DATA && type != ARC_SPACE_ABD_CHUNK_WASTE) {
+		ASSERT(aggsum_compare(&arc_meta_used, space) >= 0);
+		/*
+		 * We use the upper bound here rather than the precise value
+		 * because the arc_meta_max value doesn't need to be
+		 * precise. It's only consumed by humans via arcstats.
+		 */
+		if (arc_meta_max < aggsum_upper_bound(&arc_meta_used))
+			arc_meta_max = aggsum_upper_bound(&arc_meta_used);
+		aggsum_add(&arc_meta_used, -space);
+	}
+
+	ASSERT(aggsum_compare(&arc_size, space) >= 0);
+	aggsum_add(&arc_size, -space);
+}
+
+/*
+ * Given a hdr and a buf, returns whether that buf can share its b_data buffer
+ * with the hdr's b_pabd.
+ */
+static boolean_t
+arc_can_share(arc_buf_hdr_t *hdr, arc_buf_t *buf)
+{
+	/*
+	 * The criteria for sharing a hdr's data are:
+	 * 1. the buffer is not encrypted
+	 * 2. the hdr's compression matches the buf's compression
+	 * 3. the hdr doesn't need to be byteswapped
+	 * 4. the hdr isn't already being shared
+	 * 5. the buf is either compressed or it is the last buf in the hdr list
+	 *
+	 * Criterion #5 maintains the invariant that shared uncompressed
+	 * bufs must be the final buf in the hdr's b_buf list. Reading this, you
+	 * might ask, "if a compressed buf is allocated first, won't that be the
+	 * last thing in the list?", but in that case it's impossible to create
+	 * a shared uncompressed buf anyway (because the hdr must be compressed
+	 * to have the compressed buf). You might also think that #3 is
+	 * sufficient to make this guarantee, however it's possible
+	 * (specifically in the rare L2ARC write race mentioned in
+	 * arc_buf_alloc_impl()) there will be an existing uncompressed buf that
+	 * is shareable, but wasn't at the time of its allocation. Rather than
+	 * allow a new shared uncompressed buf to be created and then shuffle
+	 * the list around to make it the last element, this simply disallows
+	 * sharing if the new buf isn't the first to be added.
+	 */
+	ASSERT3P(buf->b_hdr, ==, hdr);
+	boolean_t hdr_compressed =
+	    arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF;
+	boolean_t buf_compressed = ARC_BUF_COMPRESSED(buf) != 0;
+	return (!ARC_BUF_ENCRYPTED(buf) &&
+	    buf_compressed == hdr_compressed &&
+	    hdr->b_l1hdr.b_byteswap == DMU_BSWAP_NUMFUNCS &&
+	    !HDR_SHARED_DATA(hdr) &&
+	    (ARC_BUF_LAST(buf) || ARC_BUF_COMPRESSED(buf)));
+}
+
+/*
+ * Allocate a buf for this hdr. If you care about the data that's in the hdr,
+ * or if you want a compressed buffer, pass those flags in. Returns 0 if the
+ * copy was made successfully, or an error code otherwise.
+ */
+static int
+arc_buf_alloc_impl(arc_buf_hdr_t *hdr, spa_t *spa, const zbookmark_phys_t *zb,
+    void *tag, boolean_t encrypted, boolean_t compressed, boolean_t noauth,
+    boolean_t fill, arc_buf_t **ret)
+{
+	arc_buf_t *buf;
+	arc_fill_flags_t flags = ARC_FILL_LOCKED;
+
+	ASSERT(HDR_HAS_L1HDR(hdr));
+	ASSERT3U(HDR_GET_LSIZE(hdr), >, 0);
+	VERIFY(hdr->b_type == ARC_BUFC_DATA ||
+	    hdr->b_type == ARC_BUFC_METADATA);
+	ASSERT3P(ret, !=, NULL);
+	ASSERT3P(*ret, ==, NULL);
+	IMPLY(encrypted, compressed);
+
+	hdr->b_l1hdr.b_mru_hits = 0;
+	hdr->b_l1hdr.b_mru_ghost_hits = 0;
+	hdr->b_l1hdr.b_mfu_hits = 0;
+	hdr->b_l1hdr.b_mfu_ghost_hits = 0;
+	hdr->b_l1hdr.b_l2_hits = 0;
+
+	buf = *ret = kmem_cache_alloc(buf_cache, KM_PUSHPAGE);
+	buf->b_hdr = hdr;
+	buf->b_data = NULL;
+	buf->b_next = hdr->b_l1hdr.b_buf;
+	buf->b_flags = 0;
+
+	add_reference(hdr, tag);
+
+	/*
+	 * We're about to change the hdr's b_flags. We must either
+	 * hold the hash_lock or be undiscoverable.
+	 */
+	ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
+
+	/*
+	 * Only honor requests for compressed bufs if the hdr is actually
+	 * compressed. This must be overridden if the buffer is encrypted since
+	 * encrypted buffers cannot be decompressed.
+	 */
+	if (encrypted) {
+		buf->b_flags |= ARC_BUF_FLAG_COMPRESSED;
+		buf->b_flags |= ARC_BUF_FLAG_ENCRYPTED;
+		flags |= ARC_FILL_COMPRESSED | ARC_FILL_ENCRYPTED;
+	} else if (compressed &&
+	    arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF) {
+		buf->b_flags |= ARC_BUF_FLAG_COMPRESSED;
+		flags |= ARC_FILL_COMPRESSED;
+	}
+
+	if (noauth) {
+		ASSERT0(encrypted);
+		flags |= ARC_FILL_NOAUTH;
+	}
+
+	/*
+	 * If the hdr's data can be shared then we share the data buffer and
+	 * set the appropriate bit in the hdr's b_flags to indicate the hdr is
+	 * sharing it's b_pabd with the arc_buf_t. Otherwise, we allocate a new
+	 * buffer to store the buf's data.
+	 *
+	 * There are two additional restrictions here because we're sharing
+	 * hdr -> buf instead of the usual buf -> hdr. First, the hdr can't be
+	 * actively involved in an L2ARC write, because if this buf is used by
+	 * an arc_write() then the hdr's data buffer will be released when the
+	 * write completes, even though the L2ARC write might still be using it.
+	 * Second, the hdr's ABD must be linear so that the buf's user doesn't
+	 * need to be ABD-aware.  It must be allocated via
+	 * zio_[data_]buf_alloc(), not as a page, because we need to be able
+	 * to abd_release_ownership_of_buf(), which isn't allowed on "linear
+	 * page" buffers because the ABD code needs to handle freeing them
+	 * specially.
+	 */
+	boolean_t can_share = arc_can_share(hdr, buf) &&
+	    !HDR_L2_WRITING(hdr) &&
+	    hdr->b_l1hdr.b_pabd != NULL &&
+	    abd_is_linear(hdr->b_l1hdr.b_pabd) &&
+	    !abd_is_linear_page(hdr->b_l1hdr.b_pabd);
+
+	/* Set up b_data and sharing */
+	if (can_share) {
+		buf->b_data = abd_to_buf(hdr->b_l1hdr.b_pabd);
+		buf->b_flags |= ARC_BUF_FLAG_SHARED;
+		arc_hdr_set_flags(hdr, ARC_FLAG_SHARED_DATA);
+	} else {
+		buf->b_data =
+		    arc_get_data_buf(hdr, arc_buf_size(buf), buf);
+		ARCSTAT_INCR(arcstat_overhead_size, arc_buf_size(buf));
+	}
+	VERIFY3P(buf->b_data, !=, NULL);
+
+	hdr->b_l1hdr.b_buf = buf;
+	hdr->b_l1hdr.b_bufcnt += 1;
+	if (encrypted)
+		hdr->b_crypt_hdr.b_ebufcnt += 1;
+
+	/*
+	 * If the user wants the data from the hdr, we need to either copy or
+	 * decompress the data.
+	 */
+	if (fill) {
+		ASSERT3P(zb, !=, NULL);
+		return (arc_buf_fill(buf, spa, zb, flags));
+	}
+
+	return (0);
+}
+
+static char *arc_onloan_tag = "onloan";
+
+static inline void
+arc_loaned_bytes_update(int64_t delta)
+{
+	atomic_add_64(&arc_loaned_bytes, delta);
+
+	/* assert that it did not wrap around */
+	ASSERT3S(atomic_add_64_nv(&arc_loaned_bytes, 0), >=, 0);
+}
+
+/*
+ * Loan out an anonymous arc buffer. Loaned buffers are not counted as in
+ * flight data by arc_tempreserve_space() until they are "returned". Loaned
+ * buffers must be returned to the arc before they can be used by the DMU or
+ * freed.
+ */
+arc_buf_t *
+arc_loan_buf(spa_t *spa, boolean_t is_metadata, int size)
+{
+	arc_buf_t *buf = arc_alloc_buf(spa, arc_onloan_tag,
+	    is_metadata ? ARC_BUFC_METADATA : ARC_BUFC_DATA, size);
+
+	arc_loaned_bytes_update(arc_buf_size(buf));
+
+	return (buf);
+}
+
+arc_buf_t *
+arc_loan_compressed_buf(spa_t *spa, uint64_t psize, uint64_t lsize,
+    enum zio_compress compression_type, uint8_t complevel)
+{
+	arc_buf_t *buf = arc_alloc_compressed_buf(spa, arc_onloan_tag,
+	    psize, lsize, compression_type, complevel);
+
+	arc_loaned_bytes_update(arc_buf_size(buf));
+
+	return (buf);
+}
+
+arc_buf_t *
+arc_loan_raw_buf(spa_t *spa, uint64_t dsobj, boolean_t byteorder,
+    const uint8_t *salt, const uint8_t *iv, const uint8_t *mac,
+    dmu_object_type_t ot, uint64_t psize, uint64_t lsize,
+    enum zio_compress compression_type, uint8_t complevel)
+{
+	arc_buf_t *buf = arc_alloc_raw_buf(spa, arc_onloan_tag, dsobj,
+	    byteorder, salt, iv, mac, ot, psize, lsize, compression_type,
+	    complevel);
+
+	atomic_add_64(&arc_loaned_bytes, psize);
+	return (buf);
+}
+
+
+/*
+ * Return a loaned arc buffer to the arc.
+ */
+void
+arc_return_buf(arc_buf_t *buf, void *tag)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+
+	ASSERT3P(buf->b_data, !=, NULL);
+	ASSERT(HDR_HAS_L1HDR(hdr));
+	(void) zfs_refcount_add(&hdr->b_l1hdr.b_refcnt, tag);
+	(void) zfs_refcount_remove(&hdr->b_l1hdr.b_refcnt, arc_onloan_tag);
+
+	arc_loaned_bytes_update(-arc_buf_size(buf));
+}
+
+/* Detach an arc_buf from a dbuf (tag) */
+void
+arc_loan_inuse_buf(arc_buf_t *buf, void *tag)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+
+	ASSERT3P(buf->b_data, !=, NULL);
+	ASSERT(HDR_HAS_L1HDR(hdr));
+	(void) zfs_refcount_add(&hdr->b_l1hdr.b_refcnt, arc_onloan_tag);
+	(void) zfs_refcount_remove(&hdr->b_l1hdr.b_refcnt, tag);
+
+	arc_loaned_bytes_update(arc_buf_size(buf));
+}
+
+static void
+l2arc_free_abd_on_write(abd_t *abd, size_t size, arc_buf_contents_t type)
+{
+	l2arc_data_free_t *df = kmem_alloc(sizeof (*df), KM_SLEEP);
+
+	df->l2df_abd = abd;
+	df->l2df_size = size;
+	df->l2df_type = type;
+	mutex_enter(&l2arc_free_on_write_mtx);
+	list_insert_head(l2arc_free_on_write, df);
+	mutex_exit(&l2arc_free_on_write_mtx);
+}
+
+static void
+arc_hdr_free_on_write(arc_buf_hdr_t *hdr, boolean_t free_rdata)
+{
+	arc_state_t *state = hdr->b_l1hdr.b_state;
+	arc_buf_contents_t type = arc_buf_type(hdr);
+	uint64_t size = (free_rdata) ? HDR_GET_PSIZE(hdr) : arc_hdr_size(hdr);
+
+	/* protected by hash lock, if in the hash table */
+	if (multilist_link_active(&hdr->b_l1hdr.b_arc_node)) {
+		ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
+		ASSERT(state != arc_anon && state != arc_l2c_only);
+
+		(void) zfs_refcount_remove_many(&state->arcs_esize[type],
+		    size, hdr);
+	}
+	(void) zfs_refcount_remove_many(&state->arcs_size, size, hdr);
+	if (type == ARC_BUFC_METADATA) {
+		arc_space_return(size, ARC_SPACE_META);
+	} else {
+		ASSERT(type == ARC_BUFC_DATA);
+		arc_space_return(size, ARC_SPACE_DATA);
+	}
+
+	if (free_rdata) {
+		l2arc_free_abd_on_write(hdr->b_crypt_hdr.b_rabd, size, type);
+	} else {
+		l2arc_free_abd_on_write(hdr->b_l1hdr.b_pabd, size, type);
+	}
+}
+
+/*
+ * Share the arc_buf_t's data with the hdr. Whenever we are sharing the
+ * data buffer, we transfer the refcount ownership to the hdr and update
+ * the appropriate kstats.
+ */
+static void
+arc_share_buf(arc_buf_hdr_t *hdr, arc_buf_t *buf)
+{
+	ASSERT(arc_can_share(hdr, buf));
+	ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
+	ASSERT(!ARC_BUF_ENCRYPTED(buf));
+	ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
+
+	/*
+	 * Start sharing the data buffer. We transfer the
+	 * refcount ownership to the hdr since it always owns
+	 * the refcount whenever an arc_buf_t is shared.
+	 */
+	zfs_refcount_transfer_ownership_many(&hdr->b_l1hdr.b_state->arcs_size,
+	    arc_hdr_size(hdr), buf, hdr);
+	hdr->b_l1hdr.b_pabd = abd_get_from_buf(buf->b_data, arc_buf_size(buf));
+	abd_take_ownership_of_buf(hdr->b_l1hdr.b_pabd,
+	    HDR_ISTYPE_METADATA(hdr));
+	arc_hdr_set_flags(hdr, ARC_FLAG_SHARED_DATA);
+	buf->b_flags |= ARC_BUF_FLAG_SHARED;
+
+	/*
+	 * Since we've transferred ownership to the hdr we need
+	 * to increment its compressed and uncompressed kstats and
+	 * decrement the overhead size.
+	 */
+	ARCSTAT_INCR(arcstat_compressed_size, arc_hdr_size(hdr));
+	ARCSTAT_INCR(arcstat_uncompressed_size, HDR_GET_LSIZE(hdr));
+	ARCSTAT_INCR(arcstat_overhead_size, -arc_buf_size(buf));
+}
+
+static void
+arc_unshare_buf(arc_buf_hdr_t *hdr, arc_buf_t *buf)
+{
+	ASSERT(arc_buf_is_shared(buf));
+	ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
+	ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
+
+	/*
+	 * We are no longer sharing this buffer so we need
+	 * to transfer its ownership to the rightful owner.
+	 */
+	zfs_refcount_transfer_ownership_many(&hdr->b_l1hdr.b_state->arcs_size,
+	    arc_hdr_size(hdr), hdr, buf);
+	arc_hdr_clear_flags(hdr, ARC_FLAG_SHARED_DATA);
+	abd_release_ownership_of_buf(hdr->b_l1hdr.b_pabd);
+	abd_put(hdr->b_l1hdr.b_pabd);
+	hdr->b_l1hdr.b_pabd = NULL;
+	buf->b_flags &= ~ARC_BUF_FLAG_SHARED;
+
+	/*
+	 * Since the buffer is no longer shared between
+	 * the arc buf and the hdr, count it as overhead.
+	 */
+	ARCSTAT_INCR(arcstat_compressed_size, -arc_hdr_size(hdr));
+	ARCSTAT_INCR(arcstat_uncompressed_size, -HDR_GET_LSIZE(hdr));
+	ARCSTAT_INCR(arcstat_overhead_size, arc_buf_size(buf));
+}
+
+/*
+ * Remove an arc_buf_t from the hdr's buf list and return the last
+ * arc_buf_t on the list. If no buffers remain on the list then return
+ * NULL.
+ */
+static arc_buf_t *
+arc_buf_remove(arc_buf_hdr_t *hdr, arc_buf_t *buf)
+{
+	ASSERT(HDR_HAS_L1HDR(hdr));
+	ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
+
+	arc_buf_t **bufp = &hdr->b_l1hdr.b_buf;
+	arc_buf_t *lastbuf = NULL;
+
+	/*
+	 * Remove the buf from the hdr list and locate the last
+	 * remaining buffer on the list.
+	 */
+	while (*bufp != NULL) {
+		if (*bufp == buf)
+			*bufp = buf->b_next;
+
+		/*
+		 * If we've removed a buffer in the middle of
+		 * the list then update the lastbuf and update
+		 * bufp.
+		 */
+		if (*bufp != NULL) {
+			lastbuf = *bufp;
+			bufp = &(*bufp)->b_next;
+		}
+	}
+	buf->b_next = NULL;
+	ASSERT3P(lastbuf, !=, buf);
+	IMPLY(hdr->b_l1hdr.b_bufcnt > 0, lastbuf != NULL);
+	IMPLY(hdr->b_l1hdr.b_bufcnt > 0, hdr->b_l1hdr.b_buf != NULL);
+	IMPLY(lastbuf != NULL, ARC_BUF_LAST(lastbuf));
+
+	return (lastbuf);
+}
+
+/*
+ * Free up buf->b_data and pull the arc_buf_t off of the arc_buf_hdr_t's
+ * list and free it.
+ */
+static void
+arc_buf_destroy_impl(arc_buf_t *buf)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+
+	/*
+	 * Free up the data associated with the buf but only if we're not
+	 * sharing this with the hdr. If we are sharing it with the hdr, the
+	 * hdr is responsible for doing the free.
+	 */
+	if (buf->b_data != NULL) {
+		/*
+		 * We're about to change the hdr's b_flags. We must either
+		 * hold the hash_lock or be undiscoverable.
+		 */
+		ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
+
+		arc_cksum_verify(buf);
+		arc_buf_unwatch(buf);
+
+		if (arc_buf_is_shared(buf)) {
+			arc_hdr_clear_flags(hdr, ARC_FLAG_SHARED_DATA);
+		} else {
+			uint64_t size = arc_buf_size(buf);
+			arc_free_data_buf(hdr, buf->b_data, size, buf);
+			ARCSTAT_INCR(arcstat_overhead_size, -size);
+		}
+		buf->b_data = NULL;
+
+		ASSERT(hdr->b_l1hdr.b_bufcnt > 0);
+		hdr->b_l1hdr.b_bufcnt -= 1;
+
+		if (ARC_BUF_ENCRYPTED(buf)) {
+			hdr->b_crypt_hdr.b_ebufcnt -= 1;
+
+			/*
+			 * If we have no more encrypted buffers and we've
+			 * already gotten a copy of the decrypted data we can
+			 * free b_rabd to save some space.
+			 */
+			if (hdr->b_crypt_hdr.b_ebufcnt == 0 &&
+			    HDR_HAS_RABD(hdr) && hdr->b_l1hdr.b_pabd != NULL &&
+			    !HDR_IO_IN_PROGRESS(hdr)) {
+				arc_hdr_free_abd(hdr, B_TRUE);
+			}
+		}
+	}
+
+	arc_buf_t *lastbuf = arc_buf_remove(hdr, buf);
+
+	if (ARC_BUF_SHARED(buf) && !ARC_BUF_COMPRESSED(buf)) {
+		/*
+		 * If the current arc_buf_t is sharing its data buffer with the
+		 * hdr, then reassign the hdr's b_pabd to share it with the new
+		 * buffer at the end of the list. The shared buffer is always
+		 * the last one on the hdr's buffer list.
+		 *
+		 * There is an equivalent case for compressed bufs, but since
+		 * they aren't guaranteed to be the last buf in the list and
+		 * that is an exceedingly rare case, we just allow that space be
+		 * wasted temporarily. We must also be careful not to share
+		 * encrypted buffers, since they cannot be shared.
+		 */
+		if (lastbuf != NULL && !ARC_BUF_ENCRYPTED(lastbuf)) {
+			/* Only one buf can be shared at once */
+			VERIFY(!arc_buf_is_shared(lastbuf));
+			/* hdr is uncompressed so can't have compressed buf */
+			VERIFY(!ARC_BUF_COMPRESSED(lastbuf));
+
+			ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
+			arc_hdr_free_abd(hdr, B_FALSE);
+
+			/*
+			 * We must setup a new shared block between the
+			 * last buffer and the hdr. The data would have
+			 * been allocated by the arc buf so we need to transfer
+			 * ownership to the hdr since it's now being shared.
+			 */
+			arc_share_buf(hdr, lastbuf);
+		}
+	} else if (HDR_SHARED_DATA(hdr)) {
+		/*
+		 * Uncompressed shared buffers are always at the end
+		 * of the list. Compressed buffers don't have the
+		 * same requirements. This makes it hard to
+		 * simply assert that the lastbuf is shared so
+		 * we rely on the hdr's compression flags to determine
+		 * if we have a compressed, shared buffer.
+		 */
+		ASSERT3P(lastbuf, !=, NULL);
+		ASSERT(arc_buf_is_shared(lastbuf) ||
+		    arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF);
+	}
+
+	/*
+	 * Free the checksum if we're removing the last uncompressed buf from
+	 * this hdr.
+	 */
+	if (!arc_hdr_has_uncompressed_buf(hdr)) {
+		arc_cksum_free(hdr);
+	}
+
+	/* clean up the buf */
+	buf->b_hdr = NULL;
+	kmem_cache_free(buf_cache, buf);
+}
+
+static void
+arc_hdr_alloc_abd(arc_buf_hdr_t *hdr, int alloc_flags)
+{
+	uint64_t size;
+	boolean_t alloc_rdata = ((alloc_flags & ARC_HDR_ALLOC_RDATA) != 0);
+	boolean_t do_adapt = ((alloc_flags & ARC_HDR_DO_ADAPT) != 0);
+
+	ASSERT3U(HDR_GET_LSIZE(hdr), >, 0);
+	ASSERT(HDR_HAS_L1HDR(hdr));
+	ASSERT(!HDR_SHARED_DATA(hdr) || alloc_rdata);
+	IMPLY(alloc_rdata, HDR_PROTECTED(hdr));
+
+	if (alloc_rdata) {
+		size = HDR_GET_PSIZE(hdr);
+		ASSERT3P(hdr->b_crypt_hdr.b_rabd, ==, NULL);
+		hdr->b_crypt_hdr.b_rabd = arc_get_data_abd(hdr, size, hdr,
+		    do_adapt);
+		ASSERT3P(hdr->b_crypt_hdr.b_rabd, !=, NULL);
+		ARCSTAT_INCR(arcstat_raw_size, size);
+	} else {
+		size = arc_hdr_size(hdr);
+		ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
+		hdr->b_l1hdr.b_pabd = arc_get_data_abd(hdr, size, hdr,
+		    do_adapt);
+		ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
+	}
+
+	ARCSTAT_INCR(arcstat_compressed_size, size);
+	ARCSTAT_INCR(arcstat_uncompressed_size, HDR_GET_LSIZE(hdr));
+}
+
+static void
+arc_hdr_free_abd(arc_buf_hdr_t *hdr, boolean_t free_rdata)
+{
+	uint64_t size = (free_rdata) ? HDR_GET_PSIZE(hdr) : arc_hdr_size(hdr);
+
+	ASSERT(HDR_HAS_L1HDR(hdr));
+	ASSERT(hdr->b_l1hdr.b_pabd != NULL || HDR_HAS_RABD(hdr));
+	IMPLY(free_rdata, HDR_HAS_RABD(hdr));
+
+	/*
+	 * If the hdr is currently being written to the l2arc then
+	 * we defer freeing the data by adding it to the l2arc_free_on_write
+	 * list. The l2arc will free the data once it's finished
+	 * writing it to the l2arc device.
+	 */
+	if (HDR_L2_WRITING(hdr)) {
+		arc_hdr_free_on_write(hdr, free_rdata);
+		ARCSTAT_BUMP(arcstat_l2_free_on_write);
+	} else if (free_rdata) {
+		arc_free_data_abd(hdr, hdr->b_crypt_hdr.b_rabd, size, hdr);
+	} else {
+		arc_free_data_abd(hdr, hdr->b_l1hdr.b_pabd, size, hdr);
+	}
+
+	if (free_rdata) {
+		hdr->b_crypt_hdr.b_rabd = NULL;
+		ARCSTAT_INCR(arcstat_raw_size, -size);
+	} else {
+		hdr->b_l1hdr.b_pabd = NULL;
+	}
+
+	if (hdr->b_l1hdr.b_pabd == NULL && !HDR_HAS_RABD(hdr))
+		hdr->b_l1hdr.b_byteswap = DMU_BSWAP_NUMFUNCS;
+
+	ARCSTAT_INCR(arcstat_compressed_size, -size);
+	ARCSTAT_INCR(arcstat_uncompressed_size, -HDR_GET_LSIZE(hdr));
+}
+
+static arc_buf_hdr_t *
+arc_hdr_alloc(uint64_t spa, int32_t psize, int32_t lsize,
+    boolean_t protected, enum zio_compress compression_type, uint8_t complevel,
+    arc_buf_contents_t type, boolean_t alloc_rdata)
+{
+	arc_buf_hdr_t *hdr;
+	int flags = ARC_HDR_DO_ADAPT;
+
+	VERIFY(type == ARC_BUFC_DATA || type == ARC_BUFC_METADATA);
+	if (protected) {
+		hdr = kmem_cache_alloc(hdr_full_crypt_cache, KM_PUSHPAGE);
+	} else {
+		hdr = kmem_cache_alloc(hdr_full_cache, KM_PUSHPAGE);
+	}
+	flags |= alloc_rdata ? ARC_HDR_ALLOC_RDATA : 0;
+
+	ASSERT(HDR_EMPTY(hdr));
+	ASSERT3P(hdr->b_l1hdr.b_freeze_cksum, ==, NULL);
+	HDR_SET_PSIZE(hdr, psize);
+	HDR_SET_LSIZE(hdr, lsize);
+	hdr->b_spa = spa;
+	hdr->b_type = type;
+	hdr->b_flags = 0;
+	arc_hdr_set_flags(hdr, arc_bufc_to_flags(type) | ARC_FLAG_HAS_L1HDR);
+	arc_hdr_set_compress(hdr, compression_type);
+	hdr->b_complevel = complevel;
+	if (protected)
+		arc_hdr_set_flags(hdr, ARC_FLAG_PROTECTED);
+
+	hdr->b_l1hdr.b_state = arc_anon;
+	hdr->b_l1hdr.b_arc_access = 0;
+	hdr->b_l1hdr.b_bufcnt = 0;
+	hdr->b_l1hdr.b_buf = NULL;
+
+	/*
+	 * Allocate the hdr's buffer. This will contain either
+	 * the compressed or uncompressed data depending on the block
+	 * it references and compressed arc enablement.
+	 */
+	arc_hdr_alloc_abd(hdr, flags);
+	ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
+
+	return (hdr);
+}
+
+/*
+ * Transition between the two allocation states for the arc_buf_hdr struct.
+ * The arc_buf_hdr struct can be allocated with (hdr_full_cache) or without
+ * (hdr_l2only_cache) the fields necessary for the L1 cache - the smaller
+ * version is used when a cache buffer is only in the L2ARC in order to reduce
+ * memory usage.
+ */
+static arc_buf_hdr_t *
+arc_hdr_realloc(arc_buf_hdr_t *hdr, kmem_cache_t *old, kmem_cache_t *new)
+{
+	ASSERT(HDR_HAS_L2HDR(hdr));
+
+	arc_buf_hdr_t *nhdr;
+	l2arc_dev_t *dev = hdr->b_l2hdr.b_dev;
+
+	ASSERT((old == hdr_full_cache && new == hdr_l2only_cache) ||
+	    (old == hdr_l2only_cache && new == hdr_full_cache));
+
+	/*
+	 * if the caller wanted a new full header and the header is to be
+	 * encrypted we will actually allocate the header from the full crypt
+	 * cache instead. The same applies to freeing from the old cache.
+	 */
+	if (HDR_PROTECTED(hdr) && new == hdr_full_cache)
+		new = hdr_full_crypt_cache;
+	if (HDR_PROTECTED(hdr) && old == hdr_full_cache)
+		old = hdr_full_crypt_cache;
+
+	nhdr = kmem_cache_alloc(new, KM_PUSHPAGE);
+
+	ASSERT(MUTEX_HELD(HDR_LOCK(hdr)));
+	buf_hash_remove(hdr);
+
+	bcopy(hdr, nhdr, HDR_L2ONLY_SIZE);
+
+	if (new == hdr_full_cache || new == hdr_full_crypt_cache) {
+		arc_hdr_set_flags(nhdr, ARC_FLAG_HAS_L1HDR);
+		/*
+		 * arc_access and arc_change_state need to be aware that a
+		 * header has just come out of L2ARC, so we set its state to
+		 * l2c_only even though it's about to change.
+		 */
+		nhdr->b_l1hdr.b_state = arc_l2c_only;
+
+		/* Verify previous threads set to NULL before freeing */
+		ASSERT3P(nhdr->b_l1hdr.b_pabd, ==, NULL);
+		ASSERT(!HDR_HAS_RABD(hdr));
+	} else {
+		ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
+		ASSERT0(hdr->b_l1hdr.b_bufcnt);
+		ASSERT3P(hdr->b_l1hdr.b_freeze_cksum, ==, NULL);
+
+		/*
+		 * If we've reached here, We must have been called from
+		 * arc_evict_hdr(), as such we should have already been
+		 * removed from any ghost list we were previously on
+		 * (which protects us from racing with arc_evict_state),
+		 * thus no locking is needed during this check.
+		 */
+		ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));
+
+		/*
+		 * A buffer must not be moved into the arc_l2c_only
+		 * state if it's not finished being written out to the
+		 * l2arc device. Otherwise, the b_l1hdr.b_pabd field
+		 * might try to be accessed, even though it was removed.
+		 */
+		VERIFY(!HDR_L2_WRITING(hdr));
+		VERIFY3P(hdr->b_l1hdr.b_pabd, ==, NULL);
+		ASSERT(!HDR_HAS_RABD(hdr));
+
+		arc_hdr_clear_flags(nhdr, ARC_FLAG_HAS_L1HDR);
+	}
+	/*
+	 * The header has been reallocated so we need to re-insert it into any
+	 * lists it was on.
+	 */
+	(void) buf_hash_insert(nhdr, NULL);
+
+	ASSERT(list_link_active(&hdr->b_l2hdr.b_l2node));
+
+	mutex_enter(&dev->l2ad_mtx);
+
+	/*
+	 * We must place the realloc'ed header back into the list at
+	 * the same spot. Otherwise, if it's placed earlier in the list,
+	 * l2arc_write_buffers() could find it during the function's
+	 * write phase, and try to write it out to the l2arc.
+	 */
+	list_insert_after(&dev->l2ad_buflist, hdr, nhdr);
+	list_remove(&dev->l2ad_buflist, hdr);
+
+	mutex_exit(&dev->l2ad_mtx);
+
+	/*
+	 * Since we're using the pointer address as the tag when
+	 * incrementing and decrementing the l2ad_alloc refcount, we
+	 * must remove the old pointer (that we're about to destroy) and
+	 * add the new pointer to the refcount. Otherwise we'd remove
+	 * the wrong pointer address when calling arc_hdr_destroy() later.
+	 */
+
+	(void) zfs_refcount_remove_many(&dev->l2ad_alloc,
+	    arc_hdr_size(hdr), hdr);
+	(void) zfs_refcount_add_many(&dev->l2ad_alloc,
+	    arc_hdr_size(nhdr), nhdr);
+
+	buf_discard_identity(hdr);
+	kmem_cache_free(old, hdr);
+
+	return (nhdr);
+}
+
+/*
+ * This function allows an L1 header to be reallocated as a crypt
+ * header and vice versa. If we are going to a crypt header, the
+ * new fields will be zeroed out.
+ */
+static arc_buf_hdr_t *
+arc_hdr_realloc_crypt(arc_buf_hdr_t *hdr, boolean_t need_crypt)
+{
+	arc_buf_hdr_t *nhdr;
+	arc_buf_t *buf;
+	kmem_cache_t *ncache, *ocache;
+	unsigned nsize, osize;
+
+	/*
+	 * This function requires that hdr is in the arc_anon state.
+	 * Therefore it won't have any L2ARC data for us to worry
+	 * about copying.
+	 */
+	ASSERT(HDR_HAS_L1HDR(hdr));
+	ASSERT(!HDR_HAS_L2HDR(hdr));
+	ASSERT3U(!!HDR_PROTECTED(hdr), !=, need_crypt);
+	ASSERT3P(hdr->b_l1hdr.b_state, ==, arc_anon);
+	ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));
+	ASSERT(!list_link_active(&hdr->b_l2hdr.b_l2node));
+	ASSERT3P(hdr->b_hash_next, ==, NULL);
+
+	if (need_crypt) {
+		ncache = hdr_full_crypt_cache;
+		nsize = sizeof (hdr->b_crypt_hdr);
+		ocache = hdr_full_cache;
+		osize = HDR_FULL_SIZE;
+	} else {
+		ncache = hdr_full_cache;
+		nsize = HDR_FULL_SIZE;
+		ocache = hdr_full_crypt_cache;
+		osize = sizeof (hdr->b_crypt_hdr);
+	}
+
+	nhdr = kmem_cache_alloc(ncache, KM_PUSHPAGE);
+
+	/*
+	 * Copy all members that aren't locks or condvars to the new header.
+	 * No lists are pointing to us (as we asserted above), so we don't
+	 * need to worry about the list nodes.
+	 */
+	nhdr->b_dva = hdr->b_dva;
+	nhdr->b_birth = hdr->b_birth;
+	nhdr->b_type = hdr->b_type;
+	nhdr->b_flags = hdr->b_flags;
+	nhdr->b_psize = hdr->b_psize;
+	nhdr->b_lsize = hdr->b_lsize;
+	nhdr->b_spa = hdr->b_spa;
+	nhdr->b_l1hdr.b_freeze_cksum = hdr->b_l1hdr.b_freeze_cksum;
+	nhdr->b_l1hdr.b_bufcnt = hdr->b_l1hdr.b_bufcnt;
+	nhdr->b_l1hdr.b_byteswap = hdr->b_l1hdr.b_byteswap;
+	nhdr->b_l1hdr.b_state = hdr->b_l1hdr.b_state;
+	nhdr->b_l1hdr.b_arc_access = hdr->b_l1hdr.b_arc_access;
+	nhdr->b_l1hdr.b_mru_hits = hdr->b_l1hdr.b_mru_hits;
+	nhdr->b_l1hdr.b_mru_ghost_hits = hdr->b_l1hdr.b_mru_ghost_hits;
+	nhdr->b_l1hdr.b_mfu_hits = hdr->b_l1hdr.b_mfu_hits;
+	nhdr->b_l1hdr.b_mfu_ghost_hits = hdr->b_l1hdr.b_mfu_ghost_hits;
+	nhdr->b_l1hdr.b_l2_hits = hdr->b_l1hdr.b_l2_hits;
+	nhdr->b_l1hdr.b_acb = hdr->b_l1hdr.b_acb;
+	nhdr->b_l1hdr.b_pabd = hdr->b_l1hdr.b_pabd;
+
+	/*
+	 * This zfs_refcount_add() exists only to ensure that the individual
+	 * arc buffers always point to a header that is referenced, avoiding
+	 * a small race condition that could trigger ASSERTs.
+	 */
+	(void) zfs_refcount_add(&nhdr->b_l1hdr.b_refcnt, FTAG);
+	nhdr->b_l1hdr.b_buf = hdr->b_l1hdr.b_buf;
+	for (buf = nhdr->b_l1hdr.b_buf; buf != NULL; buf = buf->b_next) {
+		mutex_enter(&buf->b_evict_lock);
+		buf->b_hdr = nhdr;
+		mutex_exit(&buf->b_evict_lock);
+	}
+
+	zfs_refcount_transfer(&nhdr->b_l1hdr.b_refcnt, &hdr->b_l1hdr.b_refcnt);
+	(void) zfs_refcount_remove(&nhdr->b_l1hdr.b_refcnt, FTAG);
+	ASSERT0(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt));
+
+	if (need_crypt) {
+		arc_hdr_set_flags(nhdr, ARC_FLAG_PROTECTED);
+	} else {
+		arc_hdr_clear_flags(nhdr, ARC_FLAG_PROTECTED);
+	}
+
+	/* unset all members of the original hdr */
+	bzero(&hdr->b_dva, sizeof (dva_t));
+	hdr->b_birth = 0;
+	hdr->b_type = ARC_BUFC_INVALID;
+	hdr->b_flags = 0;
+	hdr->b_psize = 0;
+	hdr->b_lsize = 0;
+	hdr->b_spa = 0;
+	hdr->b_l1hdr.b_freeze_cksum = NULL;
+	hdr->b_l1hdr.b_buf = NULL;
+	hdr->b_l1hdr.b_bufcnt = 0;
+	hdr->b_l1hdr.b_byteswap = 0;
+	hdr->b_l1hdr.b_state = NULL;
+	hdr->b_l1hdr.b_arc_access = 0;
+	hdr->b_l1hdr.b_mru_hits = 0;
+	hdr->b_l1hdr.b_mru_ghost_hits = 0;
+	hdr->b_l1hdr.b_mfu_hits = 0;
+	hdr->b_l1hdr.b_mfu_ghost_hits = 0;
+	hdr->b_l1hdr.b_l2_hits = 0;
+	hdr->b_l1hdr.b_acb = NULL;
+	hdr->b_l1hdr.b_pabd = NULL;
+
+	if (ocache == hdr_full_crypt_cache) {
+		ASSERT(!HDR_HAS_RABD(hdr));
+		hdr->b_crypt_hdr.b_ot = DMU_OT_NONE;
+		hdr->b_crypt_hdr.b_ebufcnt = 0;
+		hdr->b_crypt_hdr.b_dsobj = 0;
+		bzero(hdr->b_crypt_hdr.b_salt, ZIO_DATA_SALT_LEN);
+		bzero(hdr->b_crypt_hdr.b_iv, ZIO_DATA_IV_LEN);
+		bzero(hdr->b_crypt_hdr.b_mac, ZIO_DATA_MAC_LEN);
+	}
+
+	buf_discard_identity(hdr);
+	kmem_cache_free(ocache, hdr);
+
+	return (nhdr);
+}
+
+/*
+ * This function is used by the send / receive code to convert a newly
+ * allocated arc_buf_t to one that is suitable for a raw encrypted write. It
+ * is also used to allow the root objset block to be updated without altering
+ * its embedded MACs. Both block types will always be uncompressed so we do not
+ * have to worry about compression type or psize.
+ */
+void
+arc_convert_to_raw(arc_buf_t *buf, uint64_t dsobj, boolean_t byteorder,
+    dmu_object_type_t ot, const uint8_t *salt, const uint8_t *iv,
+    const uint8_t *mac)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+
+	ASSERT(ot == DMU_OT_DNODE || ot == DMU_OT_OBJSET);
+	ASSERT(HDR_HAS_L1HDR(hdr));
+	ASSERT3P(hdr->b_l1hdr.b_state, ==, arc_anon);
+
+	buf->b_flags |= (ARC_BUF_FLAG_COMPRESSED | ARC_BUF_FLAG_ENCRYPTED);
+	if (!HDR_PROTECTED(hdr))
+		hdr = arc_hdr_realloc_crypt(hdr, B_TRUE);
+	hdr->b_crypt_hdr.b_dsobj = dsobj;
+	hdr->b_crypt_hdr.b_ot = ot;
+	hdr->b_l1hdr.b_byteswap = (byteorder == ZFS_HOST_BYTEORDER) ?
+	    DMU_BSWAP_NUMFUNCS : DMU_OT_BYTESWAP(ot);
+	if (!arc_hdr_has_uncompressed_buf(hdr))
+		arc_cksum_free(hdr);
+
+	if (salt != NULL)
+		bcopy(salt, hdr->b_crypt_hdr.b_salt, ZIO_DATA_SALT_LEN);
+	if (iv != NULL)
+		bcopy(iv, hdr->b_crypt_hdr.b_iv, ZIO_DATA_IV_LEN);
+	if (mac != NULL)
+		bcopy(mac, hdr->b_crypt_hdr.b_mac, ZIO_DATA_MAC_LEN);
+}
+
+/*
+ * Allocate a new arc_buf_hdr_t and arc_buf_t and return the buf to the caller.
+ * The buf is returned thawed since we expect the consumer to modify it.
+ */
+arc_buf_t *
+arc_alloc_buf(spa_t *spa, void *tag, arc_buf_contents_t type, int32_t size)
+{
+	arc_buf_hdr_t *hdr = arc_hdr_alloc(spa_load_guid(spa), size, size,
+	    B_FALSE, ZIO_COMPRESS_OFF, 0, type, B_FALSE);
+
+	arc_buf_t *buf = NULL;
+	VERIFY0(arc_buf_alloc_impl(hdr, spa, NULL, tag, B_FALSE, B_FALSE,
+	    B_FALSE, B_FALSE, &buf));
+	arc_buf_thaw(buf);
+
+	return (buf);
+}
+
+/*
+ * Allocate a compressed buf in the same manner as arc_alloc_buf. Don't use this
+ * for bufs containing metadata.
+ */
+arc_buf_t *
+arc_alloc_compressed_buf(spa_t *spa, void *tag, uint64_t psize, uint64_t lsize,
+    enum zio_compress compression_type, uint8_t complevel)
+{
+	ASSERT3U(lsize, >, 0);
+	ASSERT3U(lsize, >=, psize);
+	ASSERT3U(compression_type, >, ZIO_COMPRESS_OFF);
+	ASSERT3U(compression_type, <, ZIO_COMPRESS_FUNCTIONS);
+
+	arc_buf_hdr_t *hdr = arc_hdr_alloc(spa_load_guid(spa), psize, lsize,
+	    B_FALSE, compression_type, complevel, ARC_BUFC_DATA, B_FALSE);
+
+	arc_buf_t *buf = NULL;
+	VERIFY0(arc_buf_alloc_impl(hdr, spa, NULL, tag, B_FALSE,
+	    B_TRUE, B_FALSE, B_FALSE, &buf));
+	arc_buf_thaw(buf);
+	ASSERT3P(hdr->b_l1hdr.b_freeze_cksum, ==, NULL);
+
+	if (!arc_buf_is_shared(buf)) {
+		/*
+		 * To ensure that the hdr has the correct data in it if we call
+		 * arc_untransform() on this buf before it's been written to
+		 * disk, it's easiest if we just set up sharing between the
+		 * buf and the hdr.
+		 */
+		arc_hdr_free_abd(hdr, B_FALSE);
+		arc_share_buf(hdr, buf);
+	}
+
+	return (buf);
+}
+
+arc_buf_t *
+arc_alloc_raw_buf(spa_t *spa, void *tag, uint64_t dsobj, boolean_t byteorder,
+    const uint8_t *salt, const uint8_t *iv, const uint8_t *mac,
+    dmu_object_type_t ot, uint64_t psize, uint64_t lsize,
+    enum zio_compress compression_type, uint8_t complevel)
+{
+	arc_buf_hdr_t *hdr;
+	arc_buf_t *buf;
+	arc_buf_contents_t type = DMU_OT_IS_METADATA(ot) ?
+	    ARC_BUFC_METADATA : ARC_BUFC_DATA;
+
+	ASSERT3U(lsize, >, 0);
+	ASSERT3U(lsize, >=, psize);
+	ASSERT3U(compression_type, >=, ZIO_COMPRESS_OFF);
+	ASSERT3U(compression_type, <, ZIO_COMPRESS_FUNCTIONS);
+
+	hdr = arc_hdr_alloc(spa_load_guid(spa), psize, lsize, B_TRUE,
+	    compression_type, complevel, type, B_TRUE);
+
+	hdr->b_crypt_hdr.b_dsobj = dsobj;
+	hdr->b_crypt_hdr.b_ot = ot;
+	hdr->b_l1hdr.b_byteswap = (byteorder == ZFS_HOST_BYTEORDER) ?
+	    DMU_BSWAP_NUMFUNCS : DMU_OT_BYTESWAP(ot);
+	bcopy(salt, hdr->b_crypt_hdr.b_salt, ZIO_DATA_SALT_LEN);
+	bcopy(iv, hdr->b_crypt_hdr.b_iv, ZIO_DATA_IV_LEN);
+	bcopy(mac, hdr->b_crypt_hdr.b_mac, ZIO_DATA_MAC_LEN);
+
+	/*
+	 * This buffer will be considered encrypted even if the ot is not an
+	 * encrypted type. It will become authenticated instead in
+	 * arc_write_ready().
+	 */
+	buf = NULL;
+	VERIFY0(arc_buf_alloc_impl(hdr, spa, NULL, tag, B_TRUE, B_TRUE,
+	    B_FALSE, B_FALSE, &buf));
+	arc_buf_thaw(buf);
+	ASSERT3P(hdr->b_l1hdr.b_freeze_cksum, ==, NULL);
+
+	return (buf);
+}
+
+static void
+arc_hdr_l2hdr_destroy(arc_buf_hdr_t *hdr)
+{
+	l2arc_buf_hdr_t *l2hdr = &hdr->b_l2hdr;
+	l2arc_dev_t *dev = l2hdr->b_dev;
+	uint64_t psize = HDR_GET_PSIZE(hdr);
+	uint64_t asize = vdev_psize_to_asize(dev->l2ad_vdev, psize);
+
+	ASSERT(MUTEX_HELD(&dev->l2ad_mtx));
+	ASSERT(HDR_HAS_L2HDR(hdr));
+
+	list_remove(&dev->l2ad_buflist, hdr);
+
+	ARCSTAT_INCR(arcstat_l2_psize, -psize);
+	ARCSTAT_INCR(arcstat_l2_lsize, -HDR_GET_LSIZE(hdr));
+
+	vdev_space_update(dev->l2ad_vdev, -asize, 0, 0);
+
+	(void) zfs_refcount_remove_many(&dev->l2ad_alloc, arc_hdr_size(hdr),
+	    hdr);
+	arc_hdr_clear_flags(hdr, ARC_FLAG_HAS_L2HDR);
+}
+
+static void
+arc_hdr_destroy(arc_buf_hdr_t *hdr)
+{
+	if (HDR_HAS_L1HDR(hdr)) {
+		ASSERT(hdr->b_l1hdr.b_buf == NULL ||
+		    hdr->b_l1hdr.b_bufcnt > 0);
+		ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
+		ASSERT3P(hdr->b_l1hdr.b_state, ==, arc_anon);
+	}
+	ASSERT(!HDR_IO_IN_PROGRESS(hdr));
+	ASSERT(!HDR_IN_HASH_TABLE(hdr));
+
+	if (HDR_HAS_L2HDR(hdr)) {
+		l2arc_dev_t *dev = hdr->b_l2hdr.b_dev;
+		boolean_t buflist_held = MUTEX_HELD(&dev->l2ad_mtx);
+
+		if (!buflist_held)
+			mutex_enter(&dev->l2ad_mtx);
+
+		/*
+		 * Even though we checked this conditional above, we
+		 * need to check this again now that we have the
+		 * l2ad_mtx. This is because we could be racing with
+		 * another thread calling l2arc_evict() which might have
+		 * destroyed this header's L2 portion as we were waiting
+		 * to acquire the l2ad_mtx. If that happens, we don't
+		 * want to re-destroy the header's L2 portion.
+		 */
+		if (HDR_HAS_L2HDR(hdr))
+			arc_hdr_l2hdr_destroy(hdr);
+
+		if (!buflist_held)
+			mutex_exit(&dev->l2ad_mtx);
+	}
+
+	/*
+	 * The header's identify can only be safely discarded once it is no
+	 * longer discoverable.  This requires removing it from the hash table
+	 * and the l2arc header list.  After this point the hash lock can not
+	 * be used to protect the header.
+	 */
+	if (!HDR_EMPTY(hdr))
+		buf_discard_identity(hdr);
+
+	if (HDR_HAS_L1HDR(hdr)) {
+		arc_cksum_free(hdr);
+
+		while (hdr->b_l1hdr.b_buf != NULL)
+			arc_buf_destroy_impl(hdr->b_l1hdr.b_buf);
+
+		if (hdr->b_l1hdr.b_pabd != NULL)
+			arc_hdr_free_abd(hdr, B_FALSE);
+
+		if (HDR_HAS_RABD(hdr))
+			arc_hdr_free_abd(hdr, B_TRUE);
+	}
+
+	ASSERT3P(hdr->b_hash_next, ==, NULL);
+	if (HDR_HAS_L1HDR(hdr)) {
+		ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));
+		ASSERT3P(hdr->b_l1hdr.b_acb, ==, NULL);
+
+		if (!HDR_PROTECTED(hdr)) {
+			kmem_cache_free(hdr_full_cache, hdr);
+		} else {
+			kmem_cache_free(hdr_full_crypt_cache, hdr);
+		}
+	} else {
+		kmem_cache_free(hdr_l2only_cache, hdr);
+	}
+}
+
+void
+arc_buf_destroy(arc_buf_t *buf, void* tag)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+
+	if (hdr->b_l1hdr.b_state == arc_anon) {
+		ASSERT3U(hdr->b_l1hdr.b_bufcnt, ==, 1);
+		ASSERT(!HDR_IO_IN_PROGRESS(hdr));
+		VERIFY0(remove_reference(hdr, NULL, tag));
+		arc_hdr_destroy(hdr);
+		return;
+	}
+
+	kmutex_t *hash_lock = HDR_LOCK(hdr);
+	mutex_enter(hash_lock);
+
+	ASSERT3P(hdr, ==, buf->b_hdr);
+	ASSERT(hdr->b_l1hdr.b_bufcnt > 0);
+	ASSERT3P(hash_lock, ==, HDR_LOCK(hdr));
+	ASSERT3P(hdr->b_l1hdr.b_state, !=, arc_anon);
+	ASSERT3P(buf->b_data, !=, NULL);
+
+	(void) remove_reference(hdr, hash_lock, tag);
+	arc_buf_destroy_impl(buf);
+	mutex_exit(hash_lock);
+}
+
+/*
+ * Evict the arc_buf_hdr that is provided as a parameter. The resultant
+ * state of the header is dependent on its state prior to entering this
+ * function. The following transitions are possible:
+ *
+ *    - arc_mru -> arc_mru_ghost
+ *    - arc_mfu -> arc_mfu_ghost
+ *    - arc_mru_ghost -> arc_l2c_only
+ *    - arc_mru_ghost -> deleted
+ *    - arc_mfu_ghost -> arc_l2c_only
+ *    - arc_mfu_ghost -> deleted
+ */
+static int64_t
+arc_evict_hdr(arc_buf_hdr_t *hdr, kmutex_t *hash_lock)
+{
+	arc_state_t *evicted_state, *state;
+	int64_t bytes_evicted = 0;
+	int min_lifetime = HDR_PRESCIENT_PREFETCH(hdr) ?
+	    arc_min_prescient_prefetch_ms : arc_min_prefetch_ms;
+
+	ASSERT(MUTEX_HELD(hash_lock));
+	ASSERT(HDR_HAS_L1HDR(hdr));
+
+	state = hdr->b_l1hdr.b_state;
+	if (GHOST_STATE(state)) {
+		ASSERT(!HDR_IO_IN_PROGRESS(hdr));
+		ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
+
+		/*
+		 * l2arc_write_buffers() relies on a header's L1 portion
+		 * (i.e. its b_pabd field) during it's write phase.
+		 * Thus, we cannot push a header onto the arc_l2c_only
+		 * state (removing its L1 piece) until the header is
+		 * done being written to the l2arc.
+		 */
+		if (HDR_HAS_L2HDR(hdr) && HDR_L2_WRITING(hdr)) {
+			ARCSTAT_BUMP(arcstat_evict_l2_skip);
+			return (bytes_evicted);
+		}
+
+		ARCSTAT_BUMP(arcstat_deleted);
+		bytes_evicted += HDR_GET_LSIZE(hdr);
+
+		DTRACE_PROBE1(arc__delete, arc_buf_hdr_t *, hdr);
+
+		if (HDR_HAS_L2HDR(hdr)) {
+			ASSERT(hdr->b_l1hdr.b_pabd == NULL);
+			ASSERT(!HDR_HAS_RABD(hdr));
+			/*
+			 * This buffer is cached on the 2nd Level ARC;
+			 * don't destroy the header.
+			 */
+			arc_change_state(arc_l2c_only, hdr, hash_lock);
+			/*
+			 * dropping from L1+L2 cached to L2-only,
+			 * realloc to remove the L1 header.
+			 */
+			hdr = arc_hdr_realloc(hdr, hdr_full_cache,
+			    hdr_l2only_cache);
+		} else {
+			arc_change_state(arc_anon, hdr, hash_lock);
+			arc_hdr_destroy(hdr);
+		}
+		return (bytes_evicted);
+	}
+
+	ASSERT(state == arc_mru || state == arc_mfu);
+	evicted_state = (state == arc_mru) ? arc_mru_ghost : arc_mfu_ghost;
+
+	/* prefetch buffers have a minimum lifespan */
+	if (HDR_IO_IN_PROGRESS(hdr) ||
+	    ((hdr->b_flags & (ARC_FLAG_PREFETCH | ARC_FLAG_INDIRECT)) &&
+	    ddi_get_lbolt() - hdr->b_l1hdr.b_arc_access <
+	    MSEC_TO_TICK(min_lifetime))) {
+		ARCSTAT_BUMP(arcstat_evict_skip);
+		return (bytes_evicted);
+	}
+
+	ASSERT0(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt));
+	while (hdr->b_l1hdr.b_buf) {
+		arc_buf_t *buf = hdr->b_l1hdr.b_buf;
+		if (!mutex_tryenter(&buf->b_evict_lock)) {
+			ARCSTAT_BUMP(arcstat_mutex_miss);
+			break;
+		}
+		if (buf->b_data != NULL)
+			bytes_evicted += HDR_GET_LSIZE(hdr);
+		mutex_exit(&buf->b_evict_lock);
+		arc_buf_destroy_impl(buf);
+	}
+
+	if (HDR_HAS_L2HDR(hdr)) {
+		ARCSTAT_INCR(arcstat_evict_l2_cached, HDR_GET_LSIZE(hdr));
+	} else {
+		if (l2arc_write_eligible(hdr->b_spa, hdr)) {
+			ARCSTAT_INCR(arcstat_evict_l2_eligible,
+			    HDR_GET_LSIZE(hdr));
+		} else {
+			ARCSTAT_INCR(arcstat_evict_l2_ineligible,
+			    HDR_GET_LSIZE(hdr));
+		}
+	}
+
+	if (hdr->b_l1hdr.b_bufcnt == 0) {
+		arc_cksum_free(hdr);
+
+		bytes_evicted += arc_hdr_size(hdr);
+
+		/*
+		 * If this hdr is being evicted and has a compressed
+		 * buffer then we discard it here before we change states.
+		 * This ensures that the accounting is updated correctly
+		 * in arc_free_data_impl().
+		 */
+		if (hdr->b_l1hdr.b_pabd != NULL)
+			arc_hdr_free_abd(hdr, B_FALSE);
+
+		if (HDR_HAS_RABD(hdr))
+			arc_hdr_free_abd(hdr, B_TRUE);
+
+		arc_change_state(evicted_state, hdr, hash_lock);
+		ASSERT(HDR_IN_HASH_TABLE(hdr));
+		arc_hdr_set_flags(hdr, ARC_FLAG_IN_HASH_TABLE);
+		DTRACE_PROBE1(arc__evict, arc_buf_hdr_t *, hdr);
+	}
+
+	return (bytes_evicted);
+}
+
+static void
+arc_set_need_free(void)
+{
+	ASSERT(MUTEX_HELD(&arc_evict_lock));
+	int64_t remaining = arc_free_memory() - arc_sys_free / 2;
+	arc_evict_waiter_t *aw = list_tail(&arc_evict_waiters);
+	if (aw == NULL) {
+		arc_need_free = MAX(-remaining, 0);
+	} else {
+		arc_need_free =
+		    MAX(-remaining, (int64_t)(aw->aew_count - arc_evict_count));
+	}
+}
+
+static uint64_t
+arc_evict_state_impl(multilist_t *ml, int idx, arc_buf_hdr_t *marker,
+    uint64_t spa, int64_t bytes)
+{
+	multilist_sublist_t *mls;
+	uint64_t bytes_evicted = 0;
+	arc_buf_hdr_t *hdr;
+	kmutex_t *hash_lock;
+	int evict_count = 0;
+
+	ASSERT3P(marker, !=, NULL);
+	IMPLY(bytes < 0, bytes == ARC_EVICT_ALL);
+
+	mls = multilist_sublist_lock(ml, idx);
+
+	for (hdr = multilist_sublist_prev(mls, marker); hdr != NULL;
+	    hdr = multilist_sublist_prev(mls, marker)) {
+		if ((bytes != ARC_EVICT_ALL && bytes_evicted >= bytes) ||
+		    (evict_count >= zfs_arc_evict_batch_limit))
+			break;
+
+		/*
+		 * To keep our iteration location, move the marker
+		 * forward. Since we're not holding hdr's hash lock, we
+		 * must be very careful and not remove 'hdr' from the
+		 * sublist. Otherwise, other consumers might mistake the
+		 * 'hdr' as not being on a sublist when they call the
+		 * multilist_link_active() function (they all rely on
+		 * the hash lock protecting concurrent insertions and
+		 * removals). multilist_sublist_move_forward() was
+		 * specifically implemented to ensure this is the case
+		 * (only 'marker' will be removed and re-inserted).
+		 */
+		multilist_sublist_move_forward(mls, marker);
+
+		/*
+		 * The only case where the b_spa field should ever be
+		 * zero, is the marker headers inserted by
+		 * arc_evict_state(). It's possible for multiple threads
+		 * to be calling arc_evict_state() concurrently (e.g.
+		 * dsl_pool_close() and zio_inject_fault()), so we must
+		 * skip any markers we see from these other threads.
+		 */
+		if (hdr->b_spa == 0)
+			continue;
+
+		/* we're only interested in evicting buffers of a certain spa */
+		if (spa != 0 && hdr->b_spa != spa) {
+			ARCSTAT_BUMP(arcstat_evict_skip);
+			continue;
+		}
+
+		hash_lock = HDR_LOCK(hdr);
+
+		/*
+		 * We aren't calling this function from any code path
+		 * that would already be holding a hash lock, so we're
+		 * asserting on this assumption to be defensive in case
+		 * this ever changes. Without this check, it would be
+		 * possible to incorrectly increment arcstat_mutex_miss
+		 * below (e.g. if the code changed such that we called
+		 * this function with a hash lock held).
+		 */
+		ASSERT(!MUTEX_HELD(hash_lock));
+
+		if (mutex_tryenter(hash_lock)) {
+			uint64_t evicted = arc_evict_hdr(hdr, hash_lock);
+			mutex_exit(hash_lock);
+
+			bytes_evicted += evicted;
+
+			/*
+			 * If evicted is zero, arc_evict_hdr() must have
+			 * decided to skip this header, don't increment
+			 * evict_count in this case.
+			 */
+			if (evicted != 0)
+				evict_count++;
+
+		} else {
+			ARCSTAT_BUMP(arcstat_mutex_miss);
+		}
+	}
+
+	multilist_sublist_unlock(mls);
+
+	/*
+	 * Increment the count of evicted bytes, and wake up any threads that
+	 * are waiting for the count to reach this value.  Since the list is
+	 * ordered by ascending aew_count, we pop off the beginning of the
+	 * list until we reach the end, or a waiter that's past the current
+	 * "count".  Doing this outside the loop reduces the number of times
+	 * we need to acquire the global arc_evict_lock.
+	 *
+	 * Only wake when there's sufficient free memory in the system
+	 * (specifically, arc_sys_free/2, which by default is a bit more than
+	 * 1/64th of RAM).  See the comments in arc_wait_for_eviction().
+	 */
+	mutex_enter(&arc_evict_lock);
+	arc_evict_count += bytes_evicted;
+
+	if ((int64_t)(arc_free_memory() - arc_sys_free / 2) > 0) {
+		arc_evict_waiter_t *aw;
+		while ((aw = list_head(&arc_evict_waiters)) != NULL &&
+		    aw->aew_count <= arc_evict_count) {
+			list_remove(&arc_evict_waiters, aw);
+			cv_broadcast(&aw->aew_cv);
+		}
+	}
+	arc_set_need_free();
+	mutex_exit(&arc_evict_lock);
+
+	/*
+	 * If the ARC size is reduced from arc_c_max to arc_c_min (especially
+	 * if the average cached block is small), eviction can be on-CPU for
+	 * many seconds.  To ensure that other threads that may be bound to
+	 * this CPU are able to make progress, make a voluntary preemption
+	 * call here.
+	 */
+	cond_resched();
+
+	return (bytes_evicted);
+}
+
+/*
+ * Evict buffers from the given arc state, until we've removed the
+ * specified number of bytes. Move the removed buffers to the
+ * appropriate evict state.
+ *
+ * This function makes a "best effort". It skips over any buffers
+ * it can't get a hash_lock on, and so, may not catch all candidates.
+ * It may also return without evicting as much space as requested.
+ *
+ * If bytes is specified using the special value ARC_EVICT_ALL, this
+ * will evict all available (i.e. unlocked and evictable) buffers from
+ * the given arc state; which is used by arc_flush().
+ */
+static uint64_t
+arc_evict_state(arc_state_t *state, uint64_t spa, int64_t bytes,
+    arc_buf_contents_t type)
+{
+	uint64_t total_evicted = 0;
+	multilist_t *ml = state->arcs_list[type];
+	int num_sublists;
+	arc_buf_hdr_t **markers;
+
+	IMPLY(bytes < 0, bytes == ARC_EVICT_ALL);
+
+	num_sublists = multilist_get_num_sublists(ml);
+
+	/*
+	 * If we've tried to evict from each sublist, made some
+	 * progress, but still have not hit the target number of bytes
+	 * to evict, we want to keep trying. The markers allow us to
+	 * pick up where we left off for each individual sublist, rather
+	 * than starting from the tail each time.
+	 */
+	markers = kmem_zalloc(sizeof (*markers) * num_sublists, KM_SLEEP);
+	for (int i = 0; i < num_sublists; i++) {
+		multilist_sublist_t *mls;
+
+		markers[i] = kmem_cache_alloc(hdr_full_cache, KM_SLEEP);
+
+		/*
+		 * A b_spa of 0 is used to indicate that this header is
+		 * a marker. This fact is used in arc_evict_type() and
+		 * arc_evict_state_impl().
+		 */
+		markers[i]->b_spa = 0;
+
+		mls = multilist_sublist_lock(ml, i);
+		multilist_sublist_insert_tail(mls, markers[i]);
+		multilist_sublist_unlock(mls);
+	}
+
+	/*
+	 * While we haven't hit our target number of bytes to evict, or
+	 * we're evicting all available buffers.
+	 */
+	while (total_evicted < bytes || bytes == ARC_EVICT_ALL) {
+		int sublist_idx = multilist_get_random_index(ml);
+		uint64_t scan_evicted = 0;
+
+		/*
+		 * Try to reduce pinned dnodes with a floor of arc_dnode_limit.
+		 * Request that 10% of the LRUs be scanned by the superblock
+		 * shrinker.
+		 */
+		if (type == ARC_BUFC_DATA && aggsum_compare(&astat_dnode_size,
+		    arc_dnode_size_limit) > 0) {
+			arc_prune_async((aggsum_upper_bound(&astat_dnode_size) -
+			    arc_dnode_size_limit) / sizeof (dnode_t) /
+			    zfs_arc_dnode_reduce_percent);
+		}
+
+		/*
+		 * Start eviction using a randomly selected sublist,
+		 * this is to try and evenly balance eviction across all
+		 * sublists. Always starting at the same sublist
+		 * (e.g. index 0) would cause evictions to favor certain
+		 * sublists over others.
+		 */
+		for (int i = 0; i < num_sublists; i++) {
+			uint64_t bytes_remaining;
+			uint64_t bytes_evicted;
+
+			if (bytes == ARC_EVICT_ALL)
+				bytes_remaining = ARC_EVICT_ALL;
+			else if (total_evicted < bytes)
+				bytes_remaining = bytes - total_evicted;
+			else
+				break;
+
+			bytes_evicted = arc_evict_state_impl(ml, sublist_idx,
+			    markers[sublist_idx], spa, bytes_remaining);
+
+			scan_evicted += bytes_evicted;
+			total_evicted += bytes_evicted;
+
+			/* we've reached the end, wrap to the beginning */
+			if (++sublist_idx >= num_sublists)
+				sublist_idx = 0;
+		}
+
+		/*
+		 * If we didn't evict anything during this scan, we have
+		 * no reason to believe we'll evict more during another
+		 * scan, so break the loop.
+		 */
+		if (scan_evicted == 0) {
+			/* This isn't possible, let's make that obvious */
+			ASSERT3S(bytes, !=, 0);
+
+			/*
+			 * When bytes is ARC_EVICT_ALL, the only way to
+			 * break the loop is when scan_evicted is zero.
+			 * In that case, we actually have evicted enough,
+			 * so we don't want to increment the kstat.
+			 */
+			if (bytes != ARC_EVICT_ALL) {
+				ASSERT3S(total_evicted, <, bytes);
+				ARCSTAT_BUMP(arcstat_evict_not_enough);
+			}
+
+			break;
+		}
+	}
+
+	for (int i = 0; i < num_sublists; i++) {
+		multilist_sublist_t *mls = multilist_sublist_lock(ml, i);
+		multilist_sublist_remove(mls, markers[i]);
+		multilist_sublist_unlock(mls);
+
+		kmem_cache_free(hdr_full_cache, markers[i]);
+	}
+	kmem_free(markers, sizeof (*markers) * num_sublists);
+
+	return (total_evicted);
+}
+
+/*
+ * Flush all "evictable" data of the given type from the arc state
+ * specified. This will not evict any "active" buffers (i.e. referenced).
+ *
+ * When 'retry' is set to B_FALSE, the function will make a single pass
+ * over the state and evict any buffers that it can. Since it doesn't
+ * continually retry the eviction, it might end up leaving some buffers
+ * in the ARC due to lock misses.
+ *
+ * When 'retry' is set to B_TRUE, the function will continually retry the
+ * eviction until *all* evictable buffers have been removed from the
+ * state. As a result, if concurrent insertions into the state are
+ * allowed (e.g. if the ARC isn't shutting down), this function might
+ * wind up in an infinite loop, continually trying to evict buffers.
+ */
+static uint64_t
+arc_flush_state(arc_state_t *state, uint64_t spa, arc_buf_contents_t type,
+    boolean_t retry)
+{
+	uint64_t evicted = 0;
+
+	while (zfs_refcount_count(&state->arcs_esize[type]) != 0) {
+		evicted += arc_evict_state(state, spa, ARC_EVICT_ALL, type);
+
+		if (!retry)
+			break;
+	}
+
+	return (evicted);
+}
+
+/*
+ * Evict the specified number of bytes from the state specified,
+ * restricting eviction to the spa and type given. This function
+ * prevents us from trying to evict more from a state's list than
+ * is "evictable", and to skip evicting altogether when passed a
+ * negative value for "bytes". In contrast, arc_evict_state() will
+ * evict everything it can, when passed a negative value for "bytes".
+ */
+static uint64_t
+arc_evict_impl(arc_state_t *state, uint64_t spa, int64_t bytes,
+    arc_buf_contents_t type)
+{
+	int64_t delta;
+
+	if (bytes > 0 && zfs_refcount_count(&state->arcs_esize[type]) > 0) {
+		delta = MIN(zfs_refcount_count(&state->arcs_esize[type]),
+		    bytes);
+		return (arc_evict_state(state, spa, delta, type));
+	}
+
+	return (0);
+}
+
+/*
+ * The goal of this function is to evict enough meta data buffers from the
+ * ARC in order to enforce the arc_meta_limit.  Achieving this is slightly
+ * more complicated than it appears because it is common for data buffers
+ * to have holds on meta data buffers.  In addition, dnode meta data buffers
+ * will be held by the dnodes in the block preventing them from being freed.
+ * This means we can't simply traverse the ARC and expect to always find
+ * enough unheld meta data buffer to release.
+ *
+ * Therefore, this function has been updated to make alternating passes
+ * over the ARC releasing data buffers and then newly unheld meta data
+ * buffers.  This ensures forward progress is maintained and meta_used
+ * will decrease.  Normally this is sufficient, but if required the ARC
+ * will call the registered prune callbacks causing dentry and inodes to
+ * be dropped from the VFS cache.  This will make dnode meta data buffers
+ * available for reclaim.
+ */
+static uint64_t
+arc_evict_meta_balanced(uint64_t meta_used)
+{
+	int64_t delta, prune = 0, adjustmnt;
+	uint64_t total_evicted = 0;
+	arc_buf_contents_t type = ARC_BUFC_DATA;
+	int restarts = MAX(zfs_arc_meta_adjust_restarts, 0);
+
+restart:
+	/*
+	 * This slightly differs than the way we evict from the mru in
+	 * arc_evict because we don't have a "target" value (i.e. no
+	 * "meta" arc_p). As a result, I think we can completely
+	 * cannibalize the metadata in the MRU before we evict the
+	 * metadata from the MFU. I think we probably need to implement a
+	 * "metadata arc_p" value to do this properly.
+	 */
+	adjustmnt = meta_used - arc_meta_limit;
+
+	if (adjustmnt > 0 &&
+	    zfs_refcount_count(&arc_mru->arcs_esize[type]) > 0) {
+		delta = MIN(zfs_refcount_count(&arc_mru->arcs_esize[type]),
+		    adjustmnt);
+		total_evicted += arc_evict_impl(arc_mru, 0, delta, type);
+		adjustmnt -= delta;
+	}
+
+	/*
+	 * We can't afford to recalculate adjustmnt here. If we do,
+	 * new metadata buffers can sneak into the MRU or ANON lists,
+	 * thus penalize the MFU metadata. Although the fudge factor is
+	 * small, it has been empirically shown to be significant for
+	 * certain workloads (e.g. creating many empty directories). As
+	 * such, we use the original calculation for adjustmnt, and
+	 * simply decrement the amount of data evicted from the MRU.
+	 */
+
+	if (adjustmnt > 0 &&
+	    zfs_refcount_count(&arc_mfu->arcs_esize[type]) > 0) {
+		delta = MIN(zfs_refcount_count(&arc_mfu->arcs_esize[type]),
+		    adjustmnt);
+		total_evicted += arc_evict_impl(arc_mfu, 0, delta, type);
+	}
+
+	adjustmnt = meta_used - arc_meta_limit;
+
+	if (adjustmnt > 0 &&
+	    zfs_refcount_count(&arc_mru_ghost->arcs_esize[type]) > 0) {
+		delta = MIN(adjustmnt,
+		    zfs_refcount_count(&arc_mru_ghost->arcs_esize[type]));
+		total_evicted += arc_evict_impl(arc_mru_ghost, 0, delta, type);
+		adjustmnt -= delta;
+	}
+
+	if (adjustmnt > 0 &&
+	    zfs_refcount_count(&arc_mfu_ghost->arcs_esize[type]) > 0) {
+		delta = MIN(adjustmnt,
+		    zfs_refcount_count(&arc_mfu_ghost->arcs_esize[type]));
+		total_evicted += arc_evict_impl(arc_mfu_ghost, 0, delta, type);
+	}
+
+	/*
+	 * If after attempting to make the requested adjustment to the ARC
+	 * the meta limit is still being exceeded then request that the
+	 * higher layers drop some cached objects which have holds on ARC
+	 * meta buffers.  Requests to the upper layers will be made with
+	 * increasingly large scan sizes until the ARC is below the limit.
+	 */
+	if (meta_used > arc_meta_limit) {
+		if (type == ARC_BUFC_DATA) {
+			type = ARC_BUFC_METADATA;
+		} else {
+			type = ARC_BUFC_DATA;
+
+			if (zfs_arc_meta_prune) {
+				prune += zfs_arc_meta_prune;
+				arc_prune_async(prune);
+			}
+		}
+
+		if (restarts > 0) {
+			restarts--;
+			goto restart;
+		}
+	}
+	return (total_evicted);
+}
+
+/*
+ * Evict metadata buffers from the cache, such that arc_meta_used is
+ * capped by the arc_meta_limit tunable.
+ */
+static uint64_t
+arc_evict_meta_only(uint64_t meta_used)
+{
+	uint64_t total_evicted = 0;
+	int64_t target;
+
+	/*
+	 * If we're over the meta limit, we want to evict enough
+	 * metadata to get back under the meta limit. We don't want to
+	 * evict so much that we drop the MRU below arc_p, though. If
+	 * we're over the meta limit more than we're over arc_p, we
+	 * evict some from the MRU here, and some from the MFU below.
+	 */
+	target = MIN((int64_t)(meta_used - arc_meta_limit),
+	    (int64_t)(zfs_refcount_count(&arc_anon->arcs_size) +
+	    zfs_refcount_count(&arc_mru->arcs_size) - arc_p));
+
+	total_evicted += arc_evict_impl(arc_mru, 0, target, ARC_BUFC_METADATA);
+
+	/*
+	 * Similar to the above, we want to evict enough bytes to get us
+	 * below the meta limit, but not so much as to drop us below the
+	 * space allotted to the MFU (which is defined as arc_c - arc_p).
+	 */
+	target = MIN((int64_t)(meta_used - arc_meta_limit),
+	    (int64_t)(zfs_refcount_count(&arc_mfu->arcs_size) -
+	    (arc_c - arc_p)));
+
+	total_evicted += arc_evict_impl(arc_mfu, 0, target, ARC_BUFC_METADATA);
+
+	return (total_evicted);
+}
+
+static uint64_t
+arc_evict_meta(uint64_t meta_used)
+{
+	if (zfs_arc_meta_strategy == ARC_STRATEGY_META_ONLY)
+		return (arc_evict_meta_only(meta_used));
+	else
+		return (arc_evict_meta_balanced(meta_used));
+}
+
+/*
+ * Return the type of the oldest buffer in the given arc state
+ *
+ * This function will select a random sublist of type ARC_BUFC_DATA and
+ * a random sublist of type ARC_BUFC_METADATA. The tail of each sublist
+ * is compared, and the type which contains the "older" buffer will be
+ * returned.
+ */
+static arc_buf_contents_t
+arc_evict_type(arc_state_t *state)
+{
+	multilist_t *data_ml = state->arcs_list[ARC_BUFC_DATA];
+	multilist_t *meta_ml = state->arcs_list[ARC_BUFC_METADATA];
+	int data_idx = multilist_get_random_index(data_ml);
+	int meta_idx = multilist_get_random_index(meta_ml);
+	multilist_sublist_t *data_mls;
+	multilist_sublist_t *meta_mls;
+	arc_buf_contents_t type;
+	arc_buf_hdr_t *data_hdr;
+	arc_buf_hdr_t *meta_hdr;
+
+	/*
+	 * We keep the sublist lock until we're finished, to prevent
+	 * the headers from being destroyed via arc_evict_state().
+	 */
+	data_mls = multilist_sublist_lock(data_ml, data_idx);
+	meta_mls = multilist_sublist_lock(meta_ml, meta_idx);
+
+	/*
+	 * These two loops are to ensure we skip any markers that
+	 * might be at the tail of the lists due to arc_evict_state().
+	 */
+
+	for (data_hdr = multilist_sublist_tail(data_mls); data_hdr != NULL;
+	    data_hdr = multilist_sublist_prev(data_mls, data_hdr)) {
+		if (data_hdr->b_spa != 0)
+			break;
+	}
+
+	for (meta_hdr = multilist_sublist_tail(meta_mls); meta_hdr != NULL;
+	    meta_hdr = multilist_sublist_prev(meta_mls, meta_hdr)) {
+		if (meta_hdr->b_spa != 0)
+			break;
+	}
+
+	if (data_hdr == NULL && meta_hdr == NULL) {
+		type = ARC_BUFC_DATA;
+	} else if (data_hdr == NULL) {
+		ASSERT3P(meta_hdr, !=, NULL);
+		type = ARC_BUFC_METADATA;
+	} else if (meta_hdr == NULL) {
+		ASSERT3P(data_hdr, !=, NULL);
+		type = ARC_BUFC_DATA;
+	} else {
+		ASSERT3P(data_hdr, !=, NULL);
+		ASSERT3P(meta_hdr, !=, NULL);
+
+		/* The headers can't be on the sublist without an L1 header */
+		ASSERT(HDR_HAS_L1HDR(data_hdr));
+		ASSERT(HDR_HAS_L1HDR(meta_hdr));
+
+		if (data_hdr->b_l1hdr.b_arc_access <
+		    meta_hdr->b_l1hdr.b_arc_access) {
+			type = ARC_BUFC_DATA;
+		} else {
+			type = ARC_BUFC_METADATA;
+		}
+	}
+
+	multilist_sublist_unlock(meta_mls);
+	multilist_sublist_unlock(data_mls);
+
+	return (type);
+}
+
+/*
+ * Evict buffers from the cache, such that arc_size is capped by arc_c.
+ */
+static uint64_t
+arc_evict(void)
+{
+	uint64_t total_evicted = 0;
+	uint64_t bytes;
+	int64_t target;
+	uint64_t asize = aggsum_value(&arc_size);
+	uint64_t ameta = aggsum_value(&arc_meta_used);
+
+	/*
+	 * If we're over arc_meta_limit, we want to correct that before
+	 * potentially evicting data buffers below.
+	 */
+	total_evicted += arc_evict_meta(ameta);
+
+	/*
+	 * Adjust MRU size
+	 *
+	 * If we're over the target cache size, we want to evict enough
+	 * from the list to get back to our target size. We don't want
+	 * to evict too much from the MRU, such that it drops below
+	 * arc_p. So, if we're over our target cache size more than
+	 * the MRU is over arc_p, we'll evict enough to get back to
+	 * arc_p here, and then evict more from the MFU below.
+	 */
+	target = MIN((int64_t)(asize - arc_c),
+	    (int64_t)(zfs_refcount_count(&arc_anon->arcs_size) +
+	    zfs_refcount_count(&arc_mru->arcs_size) + ameta - arc_p));
+
+	/*
+	 * If we're below arc_meta_min, always prefer to evict data.
+	 * Otherwise, try to satisfy the requested number of bytes to
+	 * evict from the type which contains older buffers; in an
+	 * effort to keep newer buffers in the cache regardless of their
+	 * type. If we cannot satisfy the number of bytes from this
+	 * type, spill over into the next type.
+	 */
+	if (arc_evict_type(arc_mru) == ARC_BUFC_METADATA &&
+	    ameta > arc_meta_min) {
+		bytes = arc_evict_impl(arc_mru, 0, target, ARC_BUFC_METADATA);
+		total_evicted += bytes;
+
+		/*
+		 * If we couldn't evict our target number of bytes from
+		 * metadata, we try to get the rest from data.
+		 */
+		target -= bytes;
+
+		total_evicted +=
+		    arc_evict_impl(arc_mru, 0, target, ARC_BUFC_DATA);
+	} else {
+		bytes = arc_evict_impl(arc_mru, 0, target, ARC_BUFC_DATA);
+		total_evicted += bytes;
+
+		/*
+		 * If we couldn't evict our target number of bytes from
+		 * data, we try to get the rest from metadata.
+		 */
+		target -= bytes;
+
+		total_evicted +=
+		    arc_evict_impl(arc_mru, 0, target, ARC_BUFC_METADATA);
+	}
+
+	/*
+	 * Re-sum ARC stats after the first round of evictions.
+	 */
+	asize = aggsum_value(&arc_size);
+	ameta = aggsum_value(&arc_meta_used);
+
+
+	/*
+	 * Adjust MFU size
+	 *
+	 * Now that we've tried to evict enough from the MRU to get its
+	 * size back to arc_p, if we're still above the target cache
+	 * size, we evict the rest from the MFU.
+	 */
+	target = asize - arc_c;
+
+	if (arc_evict_type(arc_mfu) == ARC_BUFC_METADATA &&
+	    ameta > arc_meta_min) {
+		bytes = arc_evict_impl(arc_mfu, 0, target, ARC_BUFC_METADATA);
+		total_evicted += bytes;
+
+		/*
+		 * If we couldn't evict our target number of bytes from
+		 * metadata, we try to get the rest from data.
+		 */
+		target -= bytes;
+
+		total_evicted +=
+		    arc_evict_impl(arc_mfu, 0, target, ARC_BUFC_DATA);
+	} else {
+		bytes = arc_evict_impl(arc_mfu, 0, target, ARC_BUFC_DATA);
+		total_evicted += bytes;
+
+		/*
+		 * If we couldn't evict our target number of bytes from
+		 * data, we try to get the rest from data.
+		 */
+		target -= bytes;
+
+		total_evicted +=
+		    arc_evict_impl(arc_mfu, 0, target, ARC_BUFC_METADATA);
+	}
+
+	/*
+	 * Adjust ghost lists
+	 *
+	 * In addition to the above, the ARC also defines target values
+	 * for the ghost lists. The sum of the mru list and mru ghost
+	 * list should never exceed the target size of the cache, and
+	 * the sum of the mru list, mfu list, mru ghost list, and mfu
+	 * ghost list should never exceed twice the target size of the
+	 * cache. The following logic enforces these limits on the ghost
+	 * caches, and evicts from them as needed.
+	 */
+	target = zfs_refcount_count(&arc_mru->arcs_size) +
+	    zfs_refcount_count(&arc_mru_ghost->arcs_size) - arc_c;
+
+	bytes = arc_evict_impl(arc_mru_ghost, 0, target, ARC_BUFC_DATA);
+	total_evicted += bytes;
+
+	target -= bytes;
+
+	total_evicted +=
+	    arc_evict_impl(arc_mru_ghost, 0, target, ARC_BUFC_METADATA);
+
+	/*
+	 * We assume the sum of the mru list and mfu list is less than
+	 * or equal to arc_c (we enforced this above), which means we
+	 * can use the simpler of the two equations below:
+	 *
+	 *	mru + mfu + mru ghost + mfu ghost <= 2 * arc_c
+	 *		    mru ghost + mfu ghost <= arc_c
+	 */
+	target = zfs_refcount_count(&arc_mru_ghost->arcs_size) +
+	    zfs_refcount_count(&arc_mfu_ghost->arcs_size) - arc_c;
+
+	bytes = arc_evict_impl(arc_mfu_ghost, 0, target, ARC_BUFC_DATA);
+	total_evicted += bytes;
+
+	target -= bytes;
+
+	total_evicted +=
+	    arc_evict_impl(arc_mfu_ghost, 0, target, ARC_BUFC_METADATA);
+
+	return (total_evicted);
+}
+
+void
+arc_flush(spa_t *spa, boolean_t retry)
+{
+	uint64_t guid = 0;
+
+	/*
+	 * If retry is B_TRUE, a spa must not be specified since we have
+	 * no good way to determine if all of a spa's buffers have been
+	 * evicted from an arc state.
+	 */
+	ASSERT(!retry || spa == 0);
+
+	if (spa != NULL)
+		guid = spa_load_guid(spa);
+
+	(void) arc_flush_state(arc_mru, guid, ARC_BUFC_DATA, retry);
+	(void) arc_flush_state(arc_mru, guid, ARC_BUFC_METADATA, retry);
+
+	(void) arc_flush_state(arc_mfu, guid, ARC_BUFC_DATA, retry);
+	(void) arc_flush_state(arc_mfu, guid, ARC_BUFC_METADATA, retry);
+
+	(void) arc_flush_state(arc_mru_ghost, guid, ARC_BUFC_DATA, retry);
+	(void) arc_flush_state(arc_mru_ghost, guid, ARC_BUFC_METADATA, retry);
+
+	(void) arc_flush_state(arc_mfu_ghost, guid, ARC_BUFC_DATA, retry);
+	(void) arc_flush_state(arc_mfu_ghost, guid, ARC_BUFC_METADATA, retry);
+}
+
+void
+arc_reduce_target_size(int64_t to_free)
+{
+	uint64_t asize = aggsum_value(&arc_size);
+
+	/*
+	 * All callers want the ARC to actually evict (at least) this much
+	 * memory.  Therefore we reduce from the lower of the current size and
+	 * the target size.  This way, even if arc_c is much higher than
+	 * arc_size (as can be the case after many calls to arc_freed(), we will
+	 * immediately have arc_c < arc_size and therefore the arc_evict_zthr
+	 * will evict.
+	 */
+	uint64_t c = MIN(arc_c, asize);
+
+	if (c > to_free && c - to_free > arc_c_min) {
+		arc_c = c - to_free;
+		atomic_add_64(&arc_p, -(arc_p >> arc_shrink_shift));
+		if (arc_p > arc_c)
+			arc_p = (arc_c >> 1);
+		ASSERT(arc_c >= arc_c_min);
+		ASSERT((int64_t)arc_p >= 0);
+	} else {
+		arc_c = arc_c_min;
+	}
+
+	if (asize > arc_c) {
+		/* See comment in arc_evict_cb_check() on why lock+flag */
+		mutex_enter(&arc_evict_lock);
+		arc_evict_needed = B_TRUE;
+		mutex_exit(&arc_evict_lock);
+		zthr_wakeup(arc_evict_zthr);
+	}
+}
+
+/*
+ * Determine if the system is under memory pressure and is asking
+ * to reclaim memory. A return value of B_TRUE indicates that the system
+ * is under memory pressure and that the arc should adjust accordingly.
+ */
+boolean_t
+arc_reclaim_needed(void)
+{
+	return (arc_available_memory() < 0);
+}
+
+void
+arc_kmem_reap_soon(void)
+{
+	size_t			i;
+	kmem_cache_t		*prev_cache = NULL;
+	kmem_cache_t		*prev_data_cache = NULL;
+	extern kmem_cache_t	*zio_buf_cache[];
+	extern kmem_cache_t	*zio_data_buf_cache[];
+
+#ifdef _KERNEL
+	if ((aggsum_compare(&arc_meta_used, arc_meta_limit) >= 0) &&
+	    zfs_arc_meta_prune) {
+		/*
+		 * We are exceeding our meta-data cache limit.
+		 * Prune some entries to release holds on meta-data.
+		 */
+		arc_prune_async(zfs_arc_meta_prune);
+	}
+#if defined(_ILP32)
+	/*
+	 * Reclaim unused memory from all kmem caches.
+	 */
+	kmem_reap();
+#endif
+#endif
+
+	for (i = 0; i < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT; i++) {
+#if defined(_ILP32)
+		/* reach upper limit of cache size on 32-bit */
+		if (zio_buf_cache[i] == NULL)
+			break;
+#endif
+		if (zio_buf_cache[i] != prev_cache) {
+			prev_cache = zio_buf_cache[i];
+			kmem_cache_reap_now(zio_buf_cache[i]);
+		}
+		if (zio_data_buf_cache[i] != prev_data_cache) {
+			prev_data_cache = zio_data_buf_cache[i];
+			kmem_cache_reap_now(zio_data_buf_cache[i]);
+		}
+	}
+	kmem_cache_reap_now(buf_cache);
+	kmem_cache_reap_now(hdr_full_cache);
+	kmem_cache_reap_now(hdr_l2only_cache);
+	kmem_cache_reap_now(zfs_btree_leaf_cache);
+	abd_cache_reap_now();
+}
+
+/* ARGSUSED */
+static boolean_t
+arc_evict_cb_check(void *arg, zthr_t *zthr)
+{
+	/*
+	 * This is necessary so that any changes which may have been made to
+	 * many of the zfs_arc_* module parameters will be propagated to
+	 * their actual internal variable counterparts. Without this,
+	 * changing those module params at runtime would have no effect.
+	 */
+	arc_tuning_update(B_FALSE);
+
+	/*
+	 * This is necessary in order to keep the kstat information
+	 * up to date for tools that display kstat data such as the
+	 * mdb ::arc dcmd and the Linux crash utility.  These tools
+	 * typically do not call kstat's update function, but simply
+	 * dump out stats from the most recent update.  Without
+	 * this call, these commands may show stale stats for the
+	 * anon, mru, mru_ghost, mfu, and mfu_ghost lists. Even
+	 * with this change, the data might be up to 1 second
+	 * out of date(the arc_evict_zthr has a maximum sleep
+	 * time of 1 second); but that should suffice.  The
+	 * arc_state_t structures can be queried directly if more
+	 * accurate information is needed.
+	 */
+	if (arc_ksp != NULL)
+		arc_ksp->ks_update(arc_ksp, KSTAT_READ);
+
+	/*
+	 * We have to rely on arc_wait_for_eviction() to tell us when to
+	 * evict, rather than checking if we are overflowing here, so that we
+	 * are sure to not leave arc_wait_for_eviction() waiting on aew_cv.
+	 * If we have become "not overflowing" since arc_wait_for_eviction()
+	 * checked, we need to wake it up.  We could broadcast the CV here,
+	 * but arc_wait_for_eviction() may have not yet gone to sleep.  We
+	 * would need to use a mutex to ensure that this function doesn't
+	 * broadcast until arc_wait_for_eviction() has gone to sleep (e.g.
+	 * the arc_evict_lock).  However, the lock ordering of such a lock
+	 * would necessarily be incorrect with respect to the zthr_lock,
+	 * which is held before this function is called, and is held by
+	 * arc_wait_for_eviction() when it calls zthr_wakeup().
+	 */
+	return (arc_evict_needed);
+}
+
+/*
+ * Keep arc_size under arc_c by running arc_evict which evicts data
+ * from the ARC.
+ */
+/* ARGSUSED */
+static void
+arc_evict_cb(void *arg, zthr_t *zthr)
+{
+	uint64_t evicted = 0;
+	fstrans_cookie_t cookie = spl_fstrans_mark();
+
+	/* Evict from cache */
+	evicted = arc_evict();
+
+	/*
+	 * If evicted is zero, we couldn't evict anything
+	 * via arc_evict(). This could be due to hash lock
+	 * collisions, but more likely due to the majority of
+	 * arc buffers being unevictable. Therefore, even if
+	 * arc_size is above arc_c, another pass is unlikely to
+	 * be helpful and could potentially cause us to enter an
+	 * infinite loop.  Additionally, zthr_iscancelled() is
+	 * checked here so that if the arc is shutting down, the
+	 * broadcast will wake any remaining arc evict waiters.
+	 */
+	mutex_enter(&arc_evict_lock);
+	arc_evict_needed = !zthr_iscancelled(arc_evict_zthr) &&
+	    evicted > 0 && aggsum_compare(&arc_size, arc_c) > 0;
+	if (!arc_evict_needed) {
+		/*
+		 * We're either no longer overflowing, or we
+		 * can't evict anything more, so we should wake
+		 * arc_get_data_impl() sooner.
+		 */
+		arc_evict_waiter_t *aw;
+		while ((aw = list_remove_head(&arc_evict_waiters)) != NULL) {
+			cv_broadcast(&aw->aew_cv);
+		}
+		arc_set_need_free();
+	}
+	mutex_exit(&arc_evict_lock);
+	spl_fstrans_unmark(cookie);
+}
+
+/* ARGSUSED */
+static boolean_t
+arc_reap_cb_check(void *arg, zthr_t *zthr)
+{
+	int64_t free_memory = arc_available_memory();
+
+	/*
+	 * If a kmem reap is already active, don't schedule more.  We must
+	 * check for this because kmem_cache_reap_soon() won't actually
+	 * block on the cache being reaped (this is to prevent callers from
+	 * becoming implicitly blocked by a system-wide kmem reap -- which,
+	 * on a system with many, many full magazines, can take minutes).
+	 */
+	if (!kmem_cache_reap_active() && free_memory < 0) {
+
+		arc_no_grow = B_TRUE;
+		arc_warm = B_TRUE;
+		/*
+		 * Wait at least zfs_grow_retry (default 5) seconds
+		 * before considering growing.
+		 */
+		arc_growtime = gethrtime() + SEC2NSEC(arc_grow_retry);
+		return (B_TRUE);
+	} else if (free_memory < arc_c >> arc_no_grow_shift) {
+		arc_no_grow = B_TRUE;
+	} else if (gethrtime() >= arc_growtime) {
+		arc_no_grow = B_FALSE;
+	}
+
+	return (B_FALSE);
+}
+
+/*
+ * Keep enough free memory in the system by reaping the ARC's kmem
+ * caches.  To cause more slabs to be reapable, we may reduce the
+ * target size of the cache (arc_c), causing the arc_evict_cb()
+ * to free more buffers.
+ */
+/* ARGSUSED */
+static void
+arc_reap_cb(void *arg, zthr_t *zthr)
+{
+	int64_t free_memory;
+	fstrans_cookie_t cookie = spl_fstrans_mark();
+
+	/*
+	 * Kick off asynchronous kmem_reap()'s of all our caches.
+	 */
+	arc_kmem_reap_soon();
+
+	/*
+	 * Wait at least arc_kmem_cache_reap_retry_ms between
+	 * arc_kmem_reap_soon() calls. Without this check it is possible to
+	 * end up in a situation where we spend lots of time reaping
+	 * caches, while we're near arc_c_min.  Waiting here also gives the
+	 * subsequent free memory check a chance of finding that the
+	 * asynchronous reap has already freed enough memory, and we don't
+	 * need to call arc_reduce_target_size().
+	 */
+	delay((hz * arc_kmem_cache_reap_retry_ms + 999) / 1000);
+
+	/*
+	 * Reduce the target size as needed to maintain the amount of free
+	 * memory in the system at a fraction of the arc_size (1/128th by
+	 * default).  If oversubscribed (free_memory < 0) then reduce the
+	 * target arc_size by the deficit amount plus the fractional
+	 * amount.  If free memory is positive but less then the fractional
+	 * amount, reduce by what is needed to hit the fractional amount.
+	 */
+	free_memory = arc_available_memory();
+
+	int64_t to_free =
+	    (arc_c >> arc_shrink_shift) - free_memory;
+	if (to_free > 0) {
+		arc_reduce_target_size(to_free);
+	}
+	spl_fstrans_unmark(cookie);
+}
+
+#ifdef _KERNEL
+/*
+ * Determine the amount of memory eligible for eviction contained in the
+ * ARC. All clean data reported by the ghost lists can always be safely
+ * evicted. Due to arc_c_min, the same does not hold for all clean data
+ * contained by the regular mru and mfu lists.
+ *
+ * In the case of the regular mru and mfu lists, we need to report as
+ * much clean data as possible, such that evicting that same reported
+ * data will not bring arc_size below arc_c_min. Thus, in certain
+ * circumstances, the total amount of clean data in the mru and mfu
+ * lists might not actually be evictable.
+ *
+ * The following two distinct cases are accounted for:
+ *
+ * 1. The sum of the amount of dirty data contained by both the mru and
+ *    mfu lists, plus the ARC's other accounting (e.g. the anon list),
+ *    is greater than or equal to arc_c_min.
+ *    (i.e. amount of dirty data >= arc_c_min)
+ *
+ *    This is the easy case; all clean data contained by the mru and mfu
+ *    lists is evictable. Evicting all clean data can only drop arc_size
+ *    to the amount of dirty data, which is greater than arc_c_min.
+ *
+ * 2. The sum of the amount of dirty data contained by both the mru and
+ *    mfu lists, plus the ARC's other accounting (e.g. the anon list),
+ *    is less than arc_c_min.
+ *    (i.e. arc_c_min > amount of dirty data)
+ *
+ *    2.1. arc_size is greater than or equal arc_c_min.
+ *         (i.e. arc_size >= arc_c_min > amount of dirty data)
+ *
+ *         In this case, not all clean data from the regular mru and mfu
+ *         lists is actually evictable; we must leave enough clean data
+ *         to keep arc_size above arc_c_min. Thus, the maximum amount of
+ *         evictable data from the two lists combined, is exactly the
+ *         difference between arc_size and arc_c_min.
+ *
+ *    2.2. arc_size is less than arc_c_min
+ *         (i.e. arc_c_min > arc_size > amount of dirty data)
+ *
+ *         In this case, none of the data contained in the mru and mfu
+ *         lists is evictable, even if it's clean. Since arc_size is
+ *         already below arc_c_min, evicting any more would only
+ *         increase this negative difference.
+ */
+
+#endif /* _KERNEL */
+
+/*
+ * Adapt arc info given the number of bytes we are trying to add and
+ * the state that we are coming from.  This function is only called
+ * when we are adding new content to the cache.
+ */
+static void
+arc_adapt(int bytes, arc_state_t *state)
+{
+	int mult;
+	uint64_t arc_p_min = (arc_c >> arc_p_min_shift);
+	int64_t mrug_size = zfs_refcount_count(&arc_mru_ghost->arcs_size);
+	int64_t mfug_size = zfs_refcount_count(&arc_mfu_ghost->arcs_size);
+
+	if (state == arc_l2c_only)
+		return;
+
+	ASSERT(bytes > 0);
+	/*
+	 * Adapt the target size of the MRU list:
+	 *	- if we just hit in the MRU ghost list, then increase
+	 *	  the target size of the MRU list.
+	 *	- if we just hit in the MFU ghost list, then increase
+	 *	  the target size of the MFU list by decreasing the
+	 *	  target size of the MRU list.
+	 */
+	if (state == arc_mru_ghost) {
+		mult = (mrug_size >= mfug_size) ? 1 : (mfug_size / mrug_size);
+		if (!zfs_arc_p_dampener_disable)
+			mult = MIN(mult, 10); /* avoid wild arc_p adjustment */
+
+		arc_p = MIN(arc_c - arc_p_min, arc_p + bytes * mult);
+	} else if (state == arc_mfu_ghost) {
+		uint64_t delta;
+
+		mult = (mfug_size >= mrug_size) ? 1 : (mrug_size / mfug_size);
+		if (!zfs_arc_p_dampener_disable)
+			mult = MIN(mult, 10);
+
+		delta = MIN(bytes * mult, arc_p);
+		arc_p = MAX(arc_p_min, arc_p - delta);
+	}
+	ASSERT((int64_t)arc_p >= 0);
+
+	/*
+	 * Wake reap thread if we do not have any available memory
+	 */
+	if (arc_reclaim_needed()) {
+		zthr_wakeup(arc_reap_zthr);
+		return;
+	}
+
+	if (arc_no_grow)
+		return;
+
+	if (arc_c >= arc_c_max)
+		return;
+
+	/*
+	 * If we're within (2 * maxblocksize) bytes of the target
+	 * cache size, increment the target cache size
+	 */
+	ASSERT3U(arc_c, >=, 2ULL << SPA_MAXBLOCKSHIFT);
+	if (aggsum_upper_bound(&arc_size) >=
+	    arc_c - (2ULL << SPA_MAXBLOCKSHIFT)) {
+		atomic_add_64(&arc_c, (int64_t)bytes);
+		if (arc_c > arc_c_max)
+			arc_c = arc_c_max;
+		else if (state == arc_anon)
+			atomic_add_64(&arc_p, (int64_t)bytes);
+		if (arc_p > arc_c)
+			arc_p = arc_c;
+	}
+	ASSERT((int64_t)arc_p >= 0);
+}
+
+/*
+ * Check if arc_size has grown past our upper threshold, determined by
+ * zfs_arc_overflow_shift.
+ */
+boolean_t
+arc_is_overflowing(void)
+{
+	/* Always allow at least one block of overflow */
+	int64_t overflow = MAX(SPA_MAXBLOCKSIZE,
+	    arc_c >> zfs_arc_overflow_shift);
+
+	/*
+	 * We just compare the lower bound here for performance reasons. Our
+	 * primary goals are to make sure that the arc never grows without
+	 * bound, and that it can reach its maximum size. This check
+	 * accomplishes both goals. The maximum amount we could run over by is
+	 * 2 * aggsum_borrow_multiplier * NUM_CPUS * the average size of a block
+	 * in the ARC. In practice, that's in the tens of MB, which is low
+	 * enough to be safe.
+	 */
+	return (aggsum_lower_bound(&arc_size) >= (int64_t)arc_c + overflow);
+}
+
+static abd_t *
+arc_get_data_abd(arc_buf_hdr_t *hdr, uint64_t size, void *tag,
+    boolean_t do_adapt)
+{
+	arc_buf_contents_t type = arc_buf_type(hdr);
+
+	arc_get_data_impl(hdr, size, tag, do_adapt);
+	if (type == ARC_BUFC_METADATA) {
+		return (abd_alloc(size, B_TRUE));
+	} else {
+		ASSERT(type == ARC_BUFC_DATA);
+		return (abd_alloc(size, B_FALSE));
+	}
+}
+
+static void *
+arc_get_data_buf(arc_buf_hdr_t *hdr, uint64_t size, void *tag)
+{
+	arc_buf_contents_t type = arc_buf_type(hdr);
+
+	arc_get_data_impl(hdr, size, tag, B_TRUE);
+	if (type == ARC_BUFC_METADATA) {
+		return (zio_buf_alloc(size));
+	} else {
+		ASSERT(type == ARC_BUFC_DATA);
+		return (zio_data_buf_alloc(size));
+	}
+}
+
+/*
+ * Wait for the specified amount of data (in bytes) to be evicted from the
+ * ARC, and for there to be sufficient free memory in the system.  Waiting for
+ * eviction ensures that the memory used by the ARC decreases.  Waiting for
+ * free memory ensures that the system won't run out of free pages, regardless
+ * of ARC behavior and settings.  See arc_lowmem_init().
+ */
+void
+arc_wait_for_eviction(uint64_t amount)
+{
+	mutex_enter(&arc_evict_lock);
+	if (arc_is_overflowing()) {
+		arc_evict_needed = B_TRUE;
+		zthr_wakeup(arc_evict_zthr);
+
+		if (amount != 0) {
+			arc_evict_waiter_t aw;
+			list_link_init(&aw.aew_node);
+			cv_init(&aw.aew_cv, NULL, CV_DEFAULT, NULL);
+
+			arc_evict_waiter_t *last =
+			    list_tail(&arc_evict_waiters);
+			if (last != NULL) {
+				ASSERT3U(last->aew_count, >, arc_evict_count);
+				aw.aew_count = last->aew_count + amount;
+			} else {
+				aw.aew_count = arc_evict_count + amount;
+			}
+
+			list_insert_tail(&arc_evict_waiters, &aw);
+
+			arc_set_need_free();
+
+			DTRACE_PROBE3(arc__wait__for__eviction,
+			    uint64_t, amount,
+			    uint64_t, arc_evict_count,
+			    uint64_t, aw.aew_count);
+
+			/*
+			 * We will be woken up either when arc_evict_count
+			 * reaches aew_count, or when the ARC is no longer
+			 * overflowing and eviction completes.
+			 */
+			cv_wait(&aw.aew_cv, &arc_evict_lock);
+
+			/*
+			 * In case of "false" wakeup, we will still be on the
+			 * list.
+			 */
+			if (list_link_active(&aw.aew_node))
+				list_remove(&arc_evict_waiters, &aw);
+
+			cv_destroy(&aw.aew_cv);
+		}
+	}
+	mutex_exit(&arc_evict_lock);
+}
+
+/*
+ * Allocate a block and return it to the caller. If we are hitting the
+ * hard limit for the cache size, we must sleep, waiting for the eviction
+ * thread to catch up. If we're past the target size but below the hard
+ * limit, we'll only signal the reclaim thread and continue on.
+ */
+static void
+arc_get_data_impl(arc_buf_hdr_t *hdr, uint64_t size, void *tag,
+    boolean_t do_adapt)
+{
+	arc_state_t *state = hdr->b_l1hdr.b_state;
+	arc_buf_contents_t type = arc_buf_type(hdr);
+
+	if (do_adapt)
+		arc_adapt(size, state);
+
+	/*
+	 * If arc_size is currently overflowing, we must be adding data
+	 * faster than we are evicting.  To ensure we don't compound the
+	 * problem by adding more data and forcing arc_size to grow even
+	 * further past it's target size, we wait for the eviction thread to
+	 * make some progress.  We also wait for there to be sufficient free
+	 * memory in the system, as measured by arc_free_memory().
+	 *
+	 * Specifically, we wait for zfs_arc_eviction_pct percent of the
+	 * requested size to be evicted.  This should be more than 100%, to
+	 * ensure that that progress is also made towards getting arc_size
+	 * under arc_c.  See the comment above zfs_arc_eviction_pct.
+	 *
+	 * We do the overflowing check without holding the arc_evict_lock to
+	 * reduce lock contention in this hot path.  Note that
+	 * arc_wait_for_eviction() will acquire the lock and check again to
+	 * ensure we are truly overflowing before blocking.
+	 */
+	if (arc_is_overflowing()) {
+		arc_wait_for_eviction(size *
+		    zfs_arc_eviction_pct / 100);
+	}
+
+	VERIFY3U(hdr->b_type, ==, type);
+	if (type == ARC_BUFC_METADATA) {
+		arc_space_consume(size, ARC_SPACE_META);
+	} else {
+		arc_space_consume(size, ARC_SPACE_DATA);
+	}
+
+	/*
+	 * Update the state size.  Note that ghost states have a
+	 * "ghost size" and so don't need to be updated.
+	 */
+	if (!GHOST_STATE(state)) {
+
+		(void) zfs_refcount_add_many(&state->arcs_size, size, tag);
+
+		/*
+		 * If this is reached via arc_read, the link is
+		 * protected by the hash lock. If reached via
+		 * arc_buf_alloc, the header should not be accessed by
+		 * any other thread. And, if reached via arc_read_done,
+		 * the hash lock will protect it if it's found in the
+		 * hash table; otherwise no other thread should be
+		 * trying to [add|remove]_reference it.
+		 */
+		if (multilist_link_active(&hdr->b_l1hdr.b_arc_node)) {
+			ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
+			(void) zfs_refcount_add_many(&state->arcs_esize[type],
+			    size, tag);
+		}
+
+		/*
+		 * If we are growing the cache, and we are adding anonymous
+		 * data, and we have outgrown arc_p, update arc_p
+		 */
+		if (aggsum_upper_bound(&arc_size) < arc_c &&
+		    hdr->b_l1hdr.b_state == arc_anon &&
+		    (zfs_refcount_count(&arc_anon->arcs_size) +
+		    zfs_refcount_count(&arc_mru->arcs_size) > arc_p))
+			arc_p = MIN(arc_c, arc_p + size);
+	}
+}
+
+static void
+arc_free_data_abd(arc_buf_hdr_t *hdr, abd_t *abd, uint64_t size, void *tag)
+{
+	arc_free_data_impl(hdr, size, tag);
+	abd_free(abd);
+}
+
+static void
+arc_free_data_buf(arc_buf_hdr_t *hdr, void *buf, uint64_t size, void *tag)
+{
+	arc_buf_contents_t type = arc_buf_type(hdr);
+
+	arc_free_data_impl(hdr, size, tag);
+	if (type == ARC_BUFC_METADATA) {
+		zio_buf_free(buf, size);
+	} else {
+		ASSERT(type == ARC_BUFC_DATA);
+		zio_data_buf_free(buf, size);
+	}
+}
+
+/*
+ * Free the arc data buffer.
+ */
+static void
+arc_free_data_impl(arc_buf_hdr_t *hdr, uint64_t size, void *tag)
+{
+	arc_state_t *state = hdr->b_l1hdr.b_state;
+	arc_buf_contents_t type = arc_buf_type(hdr);
+
+	/* protected by hash lock, if in the hash table */
+	if (multilist_link_active(&hdr->b_l1hdr.b_arc_node)) {
+		ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
+		ASSERT(state != arc_anon && state != arc_l2c_only);
+
+		(void) zfs_refcount_remove_many(&state->arcs_esize[type],
+		    size, tag);
+	}
+	(void) zfs_refcount_remove_many(&state->arcs_size, size, tag);
+
+	VERIFY3U(hdr->b_type, ==, type);
+	if (type == ARC_BUFC_METADATA) {
+		arc_space_return(size, ARC_SPACE_META);
+	} else {
+		ASSERT(type == ARC_BUFC_DATA);
+		arc_space_return(size, ARC_SPACE_DATA);
+	}
+}
+
+/*
+ * This routine is called whenever a buffer is accessed.
+ * NOTE: the hash lock is dropped in this function.
+ */
+static void
+arc_access(arc_buf_hdr_t *hdr, kmutex_t *hash_lock)
+{
+	clock_t now;
+
+	ASSERT(MUTEX_HELD(hash_lock));
+	ASSERT(HDR_HAS_L1HDR(hdr));
+
+	if (hdr->b_l1hdr.b_state == arc_anon) {
+		/*
+		 * This buffer is not in the cache, and does not
+		 * appear in our "ghost" list.  Add the new buffer
+		 * to the MRU state.
+		 */
+
+		ASSERT0(hdr->b_l1hdr.b_arc_access);
+		hdr->b_l1hdr.b_arc_access = ddi_get_lbolt();
+		DTRACE_PROBE1(new_state__mru, arc_buf_hdr_t *, hdr);
+		arc_change_state(arc_mru, hdr, hash_lock);
+
+	} else if (hdr->b_l1hdr.b_state == arc_mru) {
+		now = ddi_get_lbolt();
+
+		/*
+		 * If this buffer is here because of a prefetch, then either:
+		 * - clear the flag if this is a "referencing" read
+		 *   (any subsequent access will bump this into the MFU state).
+		 * or
+		 * - move the buffer to the head of the list if this is
+		 *   another prefetch (to make it less likely to be evicted).
+		 */
+		if (HDR_PREFETCH(hdr) || HDR_PRESCIENT_PREFETCH(hdr)) {
+			if (zfs_refcount_count(&hdr->b_l1hdr.b_refcnt) == 0) {
+				/* link protected by hash lock */
+				ASSERT(multilist_link_active(
+				    &hdr->b_l1hdr.b_arc_node));
+			} else {
+				arc_hdr_clear_flags(hdr,
+				    ARC_FLAG_PREFETCH |
+				    ARC_FLAG_PRESCIENT_PREFETCH);
+				atomic_inc_32(&hdr->b_l1hdr.b_mru_hits);
+				ARCSTAT_BUMP(arcstat_mru_hits);
+			}
+			hdr->b_l1hdr.b_arc_access = now;
+			return;
+		}
+
+		/*
+		 * This buffer has been "accessed" only once so far,
+		 * but it is still in the cache. Move it to the MFU
+		 * state.
+		 */
+		if (ddi_time_after(now, hdr->b_l1hdr.b_arc_access +
+		    ARC_MINTIME)) {
+			/*
+			 * More than 125ms have passed since we
+			 * instantiated this buffer.  Move it to the
+			 * most frequently used state.
+			 */
+			hdr->b_l1hdr.b_arc_access = now;
+			DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, hdr);
+			arc_change_state(arc_mfu, hdr, hash_lock);
+		}
+		atomic_inc_32(&hdr->b_l1hdr.b_mru_hits);
+		ARCSTAT_BUMP(arcstat_mru_hits);
+	} else if (hdr->b_l1hdr.b_state == arc_mru_ghost) {
+		arc_state_t	*new_state;
+		/*
+		 * This buffer has been "accessed" recently, but
+		 * was evicted from the cache.  Move it to the
+		 * MFU state.
+		 */
+
+		if (HDR_PREFETCH(hdr) || HDR_PRESCIENT_PREFETCH(hdr)) {
+			new_state = arc_mru;
+			if (zfs_refcount_count(&hdr->b_l1hdr.b_refcnt) > 0) {
+				arc_hdr_clear_flags(hdr,
+				    ARC_FLAG_PREFETCH |
+				    ARC_FLAG_PRESCIENT_PREFETCH);
+			}
+			DTRACE_PROBE1(new_state__mru, arc_buf_hdr_t *, hdr);
+		} else {
+			new_state = arc_mfu;
+			DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, hdr);
+		}
+
+		hdr->b_l1hdr.b_arc_access = ddi_get_lbolt();
+		arc_change_state(new_state, hdr, hash_lock);
+
+		atomic_inc_32(&hdr->b_l1hdr.b_mru_ghost_hits);
+		ARCSTAT_BUMP(arcstat_mru_ghost_hits);
+	} else if (hdr->b_l1hdr.b_state == arc_mfu) {
+		/*
+		 * This buffer has been accessed more than once and is
+		 * still in the cache.  Keep it in the MFU state.
+		 *
+		 * NOTE: an add_reference() that occurred when we did
+		 * the arc_read() will have kicked this off the list.
+		 * If it was a prefetch, we will explicitly move it to
+		 * the head of the list now.
+		 */
+
+		atomic_inc_32(&hdr->b_l1hdr.b_mfu_hits);
+		ARCSTAT_BUMP(arcstat_mfu_hits);
+		hdr->b_l1hdr.b_arc_access = ddi_get_lbolt();
+	} else if (hdr->b_l1hdr.b_state == arc_mfu_ghost) {
+		arc_state_t	*new_state = arc_mfu;
+		/*
+		 * This buffer has been accessed more than once but has
+		 * been evicted from the cache.  Move it back to the
+		 * MFU state.
+		 */
+
+		if (HDR_PREFETCH(hdr) || HDR_PRESCIENT_PREFETCH(hdr)) {
+			/*
+			 * This is a prefetch access...
+			 * move this block back to the MRU state.
+			 */
+			new_state = arc_mru;
+		}
+
+		hdr->b_l1hdr.b_arc_access = ddi_get_lbolt();
+		DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, hdr);
+		arc_change_state(new_state, hdr, hash_lock);
+
+		atomic_inc_32(&hdr->b_l1hdr.b_mfu_ghost_hits);
+		ARCSTAT_BUMP(arcstat_mfu_ghost_hits);
+	} else if (hdr->b_l1hdr.b_state == arc_l2c_only) {
+		/*
+		 * This buffer is on the 2nd Level ARC.
+		 */
+
+		hdr->b_l1hdr.b_arc_access = ddi_get_lbolt();
+		DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, hdr);
+		arc_change_state(arc_mfu, hdr, hash_lock);
+	} else {
+		cmn_err(CE_PANIC, "invalid arc state 0x%p",
+		    hdr->b_l1hdr.b_state);
+	}
+}
+
+/*
+ * This routine is called by dbuf_hold() to update the arc_access() state
+ * which otherwise would be skipped for entries in the dbuf cache.
+ */
+void
+arc_buf_access(arc_buf_t *buf)
+{
+	mutex_enter(&buf->b_evict_lock);
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+
+	/*
+	 * Avoid taking the hash_lock when possible as an optimization.
+	 * The header must be checked again under the hash_lock in order
+	 * to handle the case where it is concurrently being released.
+	 */
+	if (hdr->b_l1hdr.b_state == arc_anon || HDR_EMPTY(hdr)) {
+		mutex_exit(&buf->b_evict_lock);
+		return;
+	}
+
+	kmutex_t *hash_lock = HDR_LOCK(hdr);
+	mutex_enter(hash_lock);
+
+	if (hdr->b_l1hdr.b_state == arc_anon || HDR_EMPTY(hdr)) {
+		mutex_exit(hash_lock);
+		mutex_exit(&buf->b_evict_lock);
+		ARCSTAT_BUMP(arcstat_access_skip);
+		return;
+	}
+
+	mutex_exit(&buf->b_evict_lock);
+
+	ASSERT(hdr->b_l1hdr.b_state == arc_mru ||
+	    hdr->b_l1hdr.b_state == arc_mfu);
+
+	DTRACE_PROBE1(arc__hit, arc_buf_hdr_t *, hdr);
+	arc_access(hdr, hash_lock);
+	mutex_exit(hash_lock);
+
+	ARCSTAT_BUMP(arcstat_hits);
+	ARCSTAT_CONDSTAT(!HDR_PREFETCH(hdr) && !HDR_PRESCIENT_PREFETCH(hdr),
+	    demand, prefetch, !HDR_ISTYPE_METADATA(hdr), data, metadata, hits);
+}
+
+/* a generic arc_read_done_func_t which you can use */
+/* ARGSUSED */
+void
+arc_bcopy_func(zio_t *zio, const zbookmark_phys_t *zb, const blkptr_t *bp,
+    arc_buf_t *buf, void *arg)
+{
+	if (buf == NULL)
+		return;
+
+	bcopy(buf->b_data, arg, arc_buf_size(buf));
+	arc_buf_destroy(buf, arg);
+}
+
+/* a generic arc_read_done_func_t */
+/* ARGSUSED */
+void
+arc_getbuf_func(zio_t *zio, const zbookmark_phys_t *zb, const blkptr_t *bp,
+    arc_buf_t *buf, void *arg)
+{
+	arc_buf_t **bufp = arg;
+
+	if (buf == NULL) {
+		ASSERT(zio == NULL || zio->io_error != 0);
+		*bufp = NULL;
+	} else {
+		ASSERT(zio == NULL || zio->io_error == 0);
+		*bufp = buf;
+		ASSERT(buf->b_data != NULL);
+	}
+}
+
+static void
+arc_hdr_verify(arc_buf_hdr_t *hdr, blkptr_t *bp)
+{
+	if (BP_IS_HOLE(bp) || BP_IS_EMBEDDED(bp)) {
+		ASSERT3U(HDR_GET_PSIZE(hdr), ==, 0);
+		ASSERT3U(arc_hdr_get_compress(hdr), ==, ZIO_COMPRESS_OFF);
+	} else {
+		if (HDR_COMPRESSION_ENABLED(hdr)) {
+			ASSERT3U(arc_hdr_get_compress(hdr), ==,
+			    BP_GET_COMPRESS(bp));
+		}
+		ASSERT3U(HDR_GET_LSIZE(hdr), ==, BP_GET_LSIZE(bp));
+		ASSERT3U(HDR_GET_PSIZE(hdr), ==, BP_GET_PSIZE(bp));
+		ASSERT3U(!!HDR_PROTECTED(hdr), ==, BP_IS_PROTECTED(bp));
+	}
+}
+
+static void
+arc_read_done(zio_t *zio)
+{
+	blkptr_t 	*bp = zio->io_bp;
+	arc_buf_hdr_t	*hdr = zio->io_private;
+	kmutex_t	*hash_lock = NULL;
+	arc_callback_t	*callback_list;
+	arc_callback_t	*acb;
+	boolean_t	freeable = B_FALSE;
+
+	/*
+	 * The hdr was inserted into hash-table and removed from lists
+	 * prior to starting I/O.  We should find this header, since
+	 * it's in the hash table, and it should be legit since it's
+	 * not possible to evict it during the I/O.  The only possible
+	 * reason for it not to be found is if we were freed during the
+	 * read.
+	 */
+	if (HDR_IN_HASH_TABLE(hdr)) {
+		arc_buf_hdr_t *found;
+
+		ASSERT3U(hdr->b_birth, ==, BP_PHYSICAL_BIRTH(zio->io_bp));
+		ASSERT3U(hdr->b_dva.dva_word[0], ==,
+		    BP_IDENTITY(zio->io_bp)->dva_word[0]);
+		ASSERT3U(hdr->b_dva.dva_word[1], ==,
+		    BP_IDENTITY(zio->io_bp)->dva_word[1]);
+
+		found = buf_hash_find(hdr->b_spa, zio->io_bp, &hash_lock);
+
+		ASSERT((found == hdr &&
+		    DVA_EQUAL(&hdr->b_dva, BP_IDENTITY(zio->io_bp))) ||
+		    (found == hdr && HDR_L2_READING(hdr)));
+		ASSERT3P(hash_lock, !=, NULL);
+	}
+
+	if (BP_IS_PROTECTED(bp)) {
+		hdr->b_crypt_hdr.b_ot = BP_GET_TYPE(bp);
+		hdr->b_crypt_hdr.b_dsobj = zio->io_bookmark.zb_objset;
+		zio_crypt_decode_params_bp(bp, hdr->b_crypt_hdr.b_salt,
+		    hdr->b_crypt_hdr.b_iv);
+
+		if (BP_GET_TYPE(bp) == DMU_OT_INTENT_LOG) {
+			void *tmpbuf;
+
+			tmpbuf = abd_borrow_buf_copy(zio->io_abd,
+			    sizeof (zil_chain_t));
+			zio_crypt_decode_mac_zil(tmpbuf,
+			    hdr->b_crypt_hdr.b_mac);
+			abd_return_buf(zio->io_abd, tmpbuf,
+			    sizeof (zil_chain_t));
+		} else {
+			zio_crypt_decode_mac_bp(bp, hdr->b_crypt_hdr.b_mac);
+		}
+	}
+
+	if (zio->io_error == 0) {
+		/* byteswap if necessary */
+		if (BP_SHOULD_BYTESWAP(zio->io_bp)) {
+			if (BP_GET_LEVEL(zio->io_bp) > 0) {
+				hdr->b_l1hdr.b_byteswap = DMU_BSWAP_UINT64;
+			} else {
+				hdr->b_l1hdr.b_byteswap =
+				    DMU_OT_BYTESWAP(BP_GET_TYPE(zio->io_bp));
+			}
+		} else {
+			hdr->b_l1hdr.b_byteswap = DMU_BSWAP_NUMFUNCS;
+		}
+		if (!HDR_L2_READING(hdr)) {
+			hdr->b_complevel = zio->io_prop.zp_complevel;
+		}
+	}
+
+	arc_hdr_clear_flags(hdr, ARC_FLAG_L2_EVICTED);
+	if (l2arc_noprefetch && HDR_PREFETCH(hdr))
+		arc_hdr_clear_flags(hdr, ARC_FLAG_L2CACHE);
+
+	callback_list = hdr->b_l1hdr.b_acb;
+	ASSERT3P(callback_list, !=, NULL);
+
+	if (hash_lock && zio->io_error == 0 &&
+	    hdr->b_l1hdr.b_state == arc_anon) {
+		/*
+		 * Only call arc_access on anonymous buffers.  This is because
+		 * if we've issued an I/O for an evicted buffer, we've already
+		 * called arc_access (to prevent any simultaneous readers from
+		 * getting confused).
+		 */
+		arc_access(hdr, hash_lock);
+	}
+
+	/*
+	 * If a read request has a callback (i.e. acb_done is not NULL), then we
+	 * make a buf containing the data according to the parameters which were
+	 * passed in. The implementation of arc_buf_alloc_impl() ensures that we
+	 * aren't needlessly decompressing the data multiple times.
+	 */
+	int callback_cnt = 0;
+	for (acb = callback_list; acb != NULL; acb = acb->acb_next) {
+		if (!acb->acb_done)
+			continue;
+
+		callback_cnt++;
+
+		if (zio->io_error != 0)
+			continue;
+
+		int error = arc_buf_alloc_impl(hdr, zio->io_spa,
+		    &acb->acb_zb, acb->acb_private, acb->acb_encrypted,
+		    acb->acb_compressed, acb->acb_noauth, B_TRUE,
+		    &acb->acb_buf);
+
+		/*
+		 * Assert non-speculative zios didn't fail because an
+		 * encryption key wasn't loaded
+		 */
+		ASSERT((zio->io_flags & ZIO_FLAG_SPECULATIVE) ||
+		    error != EACCES);
+
+		/*
+		 * If we failed to decrypt, report an error now (as the zio
+		 * layer would have done if it had done the transforms).
+		 */
+		if (error == ECKSUM) {
+			ASSERT(BP_IS_PROTECTED(bp));
+			error = SET_ERROR(EIO);
+			if ((zio->io_flags & ZIO_FLAG_SPECULATIVE) == 0) {
+				spa_log_error(zio->io_spa, &acb->acb_zb);
+				zfs_ereport_post(FM_EREPORT_ZFS_AUTHENTICATION,
+				    zio->io_spa, NULL, &acb->acb_zb, zio, 0, 0);
+			}
+		}
+
+		if (error != 0) {
+			/*
+			 * Decompression or decryption failed.  Set
+			 * io_error so that when we call acb_done
+			 * (below), we will indicate that the read
+			 * failed. Note that in the unusual case
+			 * where one callback is compressed and another
+			 * uncompressed, we will mark all of them
+			 * as failed, even though the uncompressed
+			 * one can't actually fail.  In this case,
+			 * the hdr will not be anonymous, because
+			 * if there are multiple callbacks, it's
+			 * because multiple threads found the same
+			 * arc buf in the hash table.
+			 */
+			zio->io_error = error;
+		}
+	}
+
+	/*
+	 * If there are multiple callbacks, we must have the hash lock,
+	 * because the only way for multiple threads to find this hdr is
+	 * in the hash table.  This ensures that if there are multiple
+	 * callbacks, the hdr is not anonymous.  If it were anonymous,
+	 * we couldn't use arc_buf_destroy() in the error case below.
+	 */
+	ASSERT(callback_cnt < 2 || hash_lock != NULL);
+
+	hdr->b_l1hdr.b_acb = NULL;
+	arc_hdr_clear_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);
+	if (callback_cnt == 0)
+		ASSERT(hdr->b_l1hdr.b_pabd != NULL || HDR_HAS_RABD(hdr));
+
+	ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt) ||
+	    callback_list != NULL);
+
+	if (zio->io_error == 0) {
+		arc_hdr_verify(hdr, zio->io_bp);
+	} else {
+		arc_hdr_set_flags(hdr, ARC_FLAG_IO_ERROR);
+		if (hdr->b_l1hdr.b_state != arc_anon)
+			arc_change_state(arc_anon, hdr, hash_lock);
+		if (HDR_IN_HASH_TABLE(hdr))
+			buf_hash_remove(hdr);
+		freeable = zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt);
+	}
+
+	/*
+	 * Broadcast before we drop the hash_lock to avoid the possibility
+	 * that the hdr (and hence the cv) might be freed before we get to
+	 * the cv_broadcast().
+	 */
+	cv_broadcast(&hdr->b_l1hdr.b_cv);
+
+	if (hash_lock != NULL) {
+		mutex_exit(hash_lock);
+	} else {
+		/*
+		 * This block was freed while we waited for the read to
+		 * complete.  It has been removed from the hash table and
+		 * moved to the anonymous state (so that it won't show up
+		 * in the cache).
+		 */
+		ASSERT3P(hdr->b_l1hdr.b_state, ==, arc_anon);
+		freeable = zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt);
+	}
+
+	/* execute each callback and free its structure */
+	while ((acb = callback_list) != NULL) {
+		if (acb->acb_done != NULL) {
+			if (zio->io_error != 0 && acb->acb_buf != NULL) {
+				/*
+				 * If arc_buf_alloc_impl() fails during
+				 * decompression, the buf will still be
+				 * allocated, and needs to be freed here.
+				 */
+				arc_buf_destroy(acb->acb_buf,
+				    acb->acb_private);
+				acb->acb_buf = NULL;
+			}
+			acb->acb_done(zio, &zio->io_bookmark, zio->io_bp,
+			    acb->acb_buf, acb->acb_private);
+		}
+
+		if (acb->acb_zio_dummy != NULL) {
+			acb->acb_zio_dummy->io_error = zio->io_error;
+			zio_nowait(acb->acb_zio_dummy);
+		}
+
+		callback_list = acb->acb_next;
+		kmem_free(acb, sizeof (arc_callback_t));
+	}
+
+	if (freeable)
+		arc_hdr_destroy(hdr);
+}
+
+/*
+ * "Read" the block at the specified DVA (in bp) via the
+ * cache.  If the block is found in the cache, invoke the provided
+ * callback immediately and return.  Note that the `zio' parameter
+ * in the callback will be NULL in this case, since no IO was
+ * required.  If the block is not in the cache pass the read request
+ * on to the spa with a substitute callback function, so that the
+ * requested block will be added to the cache.
+ *
+ * If a read request arrives for a block that has a read in-progress,
+ * either wait for the in-progress read to complete (and return the
+ * results); or, if this is a read with a "done" func, add a record
+ * to the read to invoke the "done" func when the read completes,
+ * and return; or just return.
+ *
+ * arc_read_done() will invoke all the requested "done" functions
+ * for readers of this block.
+ */
+int
+arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
+    arc_read_done_func_t *done, void *private, zio_priority_t priority,
+    int zio_flags, arc_flags_t *arc_flags, const zbookmark_phys_t *zb)
+{
+	arc_buf_hdr_t *hdr = NULL;
+	kmutex_t *hash_lock = NULL;
+	zio_t *rzio;
+	uint64_t guid = spa_load_guid(spa);
+	boolean_t compressed_read = (zio_flags & ZIO_FLAG_RAW_COMPRESS) != 0;
+	boolean_t encrypted_read = BP_IS_ENCRYPTED(bp) &&
+	    (zio_flags & ZIO_FLAG_RAW_ENCRYPT) != 0;
+	boolean_t noauth_read = BP_IS_AUTHENTICATED(bp) &&
+	    (zio_flags & ZIO_FLAG_RAW_ENCRYPT) != 0;
+	boolean_t embedded_bp = !!BP_IS_EMBEDDED(bp);
+	int rc = 0;
+
+	ASSERT(!embedded_bp ||
+	    BPE_GET_ETYPE(bp) == BP_EMBEDDED_TYPE_DATA);
+	ASSERT(!BP_IS_HOLE(bp));
+	ASSERT(!BP_IS_REDACTED(bp));
+
+	/*
+	 * Normally SPL_FSTRANS will already be set since kernel threads which
+	 * expect to call the DMU interfaces will set it when created.  System
+	 * calls are similarly handled by setting/cleaning the bit in the
+	 * registered callback (module/os/.../zfs/zpl_*).
+	 *
+	 * External consumers such as Lustre which call the exported DMU
+	 * interfaces may not have set SPL_FSTRANS.  To avoid a deadlock
+	 * on the hash_lock always set and clear the bit.
+	 */
+	fstrans_cookie_t cookie = spl_fstrans_mark();
+top:
+	if (!embedded_bp) {
+		/*
+		 * Embedded BP's have no DVA and require no I/O to "read".
+		 * Create an anonymous arc buf to back it.
+		 */
+		hdr = buf_hash_find(guid, bp, &hash_lock);
+	}
+
+	/*
+	 * Determine if we have an L1 cache hit or a cache miss. For simplicity
+	 * we maintain encrypted data separately from compressed / uncompressed
+	 * data. If the user is requesting raw encrypted data and we don't have
+	 * that in the header we will read from disk to guarantee that we can
+	 * get it even if the encryption keys aren't loaded.
+	 */
+	if (hdr != NULL && HDR_HAS_L1HDR(hdr) && (HDR_HAS_RABD(hdr) ||
+	    (hdr->b_l1hdr.b_pabd != NULL && !encrypted_read))) {
+		arc_buf_t *buf = NULL;
+		*arc_flags |= ARC_FLAG_CACHED;
+
+		if (HDR_IO_IN_PROGRESS(hdr)) {
+			zio_t *head_zio = hdr->b_l1hdr.b_acb->acb_zio_head;
+
+			if (*arc_flags & ARC_FLAG_CACHED_ONLY) {
+				mutex_exit(hash_lock);
+				ARCSTAT_BUMP(arcstat_cached_only_in_progress);
+				rc = SET_ERROR(ENOENT);
+				goto out;
+			}
+
+			ASSERT3P(head_zio, !=, NULL);
+			if ((hdr->b_flags & ARC_FLAG_PRIO_ASYNC_READ) &&
+			    priority == ZIO_PRIORITY_SYNC_READ) {
+				/*
+				 * This is a sync read that needs to wait for
+				 * an in-flight async read. Request that the
+				 * zio have its priority upgraded.
+				 */
+				zio_change_priority(head_zio, priority);
+				DTRACE_PROBE1(arc__async__upgrade__sync,
+				    arc_buf_hdr_t *, hdr);
+				ARCSTAT_BUMP(arcstat_async_upgrade_sync);
+			}
+			if (hdr->b_flags & ARC_FLAG_PREDICTIVE_PREFETCH) {
+				arc_hdr_clear_flags(hdr,
+				    ARC_FLAG_PREDICTIVE_PREFETCH);
+			}
+
+			if (*arc_flags & ARC_FLAG_WAIT) {
+				cv_wait(&hdr->b_l1hdr.b_cv, hash_lock);
+				mutex_exit(hash_lock);
+				goto top;
+			}
+			ASSERT(*arc_flags & ARC_FLAG_NOWAIT);
+
+			if (done) {
+				arc_callback_t *acb = NULL;
+
+				acb = kmem_zalloc(sizeof (arc_callback_t),
+				    KM_SLEEP);
+				acb->acb_done = done;
+				acb->acb_private = private;
+				acb->acb_compressed = compressed_read;
+				acb->acb_encrypted = encrypted_read;
+				acb->acb_noauth = noauth_read;
+				acb->acb_zb = *zb;
+				if (pio != NULL)
+					acb->acb_zio_dummy = zio_null(pio,
+					    spa, NULL, NULL, NULL, zio_flags);
+
+				ASSERT3P(acb->acb_done, !=, NULL);
+				acb->acb_zio_head = head_zio;
+				acb->acb_next = hdr->b_l1hdr.b_acb;
+				hdr->b_l1hdr.b_acb = acb;
+				mutex_exit(hash_lock);
+				goto out;
+			}
+			mutex_exit(hash_lock);
+			goto out;
+		}
+
+		ASSERT(hdr->b_l1hdr.b_state == arc_mru ||
+		    hdr->b_l1hdr.b_state == arc_mfu);
+
+		if (done) {
+			if (hdr->b_flags & ARC_FLAG_PREDICTIVE_PREFETCH) {
+				/*
+				 * This is a demand read which does not have to
+				 * wait for i/o because we did a predictive
+				 * prefetch i/o for it, which has completed.
+				 */
+				DTRACE_PROBE1(
+				    arc__demand__hit__predictive__prefetch,
+				    arc_buf_hdr_t *, hdr);
+				ARCSTAT_BUMP(
+				    arcstat_demand_hit_predictive_prefetch);
+				arc_hdr_clear_flags(hdr,
+				    ARC_FLAG_PREDICTIVE_PREFETCH);
+			}
+
+			if (hdr->b_flags & ARC_FLAG_PRESCIENT_PREFETCH) {
+				ARCSTAT_BUMP(
+				    arcstat_demand_hit_prescient_prefetch);
+				arc_hdr_clear_flags(hdr,
+				    ARC_FLAG_PRESCIENT_PREFETCH);
+			}
+
+			ASSERT(!embedded_bp || !BP_IS_HOLE(bp));
+
+			/* Get a buf with the desired data in it. */
+			rc = arc_buf_alloc_impl(hdr, spa, zb, private,
+			    encrypted_read, compressed_read, noauth_read,
+			    B_TRUE, &buf);
+			if (rc == ECKSUM) {
+				/*
+				 * Convert authentication and decryption errors
+				 * to EIO (and generate an ereport if needed)
+				 * before leaving the ARC.
+				 */
+				rc = SET_ERROR(EIO);
+				if ((zio_flags & ZIO_FLAG_SPECULATIVE) == 0) {
+					spa_log_error(spa, zb);
+					zfs_ereport_post(
+					    FM_EREPORT_ZFS_AUTHENTICATION,
+					    spa, NULL, zb, NULL, 0, 0);
+				}
+			}
+			if (rc != 0) {
+				(void) remove_reference(hdr, hash_lock,
+				    private);
+				arc_buf_destroy_impl(buf);
+				buf = NULL;
+			}
+
+			/* assert any errors weren't due to unloaded keys */
+			ASSERT((zio_flags & ZIO_FLAG_SPECULATIVE) ||
+			    rc != EACCES);
+		} else if (*arc_flags & ARC_FLAG_PREFETCH &&
+		    zfs_refcount_count(&hdr->b_l1hdr.b_refcnt) == 0) {
+			arc_hdr_set_flags(hdr, ARC_FLAG_PREFETCH);
+		}
+		DTRACE_PROBE1(arc__hit, arc_buf_hdr_t *, hdr);
+		arc_access(hdr, hash_lock);
+		if (*arc_flags & ARC_FLAG_PRESCIENT_PREFETCH)
+			arc_hdr_set_flags(hdr, ARC_FLAG_PRESCIENT_PREFETCH);
+		if (*arc_flags & ARC_FLAG_L2CACHE)
+			arc_hdr_set_flags(hdr, ARC_FLAG_L2CACHE);
+		mutex_exit(hash_lock);
+		ARCSTAT_BUMP(arcstat_hits);
+		ARCSTAT_CONDSTAT(!HDR_PREFETCH(hdr),
+		    demand, prefetch, !HDR_ISTYPE_METADATA(hdr),
+		    data, metadata, hits);
+
+		if (done)
+			done(NULL, zb, bp, buf, private);
+	} else {
+		uint64_t lsize = BP_GET_LSIZE(bp);
+		uint64_t psize = BP_GET_PSIZE(bp);
+		arc_callback_t *acb;
+		vdev_t *vd = NULL;
+		uint64_t addr = 0;
+		boolean_t devw = B_FALSE;
+		uint64_t size;
+		abd_t *hdr_abd;
+		int alloc_flags = encrypted_read ? ARC_HDR_ALLOC_RDATA : 0;
+
+		if (*arc_flags & ARC_FLAG_CACHED_ONLY) {
+			rc = SET_ERROR(ENOENT);
+			if (hash_lock != NULL)
+				mutex_exit(hash_lock);
+			goto out;
+		}
+
+		/*
+		 * Gracefully handle a damaged logical block size as a
+		 * checksum error.
+		 */
+		if (lsize > spa_maxblocksize(spa)) {
+			rc = SET_ERROR(ECKSUM);
+			if (hash_lock != NULL)
+				mutex_exit(hash_lock);
+			goto out;
+		}
+
+		if (hdr == NULL) {
+			/*
+			 * This block is not in the cache or it has
+			 * embedded data.
+			 */
+			arc_buf_hdr_t *exists = NULL;
+			arc_buf_contents_t type = BP_GET_BUFC_TYPE(bp);
+			hdr = arc_hdr_alloc(spa_load_guid(spa), psize, lsize,
+			    BP_IS_PROTECTED(bp), BP_GET_COMPRESS(bp), 0, type,
+			    encrypted_read);
+
+			if (!embedded_bp) {
+				hdr->b_dva = *BP_IDENTITY(bp);
+				hdr->b_birth = BP_PHYSICAL_BIRTH(bp);
+				exists = buf_hash_insert(hdr, &hash_lock);
+			}
+			if (exists != NULL) {
+				/* somebody beat us to the hash insert */
+				mutex_exit(hash_lock);
+				buf_discard_identity(hdr);
+				arc_hdr_destroy(hdr);
+				goto top; /* restart the IO request */
+			}
+		} else {
+			/*
+			 * This block is in the ghost cache or encrypted data
+			 * was requested and we didn't have it. If it was
+			 * L2-only (and thus didn't have an L1 hdr),
+			 * we realloc the header to add an L1 hdr.
+			 */
+			if (!HDR_HAS_L1HDR(hdr)) {
+				hdr = arc_hdr_realloc(hdr, hdr_l2only_cache,
+				    hdr_full_cache);
+			}
+
+			if (GHOST_STATE(hdr->b_l1hdr.b_state)) {
+				ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
+				ASSERT(!HDR_HAS_RABD(hdr));
+				ASSERT(!HDR_IO_IN_PROGRESS(hdr));
+				ASSERT0(zfs_refcount_count(
+				    &hdr->b_l1hdr.b_refcnt));
+				ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
+				ASSERT3P(hdr->b_l1hdr.b_freeze_cksum, ==, NULL);
+			} else if (HDR_IO_IN_PROGRESS(hdr)) {
+				/*
+				 * If this header already had an IO in progress
+				 * and we are performing another IO to fetch
+				 * encrypted data we must wait until the first
+				 * IO completes so as not to confuse
+				 * arc_read_done(). This should be very rare
+				 * and so the performance impact shouldn't
+				 * matter.
+				 */
+				cv_wait(&hdr->b_l1hdr.b_cv, hash_lock);
+				mutex_exit(hash_lock);
+				goto top;
+			}
+
+			/*
+			 * This is a delicate dance that we play here.
+			 * This hdr might be in the ghost list so we access
+			 * it to move it out of the ghost list before we
+			 * initiate the read. If it's a prefetch then
+			 * it won't have a callback so we'll remove the
+			 * reference that arc_buf_alloc_impl() created. We
+			 * do this after we've called arc_access() to
+			 * avoid hitting an assert in remove_reference().
+			 */
+			arc_adapt(arc_hdr_size(hdr), hdr->b_l1hdr.b_state);
+			arc_access(hdr, hash_lock);
+			arc_hdr_alloc_abd(hdr, alloc_flags);
+		}
+
+		if (encrypted_read) {
+			ASSERT(HDR_HAS_RABD(hdr));
+			size = HDR_GET_PSIZE(hdr);
+			hdr_abd = hdr->b_crypt_hdr.b_rabd;
+			zio_flags |= ZIO_FLAG_RAW;
+		} else {
+			ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
+			size = arc_hdr_size(hdr);
+			hdr_abd = hdr->b_l1hdr.b_pabd;
+
+			if (arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF) {
+				zio_flags |= ZIO_FLAG_RAW_COMPRESS;
+			}
+
+			/*
+			 * For authenticated bp's, we do not ask the ZIO layer
+			 * to authenticate them since this will cause the entire
+			 * IO to fail if the key isn't loaded. Instead, we
+			 * defer authentication until arc_buf_fill(), which will
+			 * verify the data when the key is available.
+			 */
+			if (BP_IS_AUTHENTICATED(bp))
+				zio_flags |= ZIO_FLAG_RAW_ENCRYPT;
+		}
+
+		if (*arc_flags & ARC_FLAG_PREFETCH &&
+		    zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt))
+			arc_hdr_set_flags(hdr, ARC_FLAG_PREFETCH);
+		if (*arc_flags & ARC_FLAG_PRESCIENT_PREFETCH)
+			arc_hdr_set_flags(hdr, ARC_FLAG_PRESCIENT_PREFETCH);
+		if (*arc_flags & ARC_FLAG_L2CACHE)
+			arc_hdr_set_flags(hdr, ARC_FLAG_L2CACHE);
+		if (BP_IS_AUTHENTICATED(bp))
+			arc_hdr_set_flags(hdr, ARC_FLAG_NOAUTH);
+		if (BP_GET_LEVEL(bp) > 0)
+			arc_hdr_set_flags(hdr, ARC_FLAG_INDIRECT);
+		if (*arc_flags & ARC_FLAG_PREDICTIVE_PREFETCH)
+			arc_hdr_set_flags(hdr, ARC_FLAG_PREDICTIVE_PREFETCH);
+		ASSERT(!GHOST_STATE(hdr->b_l1hdr.b_state));
+
+		acb = kmem_zalloc(sizeof (arc_callback_t), KM_SLEEP);
+		acb->acb_done = done;
+		acb->acb_private = private;
+		acb->acb_compressed = compressed_read;
+		acb->acb_encrypted = encrypted_read;
+		acb->acb_noauth = noauth_read;
+		acb->acb_zb = *zb;
+
+		ASSERT3P(hdr->b_l1hdr.b_acb, ==, NULL);
+		hdr->b_l1hdr.b_acb = acb;
+		arc_hdr_set_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);
+
+		if (HDR_HAS_L2HDR(hdr) &&
+		    (vd = hdr->b_l2hdr.b_dev->l2ad_vdev) != NULL) {
+			devw = hdr->b_l2hdr.b_dev->l2ad_writing;
+			addr = hdr->b_l2hdr.b_daddr;
+			/*
+			 * Lock out L2ARC device removal.
+			 */
+			if (vdev_is_dead(vd) ||
+			    !spa_config_tryenter(spa, SCL_L2ARC, vd, RW_READER))
+				vd = NULL;
+		}
+
+		/*
+		 * We count both async reads and scrub IOs as asynchronous so
+		 * that both can be upgraded in the event of a cache hit while
+		 * the read IO is still in-flight.
+		 */
+		if (priority == ZIO_PRIORITY_ASYNC_READ ||
+		    priority == ZIO_PRIORITY_SCRUB)
+			arc_hdr_set_flags(hdr, ARC_FLAG_PRIO_ASYNC_READ);
+		else
+			arc_hdr_clear_flags(hdr, ARC_FLAG_PRIO_ASYNC_READ);
+
+		/*
+		 * At this point, we have a level 1 cache miss or a blkptr
+		 * with embedded data.  Try again in L2ARC if possible.
+		 */
+		ASSERT3U(HDR_GET_LSIZE(hdr), ==, lsize);
+
+		/*
+		 * Skip ARC stat bump for block pointers with embedded
+		 * data. The data are read from the blkptr itself via
+		 * decode_embedded_bp_compressed().
+		 */
+		if (!embedded_bp) {
+			DTRACE_PROBE4(arc__miss, arc_buf_hdr_t *, hdr,
+			    blkptr_t *, bp, uint64_t, lsize,
+			    zbookmark_phys_t *, zb);
+			ARCSTAT_BUMP(arcstat_misses);
+			ARCSTAT_CONDSTAT(!HDR_PREFETCH(hdr),
+			    demand, prefetch, !HDR_ISTYPE_METADATA(hdr), data,
+			    metadata, misses);
+		}
+
+		if (vd != NULL && l2arc_ndev != 0 && !(l2arc_norw && devw)) {
+			/*
+			 * Read from the L2ARC if the following are true:
+			 * 1. The L2ARC vdev was previously cached.
+			 * 2. This buffer still has L2ARC metadata.
+			 * 3. This buffer isn't currently writing to the L2ARC.
+			 * 4. The L2ARC entry wasn't evicted, which may
+			 *    also have invalidated the vdev.
+			 * 5. This isn't prefetch and l2arc_noprefetch is set.
+			 */
+			if (HDR_HAS_L2HDR(hdr) &&
+			    !HDR_L2_WRITING(hdr) && !HDR_L2_EVICTED(hdr) &&
+			    !(l2arc_noprefetch && HDR_PREFETCH(hdr))) {
+				l2arc_read_callback_t *cb;
+				abd_t *abd;
+				uint64_t asize;
+
+				DTRACE_PROBE1(l2arc__hit, arc_buf_hdr_t *, hdr);
+				ARCSTAT_BUMP(arcstat_l2_hits);
+				atomic_inc_32(&hdr->b_l2hdr.b_hits);
+
+				cb = kmem_zalloc(sizeof (l2arc_read_callback_t),
+				    KM_SLEEP);
+				cb->l2rcb_hdr = hdr;
+				cb->l2rcb_bp = *bp;
+				cb->l2rcb_zb = *zb;
+				cb->l2rcb_flags = zio_flags;
+
+				/*
+				 * When Compressed ARC is disabled, but the
+				 * L2ARC block is compressed, arc_hdr_size()
+				 * will have returned LSIZE rather than PSIZE.
+				 */
+				if (HDR_GET_COMPRESS(hdr) != ZIO_COMPRESS_OFF &&
+				    !HDR_COMPRESSION_ENABLED(hdr) &&
+				    HDR_GET_PSIZE(hdr) != 0) {
+					size = HDR_GET_PSIZE(hdr);
+				}
+
+				asize = vdev_psize_to_asize(vd, size);
+				if (asize != size) {
+					abd = abd_alloc_for_io(asize,
+					    HDR_ISTYPE_METADATA(hdr));
+					cb->l2rcb_abd = abd;
+				} else {
+					abd = hdr_abd;
+				}
+
+				ASSERT(addr >= VDEV_LABEL_START_SIZE &&
+				    addr + asize <= vd->vdev_psize -
+				    VDEV_LABEL_END_SIZE);
+
+				/*
+				 * l2arc read.  The SCL_L2ARC lock will be
+				 * released by l2arc_read_done().
+				 * Issue a null zio if the underlying buffer
+				 * was squashed to zero size by compression.
+				 */
+				ASSERT3U(arc_hdr_get_compress(hdr), !=,
+				    ZIO_COMPRESS_EMPTY);
+				rzio = zio_read_phys(pio, vd, addr,
+				    asize, abd,
+				    ZIO_CHECKSUM_OFF,
+				    l2arc_read_done, cb, priority,
+				    zio_flags | ZIO_FLAG_DONT_CACHE |
+				    ZIO_FLAG_CANFAIL |
+				    ZIO_FLAG_DONT_PROPAGATE |
+				    ZIO_FLAG_DONT_RETRY, B_FALSE);
+				acb->acb_zio_head = rzio;
+
+				if (hash_lock != NULL)
+					mutex_exit(hash_lock);
+
+				DTRACE_PROBE2(l2arc__read, vdev_t *, vd,
+				    zio_t *, rzio);
+				ARCSTAT_INCR(arcstat_l2_read_bytes,
+				    HDR_GET_PSIZE(hdr));
+
+				if (*arc_flags & ARC_FLAG_NOWAIT) {
+					zio_nowait(rzio);
+					goto out;
+				}
+
+				ASSERT(*arc_flags & ARC_FLAG_WAIT);
+				if (zio_wait(rzio) == 0)
+					goto out;
+
+				/* l2arc read error; goto zio_read() */
+				if (hash_lock != NULL)
+					mutex_enter(hash_lock);
+			} else {
+				DTRACE_PROBE1(l2arc__miss,
+				    arc_buf_hdr_t *, hdr);
+				ARCSTAT_BUMP(arcstat_l2_misses);
+				if (HDR_L2_WRITING(hdr))
+					ARCSTAT_BUMP(arcstat_l2_rw_clash);
+				spa_config_exit(spa, SCL_L2ARC, vd);
+			}
+		} else {
+			if (vd != NULL)
+				spa_config_exit(spa, SCL_L2ARC, vd);
+			/*
+			 * Skip ARC stat bump for block pointers with
+			 * embedded data. The data are read from the blkptr
+			 * itself via decode_embedded_bp_compressed().
+			 */
+			if (l2arc_ndev != 0 && !embedded_bp) {
+				DTRACE_PROBE1(l2arc__miss,
+				    arc_buf_hdr_t *, hdr);
+				ARCSTAT_BUMP(arcstat_l2_misses);
+			}
+		}
+
+		rzio = zio_read(pio, spa, bp, hdr_abd, size,
+		    arc_read_done, hdr, priority, zio_flags, zb);
+		acb->acb_zio_head = rzio;
+
+		if (hash_lock != NULL)
+			mutex_exit(hash_lock);
+
+		if (*arc_flags & ARC_FLAG_WAIT) {
+			rc = zio_wait(rzio);
+			goto out;
+		}
+
+		ASSERT(*arc_flags & ARC_FLAG_NOWAIT);
+		zio_nowait(rzio);
+	}
+
+out:
+	/* embedded bps don't actually go to disk */
+	if (!embedded_bp)
+		spa_read_history_add(spa, zb, *arc_flags);
+	spl_fstrans_unmark(cookie);
+	return (rc);
+}
+
+arc_prune_t *
+arc_add_prune_callback(arc_prune_func_t *func, void *private)
+{
+	arc_prune_t *p;
+
+	p = kmem_alloc(sizeof (*p), KM_SLEEP);
+	p->p_pfunc = func;
+	p->p_private = private;
+	list_link_init(&p->p_node);
+	zfs_refcount_create(&p->p_refcnt);
+
+	mutex_enter(&arc_prune_mtx);
+	zfs_refcount_add(&p->p_refcnt, &arc_prune_list);
+	list_insert_head(&arc_prune_list, p);
+	mutex_exit(&arc_prune_mtx);
+
+	return (p);
+}
+
+void
+arc_remove_prune_callback(arc_prune_t *p)
+{
+	boolean_t wait = B_FALSE;
+	mutex_enter(&arc_prune_mtx);
+	list_remove(&arc_prune_list, p);
+	if (zfs_refcount_remove(&p->p_refcnt, &arc_prune_list) > 0)
+		wait = B_TRUE;
+	mutex_exit(&arc_prune_mtx);
+
+	/* wait for arc_prune_task to finish */
+	if (wait)
+		taskq_wait_outstanding(arc_prune_taskq, 0);
+	ASSERT0(zfs_refcount_count(&p->p_refcnt));
+	zfs_refcount_destroy(&p->p_refcnt);
+	kmem_free(p, sizeof (*p));
+}
+
+/*
+ * Notify the arc that a block was freed, and thus will never be used again.
+ */
+void
+arc_freed(spa_t *spa, const blkptr_t *bp)
+{
+	arc_buf_hdr_t *hdr;
+	kmutex_t *hash_lock;
+	uint64_t guid = spa_load_guid(spa);
+
+	ASSERT(!BP_IS_EMBEDDED(bp));
+
+	hdr = buf_hash_find(guid, bp, &hash_lock);
+	if (hdr == NULL)
+		return;
+
+	/*
+	 * We might be trying to free a block that is still doing I/O
+	 * (i.e. prefetch) or has a reference (i.e. a dedup-ed,
+	 * dmu_sync-ed block). If this block is being prefetched, then it
+	 * would still have the ARC_FLAG_IO_IN_PROGRESS flag set on the hdr
+	 * until the I/O completes. A block may also have a reference if it is
+	 * part of a dedup-ed, dmu_synced write. The dmu_sync() function would
+	 * have written the new block to its final resting place on disk but
+	 * without the dedup flag set. This would have left the hdr in the MRU
+	 * state and discoverable. When the txg finally syncs it detects that
+	 * the block was overridden in open context and issues an override I/O.
+	 * Since this is a dedup block, the override I/O will determine if the
+	 * block is already in the DDT. If so, then it will replace the io_bp
+	 * with the bp from the DDT and allow the I/O to finish. When the I/O
+	 * reaches the done callback, dbuf_write_override_done, it will
+	 * check to see if the io_bp and io_bp_override are identical.
+	 * If they are not, then it indicates that the bp was replaced with
+	 * the bp in the DDT and the override bp is freed. This allows
+	 * us to arrive here with a reference on a block that is being
+	 * freed. So if we have an I/O in progress, or a reference to
+	 * this hdr, then we don't destroy the hdr.
+	 */
+	if (!HDR_HAS_L1HDR(hdr) || (!HDR_IO_IN_PROGRESS(hdr) &&
+	    zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt))) {
+		arc_change_state(arc_anon, hdr, hash_lock);
+		arc_hdr_destroy(hdr);
+		mutex_exit(hash_lock);
+	} else {
+		mutex_exit(hash_lock);
+	}
+
+}
+
+/*
+ * Release this buffer from the cache, making it an anonymous buffer.  This
+ * must be done after a read and prior to modifying the buffer contents.
+ * If the buffer has more than one reference, we must make
+ * a new hdr for the buffer.
+ */
+void
+arc_release(arc_buf_t *buf, void *tag)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+
+	/*
+	 * It would be nice to assert that if its DMU metadata (level >
+	 * 0 || it's the dnode file), then it must be syncing context.
+	 * But we don't know that information at this level.
+	 */
+
+	mutex_enter(&buf->b_evict_lock);
+
+	ASSERT(HDR_HAS_L1HDR(hdr));
+
+	/*
+	 * We don't grab the hash lock prior to this check, because if
+	 * the buffer's header is in the arc_anon state, it won't be
+	 * linked into the hash table.
+	 */
+	if (hdr->b_l1hdr.b_state == arc_anon) {
+		mutex_exit(&buf->b_evict_lock);
+		ASSERT(!HDR_IO_IN_PROGRESS(hdr));
+		ASSERT(!HDR_IN_HASH_TABLE(hdr));
+		ASSERT(!HDR_HAS_L2HDR(hdr));
+		ASSERT(HDR_EMPTY(hdr));
+
+		ASSERT3U(hdr->b_l1hdr.b_bufcnt, ==, 1);
+		ASSERT3S(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt), ==, 1);
+		ASSERT(!list_link_active(&hdr->b_l1hdr.b_arc_node));
+
+		hdr->b_l1hdr.b_arc_access = 0;
+
+		/*
+		 * If the buf is being overridden then it may already
+		 * have a hdr that is not empty.
+		 */
+		buf_discard_identity(hdr);
+		arc_buf_thaw(buf);
+
+		return;
+	}
+
+	kmutex_t *hash_lock = HDR_LOCK(hdr);
+	mutex_enter(hash_lock);
+
+	/*
+	 * This assignment is only valid as long as the hash_lock is
+	 * held, we must be careful not to reference state or the
+	 * b_state field after dropping the lock.
+	 */
+	arc_state_t *state = hdr->b_l1hdr.b_state;
+	ASSERT3P(hash_lock, ==, HDR_LOCK(hdr));
+	ASSERT3P(state, !=, arc_anon);
+
+	/* this buffer is not on any list */
+	ASSERT3S(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt), >, 0);
+
+	if (HDR_HAS_L2HDR(hdr)) {
+		mutex_enter(&hdr->b_l2hdr.b_dev->l2ad_mtx);
+
+		/*
+		 * We have to recheck this conditional again now that
+		 * we're holding the l2ad_mtx to prevent a race with
+		 * another thread which might be concurrently calling
+		 * l2arc_evict(). In that case, l2arc_evict() might have
+		 * destroyed the header's L2 portion as we were waiting
+		 * to acquire the l2ad_mtx.
+		 */
+		if (HDR_HAS_L2HDR(hdr))
+			arc_hdr_l2hdr_destroy(hdr);
+
+		mutex_exit(&hdr->b_l2hdr.b_dev->l2ad_mtx);
+	}
+
+	/*
+	 * Do we have more than one buf?
+	 */
+	if (hdr->b_l1hdr.b_bufcnt > 1) {
+		arc_buf_hdr_t *nhdr;
+		uint64_t spa = hdr->b_spa;
+		uint64_t psize = HDR_GET_PSIZE(hdr);
+		uint64_t lsize = HDR_GET_LSIZE(hdr);
+		boolean_t protected = HDR_PROTECTED(hdr);
+		enum zio_compress compress = arc_hdr_get_compress(hdr);
+		arc_buf_contents_t type = arc_buf_type(hdr);
+		VERIFY3U(hdr->b_type, ==, type);
+
+		ASSERT(hdr->b_l1hdr.b_buf != buf || buf->b_next != NULL);
+		(void) remove_reference(hdr, hash_lock, tag);
+
+		if (arc_buf_is_shared(buf) && !ARC_BUF_COMPRESSED(buf)) {
+			ASSERT3P(hdr->b_l1hdr.b_buf, !=, buf);
+			ASSERT(ARC_BUF_LAST(buf));
+		}
+
+		/*
+		 * Pull the data off of this hdr and attach it to
+		 * a new anonymous hdr. Also find the last buffer
+		 * in the hdr's buffer list.
+		 */
+		arc_buf_t *lastbuf = arc_buf_remove(hdr, buf);
+		ASSERT3P(lastbuf, !=, NULL);
+
+		/*
+		 * If the current arc_buf_t and the hdr are sharing their data
+		 * buffer, then we must stop sharing that block.
+		 */
+		if (arc_buf_is_shared(buf)) {
+			ASSERT3P(hdr->b_l1hdr.b_buf, !=, buf);
+			VERIFY(!arc_buf_is_shared(lastbuf));
+
+			/*
+			 * First, sever the block sharing relationship between
+			 * buf and the arc_buf_hdr_t.
+			 */
+			arc_unshare_buf(hdr, buf);
+
+			/*
+			 * Now we need to recreate the hdr's b_pabd. Since we
+			 * have lastbuf handy, we try to share with it, but if
+			 * we can't then we allocate a new b_pabd and copy the
+			 * data from buf into it.
+			 */
+			if (arc_can_share(hdr, lastbuf)) {
+				arc_share_buf(hdr, lastbuf);
+			} else {
+				arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT);
+				abd_copy_from_buf(hdr->b_l1hdr.b_pabd,
+				    buf->b_data, psize);
+			}
+			VERIFY3P(lastbuf->b_data, !=, NULL);
+		} else if (HDR_SHARED_DATA(hdr)) {
+			/*
+			 * Uncompressed shared buffers are always at the end
+			 * of the list. Compressed buffers don't have the
+			 * same requirements. This makes it hard to
+			 * simply assert that the lastbuf is shared so
+			 * we rely on the hdr's compression flags to determine
+			 * if we have a compressed, shared buffer.
+			 */
+			ASSERT(arc_buf_is_shared(lastbuf) ||
+			    arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF);
+			ASSERT(!ARC_BUF_SHARED(buf));
+		}
+
+		ASSERT(hdr->b_l1hdr.b_pabd != NULL || HDR_HAS_RABD(hdr));
+		ASSERT3P(state, !=, arc_l2c_only);
+
+		(void) zfs_refcount_remove_many(&state->arcs_size,
+		    arc_buf_size(buf), buf);
+
+		if (zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt)) {
+			ASSERT3P(state, !=, arc_l2c_only);
+			(void) zfs_refcount_remove_many(
+			    &state->arcs_esize[type],
+			    arc_buf_size(buf), buf);
+		}
+
+		hdr->b_l1hdr.b_bufcnt -= 1;
+		if (ARC_BUF_ENCRYPTED(buf))
+			hdr->b_crypt_hdr.b_ebufcnt -= 1;
+
+		arc_cksum_verify(buf);
+		arc_buf_unwatch(buf);
+
+		/* if this is the last uncompressed buf free the checksum */
+		if (!arc_hdr_has_uncompressed_buf(hdr))
+			arc_cksum_free(hdr);
+
+		mutex_exit(hash_lock);
+
+		/*
+		 * Allocate a new hdr. The new hdr will contain a b_pabd
+		 * buffer which will be freed in arc_write().
+		 */
+		nhdr = arc_hdr_alloc(spa, psize, lsize, protected,
+		    compress, hdr->b_complevel, type, HDR_HAS_RABD(hdr));
+		ASSERT3P(nhdr->b_l1hdr.b_buf, ==, NULL);
+		ASSERT0(nhdr->b_l1hdr.b_bufcnt);
+		ASSERT0(zfs_refcount_count(&nhdr->b_l1hdr.b_refcnt));
+		VERIFY3U(nhdr->b_type, ==, type);
+		ASSERT(!HDR_SHARED_DATA(nhdr));
+
+		nhdr->b_l1hdr.b_buf = buf;
+		nhdr->b_l1hdr.b_bufcnt = 1;
+		if (ARC_BUF_ENCRYPTED(buf))
+			nhdr->b_crypt_hdr.b_ebufcnt = 1;
+		nhdr->b_l1hdr.b_mru_hits = 0;
+		nhdr->b_l1hdr.b_mru_ghost_hits = 0;
+		nhdr->b_l1hdr.b_mfu_hits = 0;
+		nhdr->b_l1hdr.b_mfu_ghost_hits = 0;
+		nhdr->b_l1hdr.b_l2_hits = 0;
+		(void) zfs_refcount_add(&nhdr->b_l1hdr.b_refcnt, tag);
+		buf->b_hdr = nhdr;
+
+		mutex_exit(&buf->b_evict_lock);
+		(void) zfs_refcount_add_many(&arc_anon->arcs_size,
+		    arc_buf_size(buf), buf);
+	} else {
+		mutex_exit(&buf->b_evict_lock);
+		ASSERT(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt) == 1);
+		/* protected by hash lock, or hdr is on arc_anon */
+		ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));
+		ASSERT(!HDR_IO_IN_PROGRESS(hdr));
+		hdr->b_l1hdr.b_mru_hits = 0;
+		hdr->b_l1hdr.b_mru_ghost_hits = 0;
+		hdr->b_l1hdr.b_mfu_hits = 0;
+		hdr->b_l1hdr.b_mfu_ghost_hits = 0;
+		hdr->b_l1hdr.b_l2_hits = 0;
+		arc_change_state(arc_anon, hdr, hash_lock);
+		hdr->b_l1hdr.b_arc_access = 0;
+
+		mutex_exit(hash_lock);
+		buf_discard_identity(hdr);
+		arc_buf_thaw(buf);
+	}
+}
+
+int
+arc_released(arc_buf_t *buf)
+{
+	int released;
+
+	mutex_enter(&buf->b_evict_lock);
+	released = (buf->b_data != NULL &&
+	    buf->b_hdr->b_l1hdr.b_state == arc_anon);
+	mutex_exit(&buf->b_evict_lock);
+	return (released);
+}
+
+#ifdef ZFS_DEBUG
+int
+arc_referenced(arc_buf_t *buf)
+{
+	int referenced;
+
+	mutex_enter(&buf->b_evict_lock);
+	referenced = (zfs_refcount_count(&buf->b_hdr->b_l1hdr.b_refcnt));
+	mutex_exit(&buf->b_evict_lock);
+	return (referenced);
+}
+#endif
+
+static void
+arc_write_ready(zio_t *zio)
+{
+	arc_write_callback_t *callback = zio->io_private;
+	arc_buf_t *buf = callback->awcb_buf;
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+	blkptr_t *bp = zio->io_bp;
+	uint64_t psize = BP_IS_HOLE(bp) ? 0 : BP_GET_PSIZE(bp);
+	fstrans_cookie_t cookie = spl_fstrans_mark();
+
+	ASSERT(HDR_HAS_L1HDR(hdr));
+	ASSERT(!zfs_refcount_is_zero(&buf->b_hdr->b_l1hdr.b_refcnt));
+	ASSERT(hdr->b_l1hdr.b_bufcnt > 0);
+
+	/*
+	 * If we're reexecuting this zio because the pool suspended, then
+	 * cleanup any state that was previously set the first time the
+	 * callback was invoked.
+	 */
+	if (zio->io_flags & ZIO_FLAG_REEXECUTED) {
+		arc_cksum_free(hdr);
+		arc_buf_unwatch(buf);
+		if (hdr->b_l1hdr.b_pabd != NULL) {
+			if (arc_buf_is_shared(buf)) {
+				arc_unshare_buf(hdr, buf);
+			} else {
+				arc_hdr_free_abd(hdr, B_FALSE);
+			}
+		}
+
+		if (HDR_HAS_RABD(hdr))
+			arc_hdr_free_abd(hdr, B_TRUE);
+	}
+	ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
+	ASSERT(!HDR_HAS_RABD(hdr));
+	ASSERT(!HDR_SHARED_DATA(hdr));
+	ASSERT(!arc_buf_is_shared(buf));
+
+	callback->awcb_ready(zio, buf, callback->awcb_private);
+
+	if (HDR_IO_IN_PROGRESS(hdr))
+		ASSERT(zio->io_flags & ZIO_FLAG_REEXECUTED);
+
+	arc_hdr_set_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);
+
+	if (BP_IS_PROTECTED(bp) != !!HDR_PROTECTED(hdr))
+		hdr = arc_hdr_realloc_crypt(hdr, BP_IS_PROTECTED(bp));
+
+	if (BP_IS_PROTECTED(bp)) {
+		/* ZIL blocks are written through zio_rewrite */
+		ASSERT3U(BP_GET_TYPE(bp), !=, DMU_OT_INTENT_LOG);
+		ASSERT(HDR_PROTECTED(hdr));
+
+		if (BP_SHOULD_BYTESWAP(bp)) {
+			if (BP_GET_LEVEL(bp) > 0) {
+				hdr->b_l1hdr.b_byteswap = DMU_BSWAP_UINT64;
+			} else {
+				hdr->b_l1hdr.b_byteswap =
+				    DMU_OT_BYTESWAP(BP_GET_TYPE(bp));
+			}
+		} else {
+			hdr->b_l1hdr.b_byteswap = DMU_BSWAP_NUMFUNCS;
+		}
+
+		hdr->b_crypt_hdr.b_ot = BP_GET_TYPE(bp);
+		hdr->b_crypt_hdr.b_dsobj = zio->io_bookmark.zb_objset;
+		zio_crypt_decode_params_bp(bp, hdr->b_crypt_hdr.b_salt,
+		    hdr->b_crypt_hdr.b_iv);
+		zio_crypt_decode_mac_bp(bp, hdr->b_crypt_hdr.b_mac);
+	}
+
+	/*
+	 * If this block was written for raw encryption but the zio layer
+	 * ended up only authenticating it, adjust the buffer flags now.
+	 */
+	if (BP_IS_AUTHENTICATED(bp) && ARC_BUF_ENCRYPTED(buf)) {
+		arc_hdr_set_flags(hdr, ARC_FLAG_NOAUTH);
+		buf->b_flags &= ~ARC_BUF_FLAG_ENCRYPTED;
+		if (BP_GET_COMPRESS(bp) == ZIO_COMPRESS_OFF)
+			buf->b_flags &= ~ARC_BUF_FLAG_COMPRESSED;
+	} else if (BP_IS_HOLE(bp) && ARC_BUF_ENCRYPTED(buf)) {
+		buf->b_flags &= ~ARC_BUF_FLAG_ENCRYPTED;
+		buf->b_flags &= ~ARC_BUF_FLAG_COMPRESSED;
+	}
+
+	/* this must be done after the buffer flags are adjusted */
+	arc_cksum_compute(buf);
+
+	enum zio_compress compress;
+	if (BP_IS_HOLE(bp) || BP_IS_EMBEDDED(bp)) {
+		compress = ZIO_COMPRESS_OFF;
+	} else {
+		ASSERT3U(HDR_GET_LSIZE(hdr), ==, BP_GET_LSIZE(bp));
+		compress = BP_GET_COMPRESS(bp);
+	}
+	HDR_SET_PSIZE(hdr, psize);
+	arc_hdr_set_compress(hdr, compress);
+	hdr->b_complevel = zio->io_prop.zp_complevel;
+
+	if (zio->io_error != 0 || psize == 0)
+		goto out;
+
+	/*
+	 * Fill the hdr with data. If the buffer is encrypted we have no choice
+	 * but to copy the data into b_radb. If the hdr is compressed, the data
+	 * we want is available from the zio, otherwise we can take it from
+	 * the buf.
+	 *
+	 * We might be able to share the buf's data with the hdr here. However,
+	 * doing so would cause the ARC to be full of linear ABDs if we write a
+	 * lot of shareable data. As a compromise, we check whether scattered
+	 * ABDs are allowed, and assume that if they are then the user wants
+	 * the ARC to be primarily filled with them regardless of the data being
+	 * written. Therefore, if they're allowed then we allocate one and copy
+	 * the data into it; otherwise, we share the data directly if we can.
+	 */
+	if (ARC_BUF_ENCRYPTED(buf)) {
+		ASSERT3U(psize, >, 0);
+		ASSERT(ARC_BUF_COMPRESSED(buf));
+		arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT|ARC_HDR_ALLOC_RDATA);
+		abd_copy(hdr->b_crypt_hdr.b_rabd, zio->io_abd, psize);
+	} else if (zfs_abd_scatter_enabled || !arc_can_share(hdr, buf)) {
+		/*
+		 * Ideally, we would always copy the io_abd into b_pabd, but the
+		 * user may have disabled compressed ARC, thus we must check the
+		 * hdr's compression setting rather than the io_bp's.
+		 */
+		if (BP_IS_ENCRYPTED(bp)) {
+			ASSERT3U(psize, >, 0);
+			arc_hdr_alloc_abd(hdr,
+			    ARC_HDR_DO_ADAPT|ARC_HDR_ALLOC_RDATA);
+			abd_copy(hdr->b_crypt_hdr.b_rabd, zio->io_abd, psize);
+		} else if (arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF &&
+		    !ARC_BUF_COMPRESSED(buf)) {
+			ASSERT3U(psize, >, 0);
+			arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT);
+			abd_copy(hdr->b_l1hdr.b_pabd, zio->io_abd, psize);
+		} else {
+			ASSERT3U(zio->io_orig_size, ==, arc_hdr_size(hdr));
+			arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT);
+			abd_copy_from_buf(hdr->b_l1hdr.b_pabd, buf->b_data,
+			    arc_buf_size(buf));
+		}
+	} else {
+		ASSERT3P(buf->b_data, ==, abd_to_buf(zio->io_orig_abd));
+		ASSERT3U(zio->io_orig_size, ==, arc_buf_size(buf));
+		ASSERT3U(hdr->b_l1hdr.b_bufcnt, ==, 1);
+
+		arc_share_buf(hdr, buf);
+	}
+
+out:
+	arc_hdr_verify(hdr, bp);
+	spl_fstrans_unmark(cookie);
+}
+
+static void
+arc_write_children_ready(zio_t *zio)
+{
+	arc_write_callback_t *callback = zio->io_private;
+	arc_buf_t *buf = callback->awcb_buf;
+
+	callback->awcb_children_ready(zio, buf, callback->awcb_private);
+}
+
+/*
+ * The SPA calls this callback for each physical write that happens on behalf
+ * of a logical write.  See the comment in dbuf_write_physdone() for details.
+ */
+static void
+arc_write_physdone(zio_t *zio)
+{
+	arc_write_callback_t *cb = zio->io_private;
+	if (cb->awcb_physdone != NULL)
+		cb->awcb_physdone(zio, cb->awcb_buf, cb->awcb_private);
+}
+
+static void
+arc_write_done(zio_t *zio)
+{
+	arc_write_callback_t *callback = zio->io_private;
+	arc_buf_t *buf = callback->awcb_buf;
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+
+	ASSERT3P(hdr->b_l1hdr.b_acb, ==, NULL);
+
+	if (zio->io_error == 0) {
+		arc_hdr_verify(hdr, zio->io_bp);
+
+		if (BP_IS_HOLE(zio->io_bp) || BP_IS_EMBEDDED(zio->io_bp)) {
+			buf_discard_identity(hdr);
+		} else {
+			hdr->b_dva = *BP_IDENTITY(zio->io_bp);
+			hdr->b_birth = BP_PHYSICAL_BIRTH(zio->io_bp);
+		}
+	} else {
+		ASSERT(HDR_EMPTY(hdr));
+	}
+
+	/*
+	 * If the block to be written was all-zero or compressed enough to be
+	 * embedded in the BP, no write was performed so there will be no
+	 * dva/birth/checksum.  The buffer must therefore remain anonymous
+	 * (and uncached).
+	 */
+	if (!HDR_EMPTY(hdr)) {
+		arc_buf_hdr_t *exists;
+		kmutex_t *hash_lock;
+
+		ASSERT3U(zio->io_error, ==, 0);
+
+		arc_cksum_verify(buf);
+
+		exists = buf_hash_insert(hdr, &hash_lock);
+		if (exists != NULL) {
+			/*
+			 * This can only happen if we overwrite for
+			 * sync-to-convergence, because we remove
+			 * buffers from the hash table when we arc_free().
+			 */
+			if (zio->io_flags & ZIO_FLAG_IO_REWRITE) {
+				if (!BP_EQUAL(&zio->io_bp_orig, zio->io_bp))
+					panic("bad overwrite, hdr=%p exists=%p",
+					    (void *)hdr, (void *)exists);
+				ASSERT(zfs_refcount_is_zero(
+				    &exists->b_l1hdr.b_refcnt));
+				arc_change_state(arc_anon, exists, hash_lock);
+				arc_hdr_destroy(exists);
+				mutex_exit(hash_lock);
+				exists = buf_hash_insert(hdr, &hash_lock);
+				ASSERT3P(exists, ==, NULL);
+			} else if (zio->io_flags & ZIO_FLAG_NOPWRITE) {
+				/* nopwrite */
+				ASSERT(zio->io_prop.zp_nopwrite);
+				if (!BP_EQUAL(&zio->io_bp_orig, zio->io_bp))
+					panic("bad nopwrite, hdr=%p exists=%p",
+					    (void *)hdr, (void *)exists);
+			} else {
+				/* Dedup */
+				ASSERT(hdr->b_l1hdr.b_bufcnt == 1);
+				ASSERT(hdr->b_l1hdr.b_state == arc_anon);
+				ASSERT(BP_GET_DEDUP(zio->io_bp));
+				ASSERT(BP_GET_LEVEL(zio->io_bp) == 0);
+			}
+		}
+		arc_hdr_clear_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);
+		/* if it's not anon, we are doing a scrub */
+		if (exists == NULL && hdr->b_l1hdr.b_state == arc_anon)
+			arc_access(hdr, hash_lock);
+		mutex_exit(hash_lock);
+	} else {
+		arc_hdr_clear_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);
+	}
+
+	ASSERT(!zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
+	callback->awcb_done(zio, buf, callback->awcb_private);
+
+	abd_put(zio->io_abd);
+	kmem_free(callback, sizeof (arc_write_callback_t));
+}
+
+zio_t *
+arc_write(zio_t *pio, spa_t *spa, uint64_t txg,
+    blkptr_t *bp, arc_buf_t *buf, boolean_t l2arc,
+    const zio_prop_t *zp, arc_write_done_func_t *ready,
+    arc_write_done_func_t *children_ready, arc_write_done_func_t *physdone,
+    arc_write_done_func_t *done, void *private, zio_priority_t priority,
+    int zio_flags, const zbookmark_phys_t *zb)
+{
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+	arc_write_callback_t *callback;
+	zio_t *zio;
+	zio_prop_t localprop = *zp;
+
+	ASSERT3P(ready, !=, NULL);
+	ASSERT3P(done, !=, NULL);
+	ASSERT(!HDR_IO_ERROR(hdr));
+	ASSERT(!HDR_IO_IN_PROGRESS(hdr));
+	ASSERT3P(hdr->b_l1hdr.b_acb, ==, NULL);
+	ASSERT3U(hdr->b_l1hdr.b_bufcnt, >, 0);
+	if (l2arc)
+		arc_hdr_set_flags(hdr, ARC_FLAG_L2CACHE);
+
+	if (ARC_BUF_ENCRYPTED(buf)) {
+		ASSERT(ARC_BUF_COMPRESSED(buf));
+		localprop.zp_encrypt = B_TRUE;
+		localprop.zp_compress = HDR_GET_COMPRESS(hdr);
+		localprop.zp_complevel = hdr->b_complevel;
+		localprop.zp_byteorder =
+		    (hdr->b_l1hdr.b_byteswap == DMU_BSWAP_NUMFUNCS) ?
+		    ZFS_HOST_BYTEORDER : !ZFS_HOST_BYTEORDER;
+		bcopy(hdr->b_crypt_hdr.b_salt, localprop.zp_salt,
+		    ZIO_DATA_SALT_LEN);
+		bcopy(hdr->b_crypt_hdr.b_iv, localprop.zp_iv,
+		    ZIO_DATA_IV_LEN);
+		bcopy(hdr->b_crypt_hdr.b_mac, localprop.zp_mac,
+		    ZIO_DATA_MAC_LEN);
+		if (DMU_OT_IS_ENCRYPTED(localprop.zp_type)) {
+			localprop.zp_nopwrite = B_FALSE;
+			localprop.zp_copies =
+			    MIN(localprop.zp_copies, SPA_DVAS_PER_BP - 1);
+		}
+		zio_flags |= ZIO_FLAG_RAW;
+	} else if (ARC_BUF_COMPRESSED(buf)) {
+		ASSERT3U(HDR_GET_LSIZE(hdr), !=, arc_buf_size(buf));
+		localprop.zp_compress = HDR_GET_COMPRESS(hdr);
+		localprop.zp_complevel = hdr->b_complevel;
+		zio_flags |= ZIO_FLAG_RAW_COMPRESS;
+	}
+	callback = kmem_zalloc(sizeof (arc_write_callback_t), KM_SLEEP);
+	callback->awcb_ready = ready;
+	callback->awcb_children_ready = children_ready;
+	callback->awcb_physdone = physdone;
+	callback->awcb_done = done;
+	callback->awcb_private = private;
+	callback->awcb_buf = buf;
+
+	/*
+	 * The hdr's b_pabd is now stale, free it now. A new data block
+	 * will be allocated when the zio pipeline calls arc_write_ready().
+	 */
+	if (hdr->b_l1hdr.b_pabd != NULL) {
+		/*
+		 * If the buf is currently sharing the data block with
+		 * the hdr then we need to break that relationship here.
+		 * The hdr will remain with a NULL data pointer and the
+		 * buf will take sole ownership of the block.
+		 */
+		if (arc_buf_is_shared(buf)) {
+			arc_unshare_buf(hdr, buf);
+		} else {
+			arc_hdr_free_abd(hdr, B_FALSE);
+		}
+		VERIFY3P(buf->b_data, !=, NULL);
+	}
+
+	if (HDR_HAS_RABD(hdr))
+		arc_hdr_free_abd(hdr, B_TRUE);
+
+	if (!(zio_flags & ZIO_FLAG_RAW))
+		arc_hdr_set_compress(hdr, ZIO_COMPRESS_OFF);
+
+	ASSERT(!arc_buf_is_shared(buf));
+	ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
+
+	zio = zio_write(pio, spa, txg, bp,
+	    abd_get_from_buf(buf->b_data, HDR_GET_LSIZE(hdr)),
+	    HDR_GET_LSIZE(hdr), arc_buf_size(buf), &localprop, arc_write_ready,
+	    (children_ready != NULL) ? arc_write_children_ready : NULL,
+	    arc_write_physdone, arc_write_done, callback,
+	    priority, zio_flags, zb);
+
+	return (zio);
+}
+
+void
+arc_tempreserve_clear(uint64_t reserve)
+{
+	atomic_add_64(&arc_tempreserve, -reserve);
+	ASSERT((int64_t)arc_tempreserve >= 0);
+}
+
+int
+arc_tempreserve_space(spa_t *spa, uint64_t reserve, uint64_t txg)
+{
+	int error;
+	uint64_t anon_size;
+
+	if (!arc_no_grow &&
+	    reserve > arc_c/4 &&
+	    reserve * 4 > (2ULL << SPA_MAXBLOCKSHIFT))
+		arc_c = MIN(arc_c_max, reserve * 4);
+
+	/*
+	 * Throttle when the calculated memory footprint for the TXG
+	 * exceeds the target ARC size.
+	 */
+	if (reserve > arc_c) {
+		DMU_TX_STAT_BUMP(dmu_tx_memory_reserve);
+		return (SET_ERROR(ERESTART));
+	}
+
+	/*
+	 * Don't count loaned bufs as in flight dirty data to prevent long
+	 * network delays from blocking transactions that are ready to be
+	 * assigned to a txg.
+	 */
+
+	/* assert that it has not wrapped around */
+	ASSERT3S(atomic_add_64_nv(&arc_loaned_bytes, 0), >=, 0);
+
+	anon_size = MAX((int64_t)(zfs_refcount_count(&arc_anon->arcs_size) -
+	    arc_loaned_bytes), 0);
+
+	/*
+	 * Writes will, almost always, require additional memory allocations
+	 * in order to compress/encrypt/etc the data.  We therefore need to
+	 * make sure that there is sufficient available memory for this.
+	 */
+	error = arc_memory_throttle(spa, reserve, txg);
+	if (error != 0)
+		return (error);
+
+	/*
+	 * Throttle writes when the amount of dirty data in the cache
+	 * gets too large.  We try to keep the cache less than half full
+	 * of dirty blocks so that our sync times don't grow too large.
+	 *
+	 * In the case of one pool being built on another pool, we want
+	 * to make sure we don't end up throttling the lower (backing)
+	 * pool when the upper pool is the majority contributor to dirty
+	 * data. To insure we make forward progress during throttling, we
+	 * also check the current pool's net dirty data and only throttle
+	 * if it exceeds zfs_arc_pool_dirty_percent of the anonymous dirty
+	 * data in the cache.
+	 *
+	 * Note: if two requests come in concurrently, we might let them
+	 * both succeed, when one of them should fail.  Not a huge deal.
+	 */
+	uint64_t total_dirty = reserve + arc_tempreserve + anon_size;
+	uint64_t spa_dirty_anon = spa_dirty_data(spa);
+
+	if (total_dirty > arc_c * zfs_arc_dirty_limit_percent / 100 &&
+	    anon_size > arc_c * zfs_arc_anon_limit_percent / 100 &&
+	    spa_dirty_anon > anon_size * zfs_arc_pool_dirty_percent / 100) {
+#ifdef ZFS_DEBUG
+		uint64_t meta_esize = zfs_refcount_count(
+		    &arc_anon->arcs_esize[ARC_BUFC_METADATA]);
+		uint64_t data_esize =
+		    zfs_refcount_count(&arc_anon->arcs_esize[ARC_BUFC_DATA]);
+		dprintf("failing, arc_tempreserve=%lluK anon_meta=%lluK "
+		    "anon_data=%lluK tempreserve=%lluK arc_c=%lluK\n",
+		    arc_tempreserve >> 10, meta_esize >> 10,
+		    data_esize >> 10, reserve >> 10, arc_c >> 10);
+#endif
+		DMU_TX_STAT_BUMP(dmu_tx_dirty_throttle);
+		return (SET_ERROR(ERESTART));
+	}
+	atomic_add_64(&arc_tempreserve, reserve);
+	return (0);
+}
+
+static void
+arc_kstat_update_state(arc_state_t *state, kstat_named_t *size,
+    kstat_named_t *evict_data, kstat_named_t *evict_metadata)
+{
+	size->value.ui64 = zfs_refcount_count(&state->arcs_size);
+	evict_data->value.ui64 =
+	    zfs_refcount_count(&state->arcs_esize[ARC_BUFC_DATA]);
+	evict_metadata->value.ui64 =
+	    zfs_refcount_count(&state->arcs_esize[ARC_BUFC_METADATA]);
+}
+
+static int
+arc_kstat_update(kstat_t *ksp, int rw)
+{
+	arc_stats_t *as = ksp->ks_data;
+
+	if (rw == KSTAT_WRITE) {
+		return (SET_ERROR(EACCES));
+	} else {
+		arc_kstat_update_state(arc_anon,
+		    &as->arcstat_anon_size,
+		    &as->arcstat_anon_evictable_data,
+		    &as->arcstat_anon_evictable_metadata);
+		arc_kstat_update_state(arc_mru,
+		    &as->arcstat_mru_size,
+		    &as->arcstat_mru_evictable_data,
+		    &as->arcstat_mru_evictable_metadata);
+		arc_kstat_update_state(arc_mru_ghost,
+		    &as->arcstat_mru_ghost_size,
+		    &as->arcstat_mru_ghost_evictable_data,
+		    &as->arcstat_mru_ghost_evictable_metadata);
+		arc_kstat_update_state(arc_mfu,
+		    &as->arcstat_mfu_size,
+		    &as->arcstat_mfu_evictable_data,
+		    &as->arcstat_mfu_evictable_metadata);
+		arc_kstat_update_state(arc_mfu_ghost,
+		    &as->arcstat_mfu_ghost_size,
+		    &as->arcstat_mfu_ghost_evictable_data,
+		    &as->arcstat_mfu_ghost_evictable_metadata);
+
+		ARCSTAT(arcstat_size) = aggsum_value(&arc_size);
+		ARCSTAT(arcstat_meta_used) = aggsum_value(&arc_meta_used);
+		ARCSTAT(arcstat_data_size) = aggsum_value(&astat_data_size);
+		ARCSTAT(arcstat_metadata_size) =
+		    aggsum_value(&astat_metadata_size);
+		ARCSTAT(arcstat_hdr_size) = aggsum_value(&astat_hdr_size);
+		ARCSTAT(arcstat_l2_hdr_size) = aggsum_value(&astat_l2_hdr_size);
+		ARCSTAT(arcstat_dbuf_size) = aggsum_value(&astat_dbuf_size);
+#if defined(COMPAT_FREEBSD11)
+		ARCSTAT(arcstat_other_size) = aggsum_value(&astat_bonus_size) +
+		    aggsum_value(&astat_dnode_size) +
+		    aggsum_value(&astat_dbuf_size);
+#endif
+		ARCSTAT(arcstat_dnode_size) = aggsum_value(&astat_dnode_size);
+		ARCSTAT(arcstat_bonus_size) = aggsum_value(&astat_bonus_size);
+		ARCSTAT(arcstat_abd_chunk_waste_size) =
+		    aggsum_value(&astat_abd_chunk_waste_size);
+
+		as->arcstat_memory_all_bytes.value.ui64 =
+		    arc_all_memory();
+		as->arcstat_memory_free_bytes.value.ui64 =
+		    arc_free_memory();
+		as->arcstat_memory_available_bytes.value.i64 =
+		    arc_available_memory();
+	}
+
+	return (0);
+}
+
+/*
+ * This function *must* return indices evenly distributed between all
+ * sublists of the multilist. This is needed due to how the ARC eviction
+ * code is laid out; arc_evict_state() assumes ARC buffers are evenly
+ * distributed between all sublists and uses this assumption when
+ * deciding which sublist to evict from and how much to evict from it.
+ */
+static unsigned int
+arc_state_multilist_index_func(multilist_t *ml, void *obj)
+{
+	arc_buf_hdr_t *hdr = obj;
+
+	/*
+	 * We rely on b_dva to generate evenly distributed index
+	 * numbers using buf_hash below. So, as an added precaution,
+	 * let's make sure we never add empty buffers to the arc lists.
+	 */
+	ASSERT(!HDR_EMPTY(hdr));
+
+	/*
+	 * The assumption here, is the hash value for a given
+	 * arc_buf_hdr_t will remain constant throughout its lifetime
+	 * (i.e. its b_spa, b_dva, and b_birth fields don't change).
+	 * Thus, we don't need to store the header's sublist index
+	 * on insertion, as this index can be recalculated on removal.
+	 *
+	 * Also, the low order bits of the hash value are thought to be
+	 * distributed evenly. Otherwise, in the case that the multilist
+	 * has a power of two number of sublists, each sublists' usage
+	 * would not be evenly distributed.
+	 */
+	return (buf_hash(hdr->b_spa, &hdr->b_dva, hdr->b_birth) %
+	    multilist_get_num_sublists(ml));
+}
+
+#define	WARN_IF_TUNING_IGNORED(tuning, value, do_warn) do {	\
+	if ((do_warn) && (tuning) && ((tuning) != (value))) {	\
+		cmn_err(CE_WARN,				\
+		    "ignoring tunable %s (using %llu instead)",	\
+		    (#tuning), (value));			\
+	}							\
+} while (0)
+
+/*
+ * Called during module initialization and periodically thereafter to
+ * apply reasonable changes to the exposed performance tunings.  Can also be
+ * called explicitly by param_set_arc_*() functions when ARC tunables are
+ * updated manually.  Non-zero zfs_* values which differ from the currently set
+ * values will be applied.
+ */
+void
+arc_tuning_update(boolean_t verbose)
+{
+	uint64_t allmem = arc_all_memory();
+	unsigned long limit;
+
+	/* Valid range: 32M - <arc_c_max> */
+	if ((zfs_arc_min) && (zfs_arc_min != arc_c_min) &&
+	    (zfs_arc_min >= 2ULL << SPA_MAXBLOCKSHIFT) &&
+	    (zfs_arc_min <= arc_c_max)) {
+		arc_c_min = zfs_arc_min;
+		arc_c = MAX(arc_c, arc_c_min);
+	}
+	WARN_IF_TUNING_IGNORED(zfs_arc_min, arc_c_min, verbose);
+
+	/* Valid range: 64M - <all physical memory> */
+	if ((zfs_arc_max) && (zfs_arc_max != arc_c_max) &&
+	    (zfs_arc_max >= 64 << 20) && (zfs_arc_max < allmem) &&
+	    (zfs_arc_max > arc_c_min)) {
+		arc_c_max = zfs_arc_max;
+		arc_c = MIN(arc_c, arc_c_max);
+		arc_p = (arc_c >> 1);
+		if (arc_meta_limit > arc_c_max)
+			arc_meta_limit = arc_c_max;
+		if (arc_dnode_size_limit > arc_meta_limit)
+			arc_dnode_size_limit = arc_meta_limit;
+	}
+	WARN_IF_TUNING_IGNORED(zfs_arc_max, arc_c_max, verbose);
+
+	/* Valid range: 16M - <arc_c_max> */
+	if ((zfs_arc_meta_min) && (zfs_arc_meta_min != arc_meta_min) &&
+	    (zfs_arc_meta_min >= 1ULL << SPA_MAXBLOCKSHIFT) &&
+	    (zfs_arc_meta_min <= arc_c_max)) {
+		arc_meta_min = zfs_arc_meta_min;
+		if (arc_meta_limit < arc_meta_min)
+			arc_meta_limit = arc_meta_min;
+		if (arc_dnode_size_limit < arc_meta_min)
+			arc_dnode_size_limit = arc_meta_min;
+	}
+	WARN_IF_TUNING_IGNORED(zfs_arc_meta_min, arc_meta_min, verbose);
+
+	/* Valid range: <arc_meta_min> - <arc_c_max> */
+	limit = zfs_arc_meta_limit ? zfs_arc_meta_limit :
+	    MIN(zfs_arc_meta_limit_percent, 100) * arc_c_max / 100;
+	if ((limit != arc_meta_limit) &&
+	    (limit >= arc_meta_min) &&
+	    (limit <= arc_c_max))
+		arc_meta_limit = limit;
+	WARN_IF_TUNING_IGNORED(zfs_arc_meta_limit, arc_meta_limit, verbose);
+
+	/* Valid range: <arc_meta_min> - <arc_meta_limit> */
+	limit = zfs_arc_dnode_limit ? zfs_arc_dnode_limit :
+	    MIN(zfs_arc_dnode_limit_percent, 100) * arc_meta_limit / 100;
+	if ((limit != arc_dnode_size_limit) &&
+	    (limit >= arc_meta_min) &&
+	    (limit <= arc_meta_limit))
+		arc_dnode_size_limit = limit;
+	WARN_IF_TUNING_IGNORED(zfs_arc_dnode_limit, arc_dnode_size_limit,
+	    verbose);
+
+	/* Valid range: 1 - N */
+	if (zfs_arc_grow_retry)
+		arc_grow_retry = zfs_arc_grow_retry;
+
+	/* Valid range: 1 - N */
+	if (zfs_arc_shrink_shift) {
+		arc_shrink_shift = zfs_arc_shrink_shift;
+		arc_no_grow_shift = MIN(arc_no_grow_shift, arc_shrink_shift -1);
+	}
+
+	/* Valid range: 1 - N */
+	if (zfs_arc_p_min_shift)
+		arc_p_min_shift = zfs_arc_p_min_shift;
+
+	/* Valid range: 1 - N ms */
+	if (zfs_arc_min_prefetch_ms)
+		arc_min_prefetch_ms = zfs_arc_min_prefetch_ms;
+
+	/* Valid range: 1 - N ms */
+	if (zfs_arc_min_prescient_prefetch_ms) {
+		arc_min_prescient_prefetch_ms =
+		    zfs_arc_min_prescient_prefetch_ms;
+	}
+
+	/* Valid range: 0 - 100 */
+	if ((zfs_arc_lotsfree_percent >= 0) &&
+	    (zfs_arc_lotsfree_percent <= 100))
+		arc_lotsfree_percent = zfs_arc_lotsfree_percent;
+	WARN_IF_TUNING_IGNORED(zfs_arc_lotsfree_percent, arc_lotsfree_percent,
+	    verbose);
+
+	/* Valid range: 0 - <all physical memory> */
+	if ((zfs_arc_sys_free) && (zfs_arc_sys_free != arc_sys_free))
+		arc_sys_free = MIN(MAX(zfs_arc_sys_free, 0), allmem);
+	WARN_IF_TUNING_IGNORED(zfs_arc_sys_free, arc_sys_free, verbose);
+}
+
+static void
+arc_state_init(void)
+{
+	arc_anon = &ARC_anon;
+	arc_mru = &ARC_mru;
+	arc_mru_ghost = &ARC_mru_ghost;
+	arc_mfu = &ARC_mfu;
+	arc_mfu_ghost = &ARC_mfu_ghost;
+	arc_l2c_only = &ARC_l2c_only;
+
+	arc_mru->arcs_list[ARC_BUFC_METADATA] =
+	    multilist_create(sizeof (arc_buf_hdr_t),
+	    offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
+	    arc_state_multilist_index_func);
+	arc_mru->arcs_list[ARC_BUFC_DATA] =
+	    multilist_create(sizeof (arc_buf_hdr_t),
+	    offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
+	    arc_state_multilist_index_func);
+	arc_mru_ghost->arcs_list[ARC_BUFC_METADATA] =
+	    multilist_create(sizeof (arc_buf_hdr_t),
+	    offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
+	    arc_state_multilist_index_func);
+	arc_mru_ghost->arcs_list[ARC_BUFC_DATA] =
+	    multilist_create(sizeof (arc_buf_hdr_t),
+	    offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
+	    arc_state_multilist_index_func);
+	arc_mfu->arcs_list[ARC_BUFC_METADATA] =
+	    multilist_create(sizeof (arc_buf_hdr_t),
+	    offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
+	    arc_state_multilist_index_func);
+	arc_mfu->arcs_list[ARC_BUFC_DATA] =
+	    multilist_create(sizeof (arc_buf_hdr_t),
+	    offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
+	    arc_state_multilist_index_func);
+	arc_mfu_ghost->arcs_list[ARC_BUFC_METADATA] =
+	    multilist_create(sizeof (arc_buf_hdr_t),
+	    offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
+	    arc_state_multilist_index_func);
+	arc_mfu_ghost->arcs_list[ARC_BUFC_DATA] =
+	    multilist_create(sizeof (arc_buf_hdr_t),
+	    offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
+	    arc_state_multilist_index_func);
+	arc_l2c_only->arcs_list[ARC_BUFC_METADATA] =
+	    multilist_create(sizeof (arc_buf_hdr_t),
+	    offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
+	    arc_state_multilist_index_func);
+	arc_l2c_only->arcs_list[ARC_BUFC_DATA] =
+	    multilist_create(sizeof (arc_buf_hdr_t),
+	    offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
+	    arc_state_multilist_index_func);
+
+	zfs_refcount_create(&arc_anon->arcs_esize[ARC_BUFC_METADATA]);
+	zfs_refcount_create(&arc_anon->arcs_esize[ARC_BUFC_DATA]);
+	zfs_refcount_create(&arc_mru->arcs_esize[ARC_BUFC_METADATA]);
+	zfs_refcount_create(&arc_mru->arcs_esize[ARC_BUFC_DATA]);
+	zfs_refcount_create(&arc_mru_ghost->arcs_esize[ARC_BUFC_METADATA]);
+	zfs_refcount_create(&arc_mru_ghost->arcs_esize[ARC_BUFC_DATA]);
+	zfs_refcount_create(&arc_mfu->arcs_esize[ARC_BUFC_METADATA]);
+	zfs_refcount_create(&arc_mfu->arcs_esize[ARC_BUFC_DATA]);
+	zfs_refcount_create(&arc_mfu_ghost->arcs_esize[ARC_BUFC_METADATA]);
+	zfs_refcount_create(&arc_mfu_ghost->arcs_esize[ARC_BUFC_DATA]);
+	zfs_refcount_create(&arc_l2c_only->arcs_esize[ARC_BUFC_METADATA]);
+	zfs_refcount_create(&arc_l2c_only->arcs_esize[ARC_BUFC_DATA]);
+
+	zfs_refcount_create(&arc_anon->arcs_size);
+	zfs_refcount_create(&arc_mru->arcs_size);
+	zfs_refcount_create(&arc_mru_ghost->arcs_size);
+	zfs_refcount_create(&arc_mfu->arcs_size);
+	zfs_refcount_create(&arc_mfu_ghost->arcs_size);
+	zfs_refcount_create(&arc_l2c_only->arcs_size);
+
+	aggsum_init(&arc_meta_used, 0);
+	aggsum_init(&arc_size, 0);
+	aggsum_init(&astat_data_size, 0);
+	aggsum_init(&astat_metadata_size, 0);
+	aggsum_init(&astat_hdr_size, 0);
+	aggsum_init(&astat_l2_hdr_size, 0);
+	aggsum_init(&astat_bonus_size, 0);
+	aggsum_init(&astat_dnode_size, 0);
+	aggsum_init(&astat_dbuf_size, 0);
+	aggsum_init(&astat_abd_chunk_waste_size, 0);
+
+	arc_anon->arcs_state = ARC_STATE_ANON;
+	arc_mru->arcs_state = ARC_STATE_MRU;
+	arc_mru_ghost->arcs_state = ARC_STATE_MRU_GHOST;
+	arc_mfu->arcs_state = ARC_STATE_MFU;
+	arc_mfu_ghost->arcs_state = ARC_STATE_MFU_GHOST;
+	arc_l2c_only->arcs_state = ARC_STATE_L2C_ONLY;
+}
+
+static void
+arc_state_fini(void)
+{
+	zfs_refcount_destroy(&arc_anon->arcs_esize[ARC_BUFC_METADATA]);
+	zfs_refcount_destroy(&arc_anon->arcs_esize[ARC_BUFC_DATA]);
+	zfs_refcount_destroy(&arc_mru->arcs_esize[ARC_BUFC_METADATA]);
+	zfs_refcount_destroy(&arc_mru->arcs_esize[ARC_BUFC_DATA]);
+	zfs_refcount_destroy(&arc_mru_ghost->arcs_esize[ARC_BUFC_METADATA]);
+	zfs_refcount_destroy(&arc_mru_ghost->arcs_esize[ARC_BUFC_DATA]);
+	zfs_refcount_destroy(&arc_mfu->arcs_esize[ARC_BUFC_METADATA]);
+	zfs_refcount_destroy(&arc_mfu->arcs_esize[ARC_BUFC_DATA]);
+	zfs_refcount_destroy(&arc_mfu_ghost->arcs_esize[ARC_BUFC_METADATA]);
+	zfs_refcount_destroy(&arc_mfu_ghost->arcs_esize[ARC_BUFC_DATA]);
+	zfs_refcount_destroy(&arc_l2c_only->arcs_esize[ARC_BUFC_METADATA]);
+	zfs_refcount_destroy(&arc_l2c_only->arcs_esize[ARC_BUFC_DATA]);
+
+	zfs_refcount_destroy(&arc_anon->arcs_size);
+	zfs_refcount_destroy(&arc_mru->arcs_size);
+	zfs_refcount_destroy(&arc_mru_ghost->arcs_size);
+	zfs_refcount_destroy(&arc_mfu->arcs_size);
+	zfs_refcount_destroy(&arc_mfu_ghost->arcs_size);
+	zfs_refcount_destroy(&arc_l2c_only->arcs_size);
+
+	multilist_destroy(arc_mru->arcs_list[ARC_BUFC_METADATA]);
+	multilist_destroy(arc_mru_ghost->arcs_list[ARC_BUFC_METADATA]);
+	multilist_destroy(arc_mfu->arcs_list[ARC_BUFC_METADATA]);
+	multilist_destroy(arc_mfu_ghost->arcs_list[ARC_BUFC_METADATA]);
+	multilist_destroy(arc_mru->arcs_list[ARC_BUFC_DATA]);
+	multilist_destroy(arc_mru_ghost->arcs_list[ARC_BUFC_DATA]);
+	multilist_destroy(arc_mfu->arcs_list[ARC_BUFC_DATA]);
+	multilist_destroy(arc_mfu_ghost->arcs_list[ARC_BUFC_DATA]);
+	multilist_destroy(arc_l2c_only->arcs_list[ARC_BUFC_METADATA]);
+	multilist_destroy(arc_l2c_only->arcs_list[ARC_BUFC_DATA]);
+
+	aggsum_fini(&arc_meta_used);
+	aggsum_fini(&arc_size);
+	aggsum_fini(&astat_data_size);
+	aggsum_fini(&astat_metadata_size);
+	aggsum_fini(&astat_hdr_size);
+	aggsum_fini(&astat_l2_hdr_size);
+	aggsum_fini(&astat_bonus_size);
+	aggsum_fini(&astat_dnode_size);
+	aggsum_fini(&astat_dbuf_size);
+	aggsum_fini(&astat_abd_chunk_waste_size);
+}
+
+uint64_t
+arc_target_bytes(void)
+{
+	return (arc_c);
+}
+
+void
+arc_init(void)
+{
+	uint64_t percent, allmem = arc_all_memory();
+	mutex_init(&arc_evict_lock, NULL, MUTEX_DEFAULT, NULL);
+	list_create(&arc_evict_waiters, sizeof (arc_evict_waiter_t),
+	    offsetof(arc_evict_waiter_t, aew_node));
+
+	arc_min_prefetch_ms = 1000;
+	arc_min_prescient_prefetch_ms = 6000;
+
+#if defined(_KERNEL)
+	arc_lowmem_init();
+#endif
+
+	/* Set min cache to 1/32 of all memory, or 32MB, whichever is more. */
+	arc_c_min = MAX(allmem / 32, 2ULL << SPA_MAXBLOCKSHIFT);
+
+	/* How to set default max varies by platform. */
+	arc_c_max = arc_default_max(arc_c_min, allmem);
+
+#ifndef _KERNEL
+	/*
+	 * In userland, there's only the memory pressure that we artificially
+	 * create (see arc_available_memory()).  Don't let arc_c get too
+	 * small, because it can cause transactions to be larger than
+	 * arc_c, causing arc_tempreserve_space() to fail.
+	 */
+	arc_c_min = MAX(arc_c_max / 2, 2ULL << SPA_MAXBLOCKSHIFT);
+#endif
+
+	arc_c = arc_c_min;
+	arc_p = (arc_c >> 1);
+
+	/* Set min to 1/2 of arc_c_min */
+	arc_meta_min = 1ULL << SPA_MAXBLOCKSHIFT;
+	/* Initialize maximum observed usage to zero */
+	arc_meta_max = 0;
+	/*
+	 * Set arc_meta_limit to a percent of arc_c_max with a floor of
+	 * arc_meta_min, and a ceiling of arc_c_max.
+	 */
+	percent = MIN(zfs_arc_meta_limit_percent, 100);
+	arc_meta_limit = MAX(arc_meta_min, (percent * arc_c_max) / 100);
+	percent = MIN(zfs_arc_dnode_limit_percent, 100);
+	arc_dnode_size_limit = (percent * arc_meta_limit) / 100;
+
+	/* Apply user specified tunings */
+	arc_tuning_update(B_TRUE);
+
+	/* if kmem_flags are set, lets try to use less memory */
+	if (kmem_debugging())
+		arc_c = arc_c / 2;
+	if (arc_c < arc_c_min)
+		arc_c = arc_c_min;
+
+	arc_state_init();
+
+	buf_init();
+
+	list_create(&arc_prune_list, sizeof (arc_prune_t),
+	    offsetof(arc_prune_t, p_node));
+	mutex_init(&arc_prune_mtx, NULL, MUTEX_DEFAULT, NULL);
+
+	arc_prune_taskq = taskq_create("arc_prune", boot_ncpus, defclsyspri,
+	    boot_ncpus, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
+
+	arc_ksp = kstat_create("zfs", 0, "arcstats", "misc", KSTAT_TYPE_NAMED,
+	    sizeof (arc_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);
+
+	if (arc_ksp != NULL) {
+		arc_ksp->ks_data = &arc_stats;
+		arc_ksp->ks_update = arc_kstat_update;
+		kstat_install(arc_ksp);
+	}
+
+	arc_evict_zthr = zthr_create_timer("arc_evict",
+	    arc_evict_cb_check, arc_evict_cb, NULL, SEC2NSEC(1));
+	arc_reap_zthr = zthr_create_timer("arc_reap",
+	    arc_reap_cb_check, arc_reap_cb, NULL, SEC2NSEC(1));
+
+	arc_warm = B_FALSE;
+
+	/*
+	 * Calculate maximum amount of dirty data per pool.
+	 *
+	 * If it has been set by a module parameter, take that.
+	 * Otherwise, use a percentage of physical memory defined by
+	 * zfs_dirty_data_max_percent (default 10%) with a cap at
+	 * zfs_dirty_data_max_max (default 4G or 25% of physical memory).
+	 */
+#ifdef __LP64__
+	if (zfs_dirty_data_max_max == 0)
+		zfs_dirty_data_max_max = MIN(4ULL * 1024 * 1024 * 1024,
+		    allmem * zfs_dirty_data_max_max_percent / 100);
+#else
+	if (zfs_dirty_data_max_max == 0)
+		zfs_dirty_data_max_max = MIN(1ULL * 1024 * 1024 * 1024,
+		    allmem * zfs_dirty_data_max_max_percent / 100);
+#endif
+
+	if (zfs_dirty_data_max == 0) {
+		zfs_dirty_data_max = allmem *
+		    zfs_dirty_data_max_percent / 100;
+		zfs_dirty_data_max = MIN(zfs_dirty_data_max,
+		    zfs_dirty_data_max_max);
+	}
+}
+
+void
+arc_fini(void)
+{
+	arc_prune_t *p;
+
+#ifdef _KERNEL
+	arc_lowmem_fini();
+#endif /* _KERNEL */
+
+	/* Use B_TRUE to ensure *all* buffers are evicted */
+	arc_flush(NULL, B_TRUE);
+
+	if (arc_ksp != NULL) {
+		kstat_delete(arc_ksp);
+		arc_ksp = NULL;
+	}
+
+	taskq_wait(arc_prune_taskq);
+	taskq_destroy(arc_prune_taskq);
+
+	mutex_enter(&arc_prune_mtx);
+	while ((p = list_head(&arc_prune_list)) != NULL) {
+		list_remove(&arc_prune_list, p);
+		zfs_refcount_remove(&p->p_refcnt, &arc_prune_list);
+		zfs_refcount_destroy(&p->p_refcnt);
+		kmem_free(p, sizeof (*p));
+	}
+	mutex_exit(&arc_prune_mtx);
+
+	list_destroy(&arc_prune_list);
+	mutex_destroy(&arc_prune_mtx);
+
+	(void) zthr_cancel(arc_evict_zthr);
+	(void) zthr_cancel(arc_reap_zthr);
+
+	mutex_destroy(&arc_evict_lock);
+	list_destroy(&arc_evict_waiters);
+
+	/*
+	 * Free any buffers that were tagged for destruction.  This needs
+	 * to occur before arc_state_fini() runs and destroys the aggsum
+	 * values which are updated when freeing scatter ABDs.
+	 */
+	l2arc_do_free_on_write();
+
+	/*
+	 * buf_fini() must proceed arc_state_fini() because buf_fin() may
+	 * trigger the release of kmem magazines, which can callback to
+	 * arc_space_return() which accesses aggsums freed in act_state_fini().
+	 */
+	buf_fini();
+	arc_state_fini();
+
+	/*
+	 * We destroy the zthrs after all the ARC state has been
+	 * torn down to avoid the case of them receiving any
+	 * wakeup() signals after they are destroyed.
+	 */
+	zthr_destroy(arc_evict_zthr);
+	zthr_destroy(arc_reap_zthr);
+
+	ASSERT0(arc_loaned_bytes);
+}
+
+/*
+ * Level 2 ARC
+ *
+ * The level 2 ARC (L2ARC) is a cache layer in-between main memory and disk.
+ * It uses dedicated storage devices to hold cached data, which are populated
+ * using large infrequent writes.  The main role of this cache is to boost
+ * the performance of random read workloads.  The intended L2ARC devices
+ * include short-stroked disks, solid state disks, and other media with
+ * substantially faster read latency than disk.
+ *
+ *                 +-----------------------+
+ *                 |         ARC           |
+ *                 +-----------------------+
+ *                    |         ^     ^
+ *                    |         |     |
+ *      l2arc_feed_thread()    arc_read()
+ *                    |         |     |
+ *                    |  l2arc read   |
+ *                    V         |     |
+ *               +---------------+    |
+ *               |     L2ARC     |    |
+ *               +---------------+    |
+ *                   |    ^           |
+ *          l2arc_write() |           |
+ *                   |    |           |
+ *                   V    |           |
+ *                 +-------+      +-------+
+ *                 | vdev  |      | vdev  |
+ *                 | cache |      | cache |
+ *                 +-------+      +-------+
+ *                 +=========+     .-----.
+ *                 :  L2ARC  :    |-_____-|
+ *                 : devices :    | Disks |
+ *                 +=========+    `-_____-'
+ *
+ * Read requests are satisfied from the following sources, in order:
+ *
+ *	1) ARC
+ *	2) vdev cache of L2ARC devices
+ *	3) L2ARC devices
+ *	4) vdev cache of disks
+ *	5) disks
+ *
+ * Some L2ARC device types exhibit extremely slow write performance.
+ * To accommodate for this there are some significant differences between
+ * the L2ARC and traditional cache design:
+ *
+ * 1. There is no eviction path from the ARC to the L2ARC.  Evictions from
+ * the ARC behave as usual, freeing buffers and placing headers on ghost
+ * lists.  The ARC does not send buffers to the L2ARC during eviction as
+ * this would add inflated write latencies for all ARC memory pressure.
+ *
+ * 2. The L2ARC attempts to cache data from the ARC before it is evicted.
+ * It does this by periodically scanning buffers from the eviction-end of
+ * the MFU and MRU ARC lists, copying them to the L2ARC devices if they are
+ * not already there. It scans until a headroom of buffers is satisfied,
+ * which itself is a buffer for ARC eviction. If a compressible buffer is
+ * found during scanning and selected for writing to an L2ARC device, we
+ * temporarily boost scanning headroom during the next scan cycle to make
+ * sure we adapt to compression effects (which might significantly reduce
+ * the data volume we write to L2ARC). The thread that does this is
+ * l2arc_feed_thread(), illustrated below; example sizes are included to
+ * provide a better sense of ratio than this diagram:
+ *
+ *	       head -->                        tail
+ *	        +---------------------+----------+
+ *	ARC_mfu |:::::#:::::::::::::::|o#o###o###|-->.   # already on L2ARC
+ *	        +---------------------+----------+   |   o L2ARC eligible
+ *	ARC_mru |:#:::::::::::::::::::|#o#ooo####|-->|   : ARC buffer
+ *	        +---------------------+----------+   |
+ *	             15.9 Gbytes      ^ 32 Mbytes    |
+ *	                           headroom          |
+ *	                                      l2arc_feed_thread()
+ *	                                             |
+ *	                 l2arc write hand <--[oooo]--'
+ *	                         |           8 Mbyte
+ *	                         |          write max
+ *	                         V
+ *		  +==============================+
+ *	L2ARC dev |####|#|###|###|    |####| ... |
+ *	          +==============================+
+ *	                     32 Gbytes
+ *
+ * 3. If an ARC buffer is copied to the L2ARC but then hit instead of
+ * evicted, then the L2ARC has cached a buffer much sooner than it probably
+ * needed to, potentially wasting L2ARC device bandwidth and storage.  It is
+ * safe to say that this is an uncommon case, since buffers at the end of
+ * the ARC lists have moved there due to inactivity.
+ *
+ * 4. If the ARC evicts faster than the L2ARC can maintain a headroom,
+ * then the L2ARC simply misses copying some buffers.  This serves as a
+ * pressure valve to prevent heavy read workloads from both stalling the ARC
+ * with waits and clogging the L2ARC with writes.  This also helps prevent
+ * the potential for the L2ARC to churn if it attempts to cache content too
+ * quickly, such as during backups of the entire pool.
+ *
+ * 5. After system boot and before the ARC has filled main memory, there are
+ * no evictions from the ARC and so the tails of the ARC_mfu and ARC_mru
+ * lists can remain mostly static.  Instead of searching from tail of these
+ * lists as pictured, the l2arc_feed_thread() will search from the list heads
+ * for eligible buffers, greatly increasing its chance of finding them.
+ *
+ * The L2ARC device write speed is also boosted during this time so that
+ * the L2ARC warms up faster.  Since there have been no ARC evictions yet,
+ * there are no L2ARC reads, and no fear of degrading read performance
+ * through increased writes.
+ *
+ * 6. Writes to the L2ARC devices are grouped and sent in-sequence, so that
+ * the vdev queue can aggregate them into larger and fewer writes.  Each
+ * device is written to in a rotor fashion, sweeping writes through
+ * available space then repeating.
+ *
+ * 7. The L2ARC does not store dirty content.  It never needs to flush
+ * write buffers back to disk based storage.
+ *
+ * 8. If an ARC buffer is written (and dirtied) which also exists in the
+ * L2ARC, the now stale L2ARC buffer is immediately dropped.
+ *
+ * The performance of the L2ARC can be tweaked by a number of tunables, which
+ * may be necessary for different workloads:
+ *
+ *	l2arc_write_max		max write bytes per interval
+ *	l2arc_write_boost	extra write bytes during device warmup
+ *	l2arc_noprefetch	skip caching prefetched buffers
+ *	l2arc_headroom		number of max device writes to precache
+ *	l2arc_headroom_boost	when we find compressed buffers during ARC
+ *				scanning, we multiply headroom by this
+ *				percentage factor for the next scan cycle,
+ *				since more compressed buffers are likely to
+ *				be present
+ *	l2arc_feed_secs		seconds between L2ARC writing
+ *
+ * Tunables may be removed or added as future performance improvements are
+ * integrated, and also may become zpool properties.
+ *
+ * There are three key functions that control how the L2ARC warms up:
+ *
+ *	l2arc_write_eligible()	check if a buffer is eligible to cache
+ *	l2arc_write_size()	calculate how much to write
+ *	l2arc_write_interval()	calculate sleep delay between writes
+ *
+ * These three functions determine what to write, how much, and how quickly
+ * to send writes.
+ *
+ * L2ARC persistence:
+ *
+ * When writing buffers to L2ARC, we periodically add some metadata to
+ * make sure we can pick them up after reboot, thus dramatically reducing
+ * the impact that any downtime has on the performance of storage systems
+ * with large caches.
+ *
+ * The implementation works fairly simply by integrating the following two
+ * modifications:
+ *
+ * *) When writing to the L2ARC, we occasionally write a "l2arc log block",
+ *    which is an additional piece of metadata which describes what's been
+ *    written. This allows us to rebuild the arc_buf_hdr_t structures of the
+ *    main ARC buffers. There are 2 linked-lists of log blocks headed by
+ *    dh_start_lbps[2]. We alternate which chain we append to, so they are
+ *    time-wise and offset-wise interleaved, but that is an optimization rather
+ *    than for correctness. The log block also includes a pointer to the
+ *    previous block in its chain.
+ *
+ * *) We reserve SPA_MINBLOCKSIZE of space at the start of each L2ARC device
+ *    for our header bookkeeping purposes. This contains a device header,
+ *    which contains our top-level reference structures. We update it each
+ *    time we write a new log block, so that we're able to locate it in the
+ *    L2ARC device. If this write results in an inconsistent device header
+ *    (e.g. due to power failure), we detect this by verifying the header's
+ *    checksum and simply fail to reconstruct the L2ARC after reboot.
+ *
+ * Implementation diagram:
+ *
+ * +=== L2ARC device (not to scale) ======================================+
+ * |       ___two newest log block pointers__.__________                  |
+ * |      /                                   \dh_start_lbps[1]           |
+ * |	 /				       \         \dh_start_lbps[0]|
+ * |.___/__.                                    V         V               |
+ * ||L2 dev|....|lb |bufs |lb |bufs |lb |bufs |lb |bufs |lb |---(empty)---|
+ * ||   hdr|      ^         /^       /^        /         /                |
+ * |+------+  ...--\-------/  \-----/--\------/         /                 |
+ * |                \--------------/    \--------------/                  |
+ * +======================================================================+
+ *
+ * As can be seen on the diagram, rather than using a simple linked list,
+ * we use a pair of linked lists with alternating elements. This is a
+ * performance enhancement due to the fact that we only find out the
+ * address of the next log block access once the current block has been
+ * completely read in. Obviously, this hurts performance, because we'd be
+ * keeping the device's I/O queue at only a 1 operation deep, thus
+ * incurring a large amount of I/O round-trip latency. Having two lists
+ * allows us to fetch two log blocks ahead of where we are currently
+ * rebuilding L2ARC buffers.
+ *
+ * On-device data structures:
+ *
+ * L2ARC device header:	l2arc_dev_hdr_phys_t
+ * L2ARC log block:	l2arc_log_blk_phys_t
+ *
+ * L2ARC reconstruction:
+ *
+ * When writing data, we simply write in the standard rotary fashion,
+ * evicting buffers as we go and simply writing new data over them (writing
+ * a new log block every now and then). This obviously means that once we
+ * loop around the end of the device, we will start cutting into an already
+ * committed log block (and its referenced data buffers), like so:
+ *
+ *    current write head__       __old tail
+ *                        \     /
+ *                        V    V
+ * <--|bufs |lb |bufs |lb |    |bufs |lb |bufs |lb |-->
+ *                         ^    ^^^^^^^^^___________________________________
+ *                         |                                                \
+ *                   <<nextwrite>> may overwrite this blk and/or its bufs --'
+ *
+ * When importing the pool, we detect this situation and use it to stop
+ * our scanning process (see l2arc_rebuild).
+ *
+ * There is one significant caveat to consider when rebuilding ARC contents
+ * from an L2ARC device: what about invalidated buffers? Given the above
+ * construction, we cannot update blocks which we've already written to amend
+ * them to remove buffers which were invalidated. Thus, during reconstruction,
+ * we might be populating the cache with buffers for data that's not on the
+ * main pool anymore, or may have been overwritten!
+ *
+ * As it turns out, this isn't a problem. Every arc_read request includes
+ * both the DVA and, crucially, the birth TXG of the BP the caller is
+ * looking for. So even if the cache were populated by completely rotten
+ * blocks for data that had been long deleted and/or overwritten, we'll
+ * never actually return bad data from the cache, since the DVA with the
+ * birth TXG uniquely identify a block in space and time - once created,
+ * a block is immutable on disk. The worst thing we have done is wasted
+ * some time and memory at l2arc rebuild to reconstruct outdated ARC
+ * entries that will get dropped from the l2arc as it is being updated
+ * with new blocks.
+ *
+ * L2ARC buffers that have been evicted by l2arc_evict() ahead of the write
+ * hand are not restored. This is done by saving the offset (in bytes)
+ * l2arc_evict() has evicted to in the L2ARC device header and taking it
+ * into account when restoring buffers.
+ */
+
+static boolean_t
+l2arc_write_eligible(uint64_t spa_guid, arc_buf_hdr_t *hdr)
+{
+	/*
+	 * A buffer is *not* eligible for the L2ARC if it:
+	 * 1. belongs to a different spa.
+	 * 2. is already cached on the L2ARC.
+	 * 3. has an I/O in progress (it may be an incomplete read).
+	 * 4. is flagged not eligible (zfs property).
+	 */
+	if (hdr->b_spa != spa_guid || HDR_HAS_L2HDR(hdr) ||
+	    HDR_IO_IN_PROGRESS(hdr) || !HDR_L2CACHE(hdr))
+		return (B_FALSE);
+
+	return (B_TRUE);
+}
+
+static uint64_t
+l2arc_write_size(l2arc_dev_t *dev)
+{
+	uint64_t size, dev_size, tsize;
+
+	/*
+	 * Make sure our globals have meaningful values in case the user
+	 * altered them.
+	 */
+	size = l2arc_write_max;
+	if (size == 0) {
+		cmn_err(CE_NOTE, "Bad value for l2arc_write_max, value must "
+		    "be greater than zero, resetting it to the default (%d)",
+		    L2ARC_WRITE_SIZE);
+		size = l2arc_write_max = L2ARC_WRITE_SIZE;
+	}
+
+	if (arc_warm == B_FALSE)
+		size += l2arc_write_boost;
+
+	/*
+	 * Make sure the write size does not exceed the size of the cache
+	 * device. This is important in l2arc_evict(), otherwise infinite
+	 * iteration can occur.
+	 */
+	dev_size = dev->l2ad_end - dev->l2ad_start;
+	tsize = size + l2arc_log_blk_overhead(size, dev);
+	if (dev->l2ad_vdev->vdev_has_trim && l2arc_trim_ahead > 0)
+		tsize += MAX(64 * 1024 * 1024,
+		    (tsize * l2arc_trim_ahead) / 100);
+
+	if (tsize >= dev_size) {
+		cmn_err(CE_NOTE, "l2arc_write_max or l2arc_write_boost "
+		    "plus the overhead of log blocks (persistent L2ARC, "
+		    "%llu bytes) exceeds the size of the cache device "
+		    "(guid %llu), resetting them to the default (%d)",
+		    l2arc_log_blk_overhead(size, dev),
+		    dev->l2ad_vdev->vdev_guid, L2ARC_WRITE_SIZE);
+		size = l2arc_write_max = l2arc_write_boost = L2ARC_WRITE_SIZE;
+
+		if (arc_warm == B_FALSE)
+			size += l2arc_write_boost;
+	}
+
+	return (size);
+
+}
+
+static clock_t
+l2arc_write_interval(clock_t began, uint64_t wanted, uint64_t wrote)
+{
+	clock_t interval, next, now;
+
+	/*
+	 * If the ARC lists are busy, increase our write rate; if the
+	 * lists are stale, idle back.  This is achieved by checking
+	 * how much we previously wrote - if it was more than half of
+	 * what we wanted, schedule the next write much sooner.
+	 */
+	if (l2arc_feed_again && wrote > (wanted / 2))
+		interval = (hz * l2arc_feed_min_ms) / 1000;
+	else
+		interval = hz * l2arc_feed_secs;
+
+	now = ddi_get_lbolt();
+	next = MAX(now, MIN(now + interval, began + interval));
+
+	return (next);
+}
+
+/*
+ * Cycle through L2ARC devices.  This is how L2ARC load balances.
+ * If a device is returned, this also returns holding the spa config lock.
+ */
+static l2arc_dev_t *
+l2arc_dev_get_next(void)
+{
+	l2arc_dev_t *first, *next = NULL;
+
+	/*
+	 * Lock out the removal of spas (spa_namespace_lock), then removal
+	 * of cache devices (l2arc_dev_mtx).  Once a device has been selected,
+	 * both locks will be dropped and a spa config lock held instead.
+	 */
+	mutex_enter(&spa_namespace_lock);
+	mutex_enter(&l2arc_dev_mtx);
+
+	/* if there are no vdevs, there is nothing to do */
+	if (l2arc_ndev == 0)
+		goto out;
+
+	first = NULL;
+	next = l2arc_dev_last;
+	do {
+		/* loop around the list looking for a non-faulted vdev */
+		if (next == NULL) {
+			next = list_head(l2arc_dev_list);
+		} else {
+			next = list_next(l2arc_dev_list, next);
+			if (next == NULL)
+				next = list_head(l2arc_dev_list);
+		}
+
+		/* if we have come back to the start, bail out */
+		if (first == NULL)
+			first = next;
+		else if (next == first)
+			break;
+
+	} while (vdev_is_dead(next->l2ad_vdev) || next->l2ad_rebuild ||
+	    next->l2ad_trim_all);
+
+	/* if we were unable to find any usable vdevs, return NULL */
+	if (vdev_is_dead(next->l2ad_vdev) || next->l2ad_rebuild ||
+	    next->l2ad_trim_all)
+		next = NULL;
+
+	l2arc_dev_last = next;
+
+out:
+	mutex_exit(&l2arc_dev_mtx);
+
+	/*
+	 * Grab the config lock to prevent the 'next' device from being
+	 * removed while we are writing to it.
+	 */
+	if (next != NULL)
+		spa_config_enter(next->l2ad_spa, SCL_L2ARC, next, RW_READER);
+	mutex_exit(&spa_namespace_lock);
+
+	return (next);
+}
+
+/*
+ * Free buffers that were tagged for destruction.
+ */
+static void
+l2arc_do_free_on_write(void)
+{
+	list_t *buflist;
+	l2arc_data_free_t *df, *df_prev;
+
+	mutex_enter(&l2arc_free_on_write_mtx);
+	buflist = l2arc_free_on_write;
+
+	for (df = list_tail(buflist); df; df = df_prev) {
+		df_prev = list_prev(buflist, df);
+		ASSERT3P(df->l2df_abd, !=, NULL);
+		abd_free(df->l2df_abd);
+		list_remove(buflist, df);
+		kmem_free(df, sizeof (l2arc_data_free_t));
+	}
+
+	mutex_exit(&l2arc_free_on_write_mtx);
+}
+
+/*
+ * A write to a cache device has completed.  Update all headers to allow
+ * reads from these buffers to begin.
+ */
+static void
+l2arc_write_done(zio_t *zio)
+{
+	l2arc_write_callback_t	*cb;
+	l2arc_lb_abd_buf_t	*abd_buf;
+	l2arc_lb_ptr_buf_t	*lb_ptr_buf;
+	l2arc_dev_t		*dev;
+	l2arc_dev_hdr_phys_t	*l2dhdr;
+	list_t			*buflist;
+	arc_buf_hdr_t		*head, *hdr, *hdr_prev;
+	kmutex_t		*hash_lock;
+	int64_t			bytes_dropped = 0;
+
+	cb = zio->io_private;
+	ASSERT3P(cb, !=, NULL);
+	dev = cb->l2wcb_dev;
+	l2dhdr = dev->l2ad_dev_hdr;
+	ASSERT3P(dev, !=, NULL);
+	head = cb->l2wcb_head;
+	ASSERT3P(head, !=, NULL);
+	buflist = &dev->l2ad_buflist;
+	ASSERT3P(buflist, !=, NULL);
+	DTRACE_PROBE2(l2arc__iodone, zio_t *, zio,
+	    l2arc_write_callback_t *, cb);
+
+	if (zio->io_error != 0)
+		ARCSTAT_BUMP(arcstat_l2_writes_error);
+
+	/*
+	 * All writes completed, or an error was hit.
+	 */
+top:
+	mutex_enter(&dev->l2ad_mtx);
+	for (hdr = list_prev(buflist, head); hdr; hdr = hdr_prev) {
+		hdr_prev = list_prev(buflist, hdr);
+
+		hash_lock = HDR_LOCK(hdr);
+
+		/*
+		 * We cannot use mutex_enter or else we can deadlock
+		 * with l2arc_write_buffers (due to swapping the order
+		 * the hash lock and l2ad_mtx are taken).
+		 */
+		if (!mutex_tryenter(hash_lock)) {
+			/*
+			 * Missed the hash lock. We must retry so we
+			 * don't leave the ARC_FLAG_L2_WRITING bit set.
+			 */
+			ARCSTAT_BUMP(arcstat_l2_writes_lock_retry);
+
+			/*
+			 * We don't want to rescan the headers we've
+			 * already marked as having been written out, so
+			 * we reinsert the head node so we can pick up
+			 * where we left off.
+			 */
+			list_remove(buflist, head);
+			list_insert_after(buflist, hdr, head);
+
+			mutex_exit(&dev->l2ad_mtx);
+
+			/*
+			 * We wait for the hash lock to become available
+			 * to try and prevent busy waiting, and increase
+			 * the chance we'll be able to acquire the lock
+			 * the next time around.
+			 */
+			mutex_enter(hash_lock);
+			mutex_exit(hash_lock);
+			goto top;
+		}
+
+		/*
+		 * We could not have been moved into the arc_l2c_only
+		 * state while in-flight due to our ARC_FLAG_L2_WRITING
+		 * bit being set. Let's just ensure that's being enforced.
+		 */
+		ASSERT(HDR_HAS_L1HDR(hdr));
+
+		/*
+		 * Skipped - drop L2ARC entry and mark the header as no
+		 * longer L2 eligibile.
+		 */
+		if (zio->io_error != 0) {
+			/*
+			 * Error - drop L2ARC entry.
+			 */
+			list_remove(buflist, hdr);
+			arc_hdr_clear_flags(hdr, ARC_FLAG_HAS_L2HDR);
+
+			uint64_t psize = HDR_GET_PSIZE(hdr);
+			ARCSTAT_INCR(arcstat_l2_psize, -psize);
+			ARCSTAT_INCR(arcstat_l2_lsize, -HDR_GET_LSIZE(hdr));
+
+			bytes_dropped +=
+			    vdev_psize_to_asize(dev->l2ad_vdev, psize);
+			(void) zfs_refcount_remove_many(&dev->l2ad_alloc,
+			    arc_hdr_size(hdr), hdr);
+		}
+
+		/*
+		 * Allow ARC to begin reads and ghost list evictions to
+		 * this L2ARC entry.
+		 */
+		arc_hdr_clear_flags(hdr, ARC_FLAG_L2_WRITING);
+
+		mutex_exit(hash_lock);
+	}
+
+	/*
+	 * Free the allocated abd buffers for writing the log blocks.
+	 * If the zio failed reclaim the allocated space and remove the
+	 * pointers to these log blocks from the log block pointer list
+	 * of the L2ARC device.
+	 */
+	while ((abd_buf = list_remove_tail(&cb->l2wcb_abd_list)) != NULL) {
+		abd_free(abd_buf->abd);
+		zio_buf_free(abd_buf, sizeof (*abd_buf));
+		if (zio->io_error != 0) {
+			lb_ptr_buf = list_remove_head(&dev->l2ad_lbptr_list);
+			/*
+			 * L2BLK_GET_PSIZE returns aligned size for log
+			 * blocks.
+			 */
+			uint64_t asize =
+			    L2BLK_GET_PSIZE((lb_ptr_buf->lb_ptr)->lbp_prop);
+			bytes_dropped += asize;
+			ARCSTAT_INCR(arcstat_l2_log_blk_asize, -asize);
+			ARCSTAT_BUMPDOWN(arcstat_l2_log_blk_count);
+			zfs_refcount_remove_many(&dev->l2ad_lb_asize, asize,
+			    lb_ptr_buf);
+			zfs_refcount_remove(&dev->l2ad_lb_count, lb_ptr_buf);
+			kmem_free(lb_ptr_buf->lb_ptr,
+			    sizeof (l2arc_log_blkptr_t));
+			kmem_free(lb_ptr_buf, sizeof (l2arc_lb_ptr_buf_t));
+		}
+	}
+	list_destroy(&cb->l2wcb_abd_list);
+
+	if (zio->io_error != 0) {
+		/*
+		 * Restore the lbps array in the header to its previous state.
+		 * If the list of log block pointers is empty, zero out the
+		 * log block pointers in the device header.
+		 */
+		lb_ptr_buf = list_head(&dev->l2ad_lbptr_list);
+		for (int i = 0; i < 2; i++) {
+			if (lb_ptr_buf == NULL) {
+				/*
+				 * If the list is empty zero out the device
+				 * header. Otherwise zero out the second log
+				 * block pointer in the header.
+				 */
+				if (i == 0) {
+					bzero(l2dhdr, dev->l2ad_dev_hdr_asize);
+				} else {
+					bzero(&l2dhdr->dh_start_lbps[i],
+					    sizeof (l2arc_log_blkptr_t));
+				}
+				break;
+			}
+			bcopy(lb_ptr_buf->lb_ptr, &l2dhdr->dh_start_lbps[i],
+			    sizeof (l2arc_log_blkptr_t));
+			lb_ptr_buf = list_next(&dev->l2ad_lbptr_list,
+			    lb_ptr_buf);
+		}
+	}
+
+	atomic_inc_64(&l2arc_writes_done);
+	list_remove(buflist, head);
+	ASSERT(!HDR_HAS_L1HDR(head));
+	kmem_cache_free(hdr_l2only_cache, head);
+	mutex_exit(&dev->l2ad_mtx);
+
+	ASSERT(dev->l2ad_vdev != NULL);
+	vdev_space_update(dev->l2ad_vdev, -bytes_dropped, 0, 0);
+
+	l2arc_do_free_on_write();
+
+	kmem_free(cb, sizeof (l2arc_write_callback_t));
+}
+
+static int
+l2arc_untransform(zio_t *zio, l2arc_read_callback_t *cb)
+{
+	int ret;
+	spa_t *spa = zio->io_spa;
+	arc_buf_hdr_t *hdr = cb->l2rcb_hdr;
+	blkptr_t *bp = zio->io_bp;
+	uint8_t salt[ZIO_DATA_SALT_LEN];
+	uint8_t iv[ZIO_DATA_IV_LEN];
+	uint8_t mac[ZIO_DATA_MAC_LEN];
+	boolean_t no_crypt = B_FALSE;
+
+	/*
+	 * ZIL data is never be written to the L2ARC, so we don't need
+	 * special handling for its unique MAC storage.
+	 */
+	ASSERT3U(BP_GET_TYPE(bp), !=, DMU_OT_INTENT_LOG);
+	ASSERT(MUTEX_HELD(HDR_LOCK(hdr)));
+	ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
+
+	/*
+	 * If the data was encrypted, decrypt it now. Note that
+	 * we must check the bp here and not the hdr, since the
+	 * hdr does not have its encryption parameters updated
+	 * until arc_read_done().
+	 */
+	if (BP_IS_ENCRYPTED(bp)) {
+		abd_t *eabd = arc_get_data_abd(hdr, arc_hdr_size(hdr), hdr,
+		    B_TRUE);
+
+		zio_crypt_decode_params_bp(bp, salt, iv);
+		zio_crypt_decode_mac_bp(bp, mac);
+
+		ret = spa_do_crypt_abd(B_FALSE, spa, &cb->l2rcb_zb,
+		    BP_GET_TYPE(bp), BP_GET_DEDUP(bp), BP_SHOULD_BYTESWAP(bp),
+		    salt, iv, mac, HDR_GET_PSIZE(hdr), eabd,
+		    hdr->b_l1hdr.b_pabd, &no_crypt);
+		if (ret != 0) {
+			arc_free_data_abd(hdr, eabd, arc_hdr_size(hdr), hdr);
+			goto error;
+		}
+
+		/*
+		 * If we actually performed decryption, replace b_pabd
+		 * with the decrypted data. Otherwise we can just throw
+		 * our decryption buffer away.
+		 */
+		if (!no_crypt) {
+			arc_free_data_abd(hdr, hdr->b_l1hdr.b_pabd,
+			    arc_hdr_size(hdr), hdr);
+			hdr->b_l1hdr.b_pabd = eabd;
+			zio->io_abd = eabd;
+		} else {
+			arc_free_data_abd(hdr, eabd, arc_hdr_size(hdr), hdr);
+		}
+	}
+
+	/*
+	 * If the L2ARC block was compressed, but ARC compression
+	 * is disabled we decompress the data into a new buffer and
+	 * replace the existing data.
+	 */
+	if (HDR_GET_COMPRESS(hdr) != ZIO_COMPRESS_OFF &&
+	    !HDR_COMPRESSION_ENABLED(hdr)) {
+		abd_t *cabd = arc_get_data_abd(hdr, arc_hdr_size(hdr), hdr,
+		    B_TRUE);
+		void *tmp = abd_borrow_buf(cabd, arc_hdr_size(hdr));
+
+		ret = zio_decompress_data(HDR_GET_COMPRESS(hdr),
+		    hdr->b_l1hdr.b_pabd, tmp, HDR_GET_PSIZE(hdr),
+		    HDR_GET_LSIZE(hdr), &hdr->b_complevel);
+		if (ret != 0) {
+			abd_return_buf_copy(cabd, tmp, arc_hdr_size(hdr));
+			arc_free_data_abd(hdr, cabd, arc_hdr_size(hdr), hdr);
+			goto error;
+		}
+
+		abd_return_buf_copy(cabd, tmp, arc_hdr_size(hdr));
+		arc_free_data_abd(hdr, hdr->b_l1hdr.b_pabd,
+		    arc_hdr_size(hdr), hdr);
+		hdr->b_l1hdr.b_pabd = cabd;
+		zio->io_abd = cabd;
+		zio->io_size = HDR_GET_LSIZE(hdr);
+	}
+
+	return (0);
+
+error:
+	return (ret);
+}
+
+
+/*
+ * A read to a cache device completed.  Validate buffer contents before
+ * handing over to the regular ARC routines.
+ */
+static void
+l2arc_read_done(zio_t *zio)
+{
+	int tfm_error = 0;
+	l2arc_read_callback_t *cb = zio->io_private;
+	arc_buf_hdr_t *hdr;
+	kmutex_t *hash_lock;
+	boolean_t valid_cksum;
+	boolean_t using_rdata = (BP_IS_ENCRYPTED(&cb->l2rcb_bp) &&
+	    (cb->l2rcb_flags & ZIO_FLAG_RAW_ENCRYPT));
+
+	ASSERT3P(zio->io_vd, !=, NULL);
+	ASSERT(zio->io_flags & ZIO_FLAG_DONT_PROPAGATE);
+
+	spa_config_exit(zio->io_spa, SCL_L2ARC, zio->io_vd);
+
+	ASSERT3P(cb, !=, NULL);
+	hdr = cb->l2rcb_hdr;
+	ASSERT3P(hdr, !=, NULL);
+
+	hash_lock = HDR_LOCK(hdr);
+	mutex_enter(hash_lock);
+	ASSERT3P(hash_lock, ==, HDR_LOCK(hdr));
+
+	/*
+	 * If the data was read into a temporary buffer,
+	 * move it and free the buffer.
+	 */
+	if (cb->l2rcb_abd != NULL) {
+		ASSERT3U(arc_hdr_size(hdr), <, zio->io_size);
+		if (zio->io_error == 0) {
+			if (using_rdata) {
+				abd_copy(hdr->b_crypt_hdr.b_rabd,
+				    cb->l2rcb_abd, arc_hdr_size(hdr));
+			} else {
+				abd_copy(hdr->b_l1hdr.b_pabd,
+				    cb->l2rcb_abd, arc_hdr_size(hdr));
+			}
+		}
+
+		/*
+		 * The following must be done regardless of whether
+		 * there was an error:
+		 * - free the temporary buffer
+		 * - point zio to the real ARC buffer
+		 * - set zio size accordingly
+		 * These are required because zio is either re-used for
+		 * an I/O of the block in the case of the error
+		 * or the zio is passed to arc_read_done() and it
+		 * needs real data.
+		 */
+		abd_free(cb->l2rcb_abd);
+		zio->io_size = zio->io_orig_size = arc_hdr_size(hdr);
+
+		if (using_rdata) {
+			ASSERT(HDR_HAS_RABD(hdr));
+			zio->io_abd = zio->io_orig_abd =
+			    hdr->b_crypt_hdr.b_rabd;
+		} else {
+			ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
+			zio->io_abd = zio->io_orig_abd = hdr->b_l1hdr.b_pabd;
+		}
+	}
+
+	ASSERT3P(zio->io_abd, !=, NULL);
+
+	/*
+	 * Check this survived the L2ARC journey.
+	 */
+	ASSERT(zio->io_abd == hdr->b_l1hdr.b_pabd ||
+	    (HDR_HAS_RABD(hdr) && zio->io_abd == hdr->b_crypt_hdr.b_rabd));
+	zio->io_bp_copy = cb->l2rcb_bp;	/* XXX fix in L2ARC 2.0	*/
+	zio->io_bp = &zio->io_bp_copy;	/* XXX fix in L2ARC 2.0	*/
+	zio->io_prop.zp_complevel = hdr->b_complevel;
+
+	valid_cksum = arc_cksum_is_equal(hdr, zio);
+
+	/*
+	 * b_rabd will always match the data as it exists on disk if it is
+	 * being used. Therefore if we are reading into b_rabd we do not
+	 * attempt to untransform the data.
+	 */
+	if (valid_cksum && !using_rdata)
+		tfm_error = l2arc_untransform(zio, cb);
+
+	if (valid_cksum && tfm_error == 0 && zio->io_error == 0 &&
+	    !HDR_L2_EVICTED(hdr)) {
+		mutex_exit(hash_lock);
+		zio->io_private = hdr;
+		arc_read_done(zio);
+	} else {
+		/*
+		 * Buffer didn't survive caching.  Increment stats and
+		 * reissue to the original storage device.
+		 */
+		if (zio->io_error != 0) {
+			ARCSTAT_BUMP(arcstat_l2_io_error);
+		} else {
+			zio->io_error = SET_ERROR(EIO);
+		}
+		if (!valid_cksum || tfm_error != 0)
+			ARCSTAT_BUMP(arcstat_l2_cksum_bad);
+
+		/*
+		 * If there's no waiter, issue an async i/o to the primary
+		 * storage now.  If there *is* a waiter, the caller must
+		 * issue the i/o in a context where it's OK to block.
+		 */
+		if (zio->io_waiter == NULL) {
+			zio_t *pio = zio_unique_parent(zio);
+			void *abd = (using_rdata) ?
+			    hdr->b_crypt_hdr.b_rabd : hdr->b_l1hdr.b_pabd;
+
+			ASSERT(!pio || pio->io_child_type == ZIO_CHILD_LOGICAL);
+
+			zio = zio_read(pio, zio->io_spa, zio->io_bp,
+			    abd, zio->io_size, arc_read_done,
+			    hdr, zio->io_priority, cb->l2rcb_flags,
+			    &cb->l2rcb_zb);
+
+			/*
+			 * Original ZIO will be freed, so we need to update
+			 * ARC header with the new ZIO pointer to be used
+			 * by zio_change_priority() in arc_read().
+			 */
+			for (struct arc_callback *acb = hdr->b_l1hdr.b_acb;
+			    acb != NULL; acb = acb->acb_next)
+				acb->acb_zio_head = zio;
+
+			mutex_exit(hash_lock);
+			zio_nowait(zio);
+		} else {
+			mutex_exit(hash_lock);
+		}
+	}
+
+	kmem_free(cb, sizeof (l2arc_read_callback_t));
+}
+
+/*
+ * This is the list priority from which the L2ARC will search for pages to
+ * cache.  This is used within loops (0..3) to cycle through lists in the
+ * desired order.  This order can have a significant effect on cache
+ * performance.
+ *
+ * Currently the metadata lists are hit first, MFU then MRU, followed by
+ * the data lists.  This function returns a locked list, and also returns
+ * the lock pointer.
+ */
+static multilist_sublist_t *
+l2arc_sublist_lock(int list_num)
+{
+	multilist_t *ml = NULL;
+	unsigned int idx;
+
+	ASSERT(list_num >= 0 && list_num < L2ARC_FEED_TYPES);
+
+	switch (list_num) {
+	case 0:
+		ml = arc_mfu->arcs_list[ARC_BUFC_METADATA];
+		break;
+	case 1:
+		ml = arc_mru->arcs_list[ARC_BUFC_METADATA];
+		break;
+	case 2:
+		ml = arc_mfu->arcs_list[ARC_BUFC_DATA];
+		break;
+	case 3:
+		ml = arc_mru->arcs_list[ARC_BUFC_DATA];
+		break;
+	default:
+		return (NULL);
+	}
+
+	/*
+	 * Return a randomly-selected sublist. This is acceptable
+	 * because the caller feeds only a little bit of data for each
+	 * call (8MB). Subsequent calls will result in different
+	 * sublists being selected.
+	 */
+	idx = multilist_get_random_index(ml);
+	return (multilist_sublist_lock(ml, idx));
+}
+
+/*
+ * Calculates the maximum overhead of L2ARC metadata log blocks for a given
+ * L2ARC write size. l2arc_evict and l2arc_write_size need to include this
+ * overhead in processing to make sure there is enough headroom available
+ * when writing buffers.
+ */
+static inline uint64_t
+l2arc_log_blk_overhead(uint64_t write_sz, l2arc_dev_t *dev)
+{
+	if (dev->l2ad_log_entries == 0) {
+		return (0);
+	} else {
+		uint64_t log_entries = write_sz >> SPA_MINBLOCKSHIFT;
+
+		uint64_t log_blocks = (log_entries +
+		    dev->l2ad_log_entries - 1) /
+		    dev->l2ad_log_entries;
+
+		return (vdev_psize_to_asize(dev->l2ad_vdev,
+		    sizeof (l2arc_log_blk_phys_t)) * log_blocks);
+	}
+}
+
+/*
+ * Evict buffers from the device write hand to the distance specified in
+ * bytes. This distance may span populated buffers, it may span nothing.
+ * This is clearing a region on the L2ARC device ready for writing.
+ * If the 'all' boolean is set, every buffer is evicted.
+ */
+static void
+l2arc_evict(l2arc_dev_t *dev, uint64_t distance, boolean_t all)
+{
+	list_t *buflist;
+	arc_buf_hdr_t *hdr, *hdr_prev;
+	kmutex_t *hash_lock;
+	uint64_t taddr;
+	l2arc_lb_ptr_buf_t *lb_ptr_buf, *lb_ptr_buf_prev;
+	vdev_t *vd = dev->l2ad_vdev;
+	boolean_t rerun;
+
+	buflist = &dev->l2ad_buflist;
+
+	/*
+	 * We need to add in the worst case scenario of log block overhead.
+	 */
+	distance += l2arc_log_blk_overhead(distance, dev);
+	if (vd->vdev_has_trim && l2arc_trim_ahead > 0) {
+		/*
+		 * Trim ahead of the write size 64MB or (l2arc_trim_ahead/100)
+		 * times the write size, whichever is greater.
+		 */
+		distance += MAX(64 * 1024 * 1024,
+		    (distance * l2arc_trim_ahead) / 100);
+	}
+
+top:
+	rerun = B_FALSE;
+	if (dev->l2ad_hand >= (dev->l2ad_end - distance)) {
+		/*
+		 * When there is no space to accommodate upcoming writes,
+		 * evict to the end. Then bump the write and evict hands
+		 * to the start and iterate. This iteration does not
+		 * happen indefinitely as we make sure in
+		 * l2arc_write_size() that when the write hand is reset,
+		 * the write size does not exceed the end of the device.
+		 */
+		rerun = B_TRUE;
+		taddr = dev->l2ad_end;
+	} else {
+		taddr = dev->l2ad_hand + distance;
+	}
+	DTRACE_PROBE4(l2arc__evict, l2arc_dev_t *, dev, list_t *, buflist,
+	    uint64_t, taddr, boolean_t, all);
+
+	if (!all) {
+		/*
+		 * This check has to be placed after deciding whether to
+		 * iterate (rerun).
+		 */
+		if (dev->l2ad_first) {
+			/*
+			 * This is the first sweep through the device. There is
+			 * nothing to evict. We have already trimmmed the
+			 * whole device.
+			 */
+			goto out;
+		} else {
+			/*
+			 * Trim the space to be evicted.
+			 */
+			if (vd->vdev_has_trim && dev->l2ad_evict < taddr &&
+			    l2arc_trim_ahead > 0) {
+				/*
+				 * We have to drop the spa_config lock because
+				 * vdev_trim_range() will acquire it.
+				 * l2ad_evict already accounts for the label
+				 * size. To prevent vdev_trim_ranges() from
+				 * adding it again, we subtract it from
+				 * l2ad_evict.
+				 */
+				spa_config_exit(dev->l2ad_spa, SCL_L2ARC, dev);
+				vdev_trim_simple(vd,
+				    dev->l2ad_evict - VDEV_LABEL_START_SIZE,
+				    taddr - dev->l2ad_evict);
+				spa_config_enter(dev->l2ad_spa, SCL_L2ARC, dev,
+				    RW_READER);
+			}
+
+			/*
+			 * When rebuilding L2ARC we retrieve the evict hand
+			 * from the header of the device. Of note, l2arc_evict()
+			 * does not actually delete buffers from the cache
+			 * device, but trimming may do so depending on the
+			 * hardware implementation. Thus keeping track of the
+			 * evict hand is useful.
+			 */
+			dev->l2ad_evict = MAX(dev->l2ad_evict, taddr);
+		}
+	}
+
+retry:
+	mutex_enter(&dev->l2ad_mtx);
+	/*
+	 * We have to account for evicted log blocks. Run vdev_space_update()
+	 * on log blocks whose offset (in bytes) is before the evicted offset
+	 * (in bytes) by searching in the list of pointers to log blocks
+	 * present in the L2ARC device.
+	 */
+	for (lb_ptr_buf = list_tail(&dev->l2ad_lbptr_list); lb_ptr_buf;
+	    lb_ptr_buf = lb_ptr_buf_prev) {
+
+		lb_ptr_buf_prev = list_prev(&dev->l2ad_lbptr_list, lb_ptr_buf);
+
+		/* L2BLK_GET_PSIZE returns aligned size for log blocks */
+		uint64_t asize = L2BLK_GET_PSIZE(
+		    (lb_ptr_buf->lb_ptr)->lbp_prop);
+
+		/*
+		 * We don't worry about log blocks left behind (ie
+		 * lbp_payload_start < l2ad_hand) because l2arc_write_buffers()
+		 * will never write more than l2arc_evict() evicts.
+		 */
+		if (!all && l2arc_log_blkptr_valid(dev, lb_ptr_buf->lb_ptr)) {
+			break;
+		} else {
+			vdev_space_update(vd, -asize, 0, 0);
+			ARCSTAT_INCR(arcstat_l2_log_blk_asize, -asize);
+			ARCSTAT_BUMPDOWN(arcstat_l2_log_blk_count);
+			zfs_refcount_remove_many(&dev->l2ad_lb_asize, asize,
+			    lb_ptr_buf);
+			zfs_refcount_remove(&dev->l2ad_lb_count, lb_ptr_buf);
+			list_remove(&dev->l2ad_lbptr_list, lb_ptr_buf);
+			kmem_free(lb_ptr_buf->lb_ptr,
+			    sizeof (l2arc_log_blkptr_t));
+			kmem_free(lb_ptr_buf, sizeof (l2arc_lb_ptr_buf_t));
+		}
+	}
+
+	for (hdr = list_tail(buflist); hdr; hdr = hdr_prev) {
+		hdr_prev = list_prev(buflist, hdr);
+
+		ASSERT(!HDR_EMPTY(hdr));
+		hash_lock = HDR_LOCK(hdr);
+
+		/*
+		 * We cannot use mutex_enter or else we can deadlock
+		 * with l2arc_write_buffers (due to swapping the order
+		 * the hash lock and l2ad_mtx are taken).
+		 */
+		if (!mutex_tryenter(hash_lock)) {
+			/*
+			 * Missed the hash lock.  Retry.
+			 */
+			ARCSTAT_BUMP(arcstat_l2_evict_lock_retry);
+			mutex_exit(&dev->l2ad_mtx);
+			mutex_enter(hash_lock);
+			mutex_exit(hash_lock);
+			goto retry;
+		}
+
+		/*
+		 * A header can't be on this list if it doesn't have L2 header.
+		 */
+		ASSERT(HDR_HAS_L2HDR(hdr));
+
+		/* Ensure this header has finished being written. */
+		ASSERT(!HDR_L2_WRITING(hdr));
+		ASSERT(!HDR_L2_WRITE_HEAD(hdr));
+
+		if (!all && (hdr->b_l2hdr.b_daddr >= dev->l2ad_evict ||
+		    hdr->b_l2hdr.b_daddr < dev->l2ad_hand)) {
+			/*
+			 * We've evicted to the target address,
+			 * or the end of the device.
+			 */
+			mutex_exit(hash_lock);
+			break;
+		}
+
+		if (!HDR_HAS_L1HDR(hdr)) {
+			ASSERT(!HDR_L2_READING(hdr));
+			/*
+			 * This doesn't exist in the ARC.  Destroy.
+			 * arc_hdr_destroy() will call list_remove()
+			 * and decrement arcstat_l2_lsize.
+			 */
+			arc_change_state(arc_anon, hdr, hash_lock);
+			arc_hdr_destroy(hdr);
+		} else {
+			ASSERT(hdr->b_l1hdr.b_state != arc_l2c_only);
+			ARCSTAT_BUMP(arcstat_l2_evict_l1cached);
+			/*
+			 * Invalidate issued or about to be issued
+			 * reads, since we may be about to write
+			 * over this location.
+			 */
+			if (HDR_L2_READING(hdr)) {
+				ARCSTAT_BUMP(arcstat_l2_evict_reading);
+				arc_hdr_set_flags(hdr, ARC_FLAG_L2_EVICTED);
+			}
+
+			arc_hdr_l2hdr_destroy(hdr);
+		}
+		mutex_exit(hash_lock);
+	}
+	mutex_exit(&dev->l2ad_mtx);
+
+out:
+	/*
+	 * We need to check if we evict all buffers, otherwise we may iterate
+	 * unnecessarily.
+	 */
+	if (!all && rerun) {
+		/*
+		 * Bump device hand to the device start if it is approaching the
+		 * end. l2arc_evict() has already evicted ahead for this case.
+		 */
+		dev->l2ad_hand = dev->l2ad_start;
+		dev->l2ad_evict = dev->l2ad_start;
+		dev->l2ad_first = B_FALSE;
+		goto top;
+	}
+
+	ASSERT3U(dev->l2ad_hand + distance, <, dev->l2ad_end);
+	if (!dev->l2ad_first)
+		ASSERT3U(dev->l2ad_hand, <, dev->l2ad_evict);
+}
+
+/*
+ * Handle any abd transforms that might be required for writing to the L2ARC.
+ * If successful, this function will always return an abd with the data
+ * transformed as it is on disk in a new abd of asize bytes.
+ */
+static int
+l2arc_apply_transforms(spa_t *spa, arc_buf_hdr_t *hdr, uint64_t asize,
+    abd_t **abd_out)
+{
+	int ret;
+	void *tmp = NULL;
+	abd_t *cabd = NULL, *eabd = NULL, *to_write = hdr->b_l1hdr.b_pabd;
+	enum zio_compress compress = HDR_GET_COMPRESS(hdr);
+	uint64_t psize = HDR_GET_PSIZE(hdr);
+	uint64_t size = arc_hdr_size(hdr);
+	boolean_t ismd = HDR_ISTYPE_METADATA(hdr);
+	boolean_t bswap = (hdr->b_l1hdr.b_byteswap != DMU_BSWAP_NUMFUNCS);
+	dsl_crypto_key_t *dck = NULL;
+	uint8_t mac[ZIO_DATA_MAC_LEN] = { 0 };
+	boolean_t no_crypt = B_FALSE;
+
+	ASSERT((HDR_GET_COMPRESS(hdr) != ZIO_COMPRESS_OFF &&
+	    !HDR_COMPRESSION_ENABLED(hdr)) ||
+	    HDR_ENCRYPTED(hdr) || HDR_SHARED_DATA(hdr) || psize != asize);
+	ASSERT3U(psize, <=, asize);
+
+	/*
+	 * If this data simply needs its own buffer, we simply allocate it
+	 * and copy the data. This may be done to eliminate a dependency on a
+	 * shared buffer or to reallocate the buffer to match asize.
+	 */
+	if (HDR_HAS_RABD(hdr) && asize != psize) {
+		ASSERT3U(asize, >=, psize);
+		to_write = abd_alloc_for_io(asize, ismd);
+		abd_copy(to_write, hdr->b_crypt_hdr.b_rabd, psize);
+		if (psize != asize)
+			abd_zero_off(to_write, psize, asize - psize);
+		goto out;
+	}
+
+	if ((compress == ZIO_COMPRESS_OFF || HDR_COMPRESSION_ENABLED(hdr)) &&
+	    !HDR_ENCRYPTED(hdr)) {
+		ASSERT3U(size, ==, psize);
+		to_write = abd_alloc_for_io(asize, ismd);
+		abd_copy(to_write, hdr->b_l1hdr.b_pabd, size);
+		if (size != asize)
+			abd_zero_off(to_write, size, asize - size);
+		goto out;
+	}
+
+	if (compress != ZIO_COMPRESS_OFF && !HDR_COMPRESSION_ENABLED(hdr)) {
+		cabd = abd_alloc_for_io(asize, ismd);
+		tmp = abd_borrow_buf(cabd, asize);
+
+		psize = zio_compress_data(compress, to_write, tmp, size,
+		    hdr->b_complevel);
+
+		if (psize >= size) {
+			abd_return_buf(cabd, tmp, asize);
+			HDR_SET_COMPRESS(hdr, ZIO_COMPRESS_OFF);
+			to_write = cabd;
+			abd_copy(to_write, hdr->b_l1hdr.b_pabd, size);
+			if (size != asize)
+				abd_zero_off(to_write, size, asize - size);
+			goto encrypt;
+		}
+		ASSERT3U(psize, <=, HDR_GET_PSIZE(hdr));
+		if (psize < asize)
+			bzero((char *)tmp + psize, asize - psize);
+		psize = HDR_GET_PSIZE(hdr);
+		abd_return_buf_copy(cabd, tmp, asize);
+		to_write = cabd;
+	}
+
+encrypt:
+	if (HDR_ENCRYPTED(hdr)) {
+		eabd = abd_alloc_for_io(asize, ismd);
+
+		/*
+		 * If the dataset was disowned before the buffer
+		 * made it to this point, the key to re-encrypt
+		 * it won't be available. In this case we simply
+		 * won't write the buffer to the L2ARC.
+		 */
+		ret = spa_keystore_lookup_key(spa, hdr->b_crypt_hdr.b_dsobj,
+		    FTAG, &dck);
+		if (ret != 0)
+			goto error;
+
+		ret = zio_do_crypt_abd(B_TRUE, &dck->dck_key,
+		    hdr->b_crypt_hdr.b_ot, bswap, hdr->b_crypt_hdr.b_salt,
+		    hdr->b_crypt_hdr.b_iv, mac, psize, to_write, eabd,
+		    &no_crypt);
+		if (ret != 0)
+			goto error;
+
+		if (no_crypt)
+			abd_copy(eabd, to_write, psize);
+
+		if (psize != asize)
+			abd_zero_off(eabd, psize, asize - psize);
+
+		/* assert that the MAC we got here matches the one we saved */
+		ASSERT0(bcmp(mac, hdr->b_crypt_hdr.b_mac, ZIO_DATA_MAC_LEN));
+		spa_keystore_dsl_key_rele(spa, dck, FTAG);
+
+		if (to_write == cabd)
+			abd_free(cabd);
+
+		to_write = eabd;
+	}
+
+out:
+	ASSERT3P(to_write, !=, hdr->b_l1hdr.b_pabd);
+	*abd_out = to_write;
+	return (0);
+
+error:
+	if (dck != NULL)
+		spa_keystore_dsl_key_rele(spa, dck, FTAG);
+	if (cabd != NULL)
+		abd_free(cabd);
+	if (eabd != NULL)
+		abd_free(eabd);
+
+	*abd_out = NULL;
+	return (ret);
+}
+
+static void
+l2arc_blk_fetch_done(zio_t *zio)
+{
+	l2arc_read_callback_t *cb;
+
+	cb = zio->io_private;
+	if (cb->l2rcb_abd != NULL)
+		abd_put(cb->l2rcb_abd);
+	kmem_free(cb, sizeof (l2arc_read_callback_t));
+}
+
+/*
+ * Find and write ARC buffers to the L2ARC device.
+ *
+ * An ARC_FLAG_L2_WRITING flag is set so that the L2ARC buffers are not valid
+ * for reading until they have completed writing.
+ * The headroom_boost is an in-out parameter used to maintain headroom boost
+ * state between calls to this function.
+ *
+ * Returns the number of bytes actually written (which may be smaller than
+ * the delta by which the device hand has changed due to alignment and the
+ * writing of log blocks).
+ */
+static uint64_t
+l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz)
+{
+	arc_buf_hdr_t 		*hdr, *hdr_prev, *head;
+	uint64_t 		write_asize, write_psize, write_lsize, headroom;
+	boolean_t		full;
+	l2arc_write_callback_t	*cb = NULL;
+	zio_t 			*pio, *wzio;
+	uint64_t 		guid = spa_load_guid(spa);
+
+	ASSERT3P(dev->l2ad_vdev, !=, NULL);
+
+	pio = NULL;
+	write_lsize = write_asize = write_psize = 0;
+	full = B_FALSE;
+	head = kmem_cache_alloc(hdr_l2only_cache, KM_PUSHPAGE);
+	arc_hdr_set_flags(head, ARC_FLAG_L2_WRITE_HEAD | ARC_FLAG_HAS_L2HDR);
+
+	/*
+	 * Copy buffers for L2ARC writing.
+	 */
+	for (int try = 0; try < L2ARC_FEED_TYPES; try++) {
+		multilist_sublist_t *mls = l2arc_sublist_lock(try);
+		uint64_t passed_sz = 0;
+
+		VERIFY3P(mls, !=, NULL);
+
+		/*
+		 * L2ARC fast warmup.
+		 *
+		 * Until the ARC is warm and starts to evict, read from the
+		 * head of the ARC lists rather than the tail.
+		 */
+		if (arc_warm == B_FALSE)
+			hdr = multilist_sublist_head(mls);
+		else
+			hdr = multilist_sublist_tail(mls);
+
+		headroom = target_sz * l2arc_headroom;
+		if (zfs_compressed_arc_enabled)
+			headroom = (headroom * l2arc_headroom_boost) / 100;
+
+		for (; hdr; hdr = hdr_prev) {
+			kmutex_t *hash_lock;
+			abd_t *to_write = NULL;
+
+			if (arc_warm == B_FALSE)
+				hdr_prev = multilist_sublist_next(mls, hdr);
+			else
+				hdr_prev = multilist_sublist_prev(mls, hdr);
+
+			hash_lock = HDR_LOCK(hdr);
+			if (!mutex_tryenter(hash_lock)) {
+				/*
+				 * Skip this buffer rather than waiting.
+				 */
+				continue;
+			}
+
+			passed_sz += HDR_GET_LSIZE(hdr);
+			if (l2arc_headroom != 0 && passed_sz > headroom) {
+				/*
+				 * Searched too far.
+				 */
+				mutex_exit(hash_lock);
+				break;
+			}
+
+			if (!l2arc_write_eligible(guid, hdr)) {
+				mutex_exit(hash_lock);
+				continue;
+			}
+
+			/*
+			 * We rely on the L1 portion of the header below, so
+			 * it's invalid for this header to have been evicted out
+			 * of the ghost cache, prior to being written out. The
+			 * ARC_FLAG_L2_WRITING bit ensures this won't happen.
+			 */
+			ASSERT(HDR_HAS_L1HDR(hdr));
+
+			ASSERT3U(HDR_GET_PSIZE(hdr), >, 0);
+			ASSERT3U(arc_hdr_size(hdr), >, 0);
+			ASSERT(hdr->b_l1hdr.b_pabd != NULL ||
+			    HDR_HAS_RABD(hdr));
+			uint64_t psize = HDR_GET_PSIZE(hdr);
+			uint64_t asize = vdev_psize_to_asize(dev->l2ad_vdev,
+			    psize);
+
+			if ((write_asize + asize) > target_sz) {
+				full = B_TRUE;
+				mutex_exit(hash_lock);
+				break;
+			}
+
+			/*
+			 * We rely on the L1 portion of the header below, so
+			 * it's invalid for this header to have been evicted out
+			 * of the ghost cache, prior to being written out. The
+			 * ARC_FLAG_L2_WRITING bit ensures this won't happen.
+			 */
+			arc_hdr_set_flags(hdr, ARC_FLAG_L2_WRITING);
+			ASSERT(HDR_HAS_L1HDR(hdr));
+
+			ASSERT3U(HDR_GET_PSIZE(hdr), >, 0);
+			ASSERT(hdr->b_l1hdr.b_pabd != NULL ||
+			    HDR_HAS_RABD(hdr));
+			ASSERT3U(arc_hdr_size(hdr), >, 0);
+
+			/*
+			 * If this header has b_rabd, we can use this since it
+			 * must always match the data exactly as it exists on
+			 * disk. Otherwise, the L2ARC can normally use the
+			 * hdr's data, but if we're sharing data between the
+			 * hdr and one of its bufs, L2ARC needs its own copy of
+			 * the data so that the ZIO below can't race with the
+			 * buf consumer. To ensure that this copy will be
+			 * available for the lifetime of the ZIO and be cleaned
+			 * up afterwards, we add it to the l2arc_free_on_write
+			 * queue. If we need to apply any transforms to the
+			 * data (compression, encryption) we will also need the
+			 * extra buffer.
+			 */
+			if (HDR_HAS_RABD(hdr) && psize == asize) {
+				to_write = hdr->b_crypt_hdr.b_rabd;
+			} else if ((HDR_COMPRESSION_ENABLED(hdr) ||
+			    HDR_GET_COMPRESS(hdr) == ZIO_COMPRESS_OFF) &&
+			    !HDR_ENCRYPTED(hdr) && !HDR_SHARED_DATA(hdr) &&
+			    psize == asize) {
+				to_write = hdr->b_l1hdr.b_pabd;
+			} else {
+				int ret;
+				arc_buf_contents_t type = arc_buf_type(hdr);
+
+				ret = l2arc_apply_transforms(spa, hdr, asize,
+				    &to_write);
+				if (ret != 0) {
+					arc_hdr_clear_flags(hdr,
+					    ARC_FLAG_L2_WRITING);
+					mutex_exit(hash_lock);
+					continue;
+				}
+
+				l2arc_free_abd_on_write(to_write, asize, type);
+			}
+
+			if (pio == NULL) {
+				/*
+				 * Insert a dummy header on the buflist so
+				 * l2arc_write_done() can find where the
+				 * write buffers begin without searching.
+				 */
+				mutex_enter(&dev->l2ad_mtx);
+				list_insert_head(&dev->l2ad_buflist, head);
+				mutex_exit(&dev->l2ad_mtx);
+
+				cb = kmem_alloc(
+				    sizeof (l2arc_write_callback_t), KM_SLEEP);
+				cb->l2wcb_dev = dev;
+				cb->l2wcb_head = head;
+				/*
+				 * Create a list to save allocated abd buffers
+				 * for l2arc_log_blk_commit().
+				 */
+				list_create(&cb->l2wcb_abd_list,
+				    sizeof (l2arc_lb_abd_buf_t),
+				    offsetof(l2arc_lb_abd_buf_t, node));
+				pio = zio_root(spa, l2arc_write_done, cb,
+				    ZIO_FLAG_CANFAIL);
+			}
+
+			hdr->b_l2hdr.b_dev = dev;
+			hdr->b_l2hdr.b_hits = 0;
+
+			hdr->b_l2hdr.b_daddr = dev->l2ad_hand;
+			arc_hdr_set_flags(hdr, ARC_FLAG_HAS_L2HDR);
+
+			mutex_enter(&dev->l2ad_mtx);
+			list_insert_head(&dev->l2ad_buflist, hdr);
+			mutex_exit(&dev->l2ad_mtx);
+
+			(void) zfs_refcount_add_many(&dev->l2ad_alloc,
+			    arc_hdr_size(hdr), hdr);
+
+			wzio = zio_write_phys(pio, dev->l2ad_vdev,
+			    hdr->b_l2hdr.b_daddr, asize, to_write,
+			    ZIO_CHECKSUM_OFF, NULL, hdr,
+			    ZIO_PRIORITY_ASYNC_WRITE,
+			    ZIO_FLAG_CANFAIL, B_FALSE);
+
+			write_lsize += HDR_GET_LSIZE(hdr);
+			DTRACE_PROBE2(l2arc__write, vdev_t *, dev->l2ad_vdev,
+			    zio_t *, wzio);
+
+			write_psize += psize;
+			write_asize += asize;
+			dev->l2ad_hand += asize;
+			vdev_space_update(dev->l2ad_vdev, asize, 0, 0);
+
+			mutex_exit(hash_lock);
+
+			/*
+			 * Append buf info to current log and commit if full.
+			 * arcstat_l2_{size,asize} kstats are updated
+			 * internally.
+			 */
+			if (l2arc_log_blk_insert(dev, hdr))
+				l2arc_log_blk_commit(dev, pio, cb);
+
+			zio_nowait(wzio);
+		}
+
+		multilist_sublist_unlock(mls);
+
+		if (full == B_TRUE)
+			break;
+	}
+
+	/* No buffers selected for writing? */
+	if (pio == NULL) {
+		ASSERT0(write_lsize);
+		ASSERT(!HDR_HAS_L1HDR(head));
+		kmem_cache_free(hdr_l2only_cache, head);
+
+		/*
+		 * Although we did not write any buffers l2ad_evict may
+		 * have advanced.
+		 */
+		l2arc_dev_hdr_update(dev);
+
+		return (0);
+	}
+
+	if (!dev->l2ad_first)
+		ASSERT3U(dev->l2ad_hand, <=, dev->l2ad_evict);
+
+	ASSERT3U(write_asize, <=, target_sz);
+	ARCSTAT_BUMP(arcstat_l2_writes_sent);
+	ARCSTAT_INCR(arcstat_l2_write_bytes, write_psize);
+	ARCSTAT_INCR(arcstat_l2_lsize, write_lsize);
+	ARCSTAT_INCR(arcstat_l2_psize, write_psize);
+
+	dev->l2ad_writing = B_TRUE;
+	(void) zio_wait(pio);
+	dev->l2ad_writing = B_FALSE;
+
+	/*
+	 * Update the device header after the zio completes as
+	 * l2arc_write_done() may have updated the memory holding the log block
+	 * pointers in the device header.
+	 */
+	l2arc_dev_hdr_update(dev);
+
+	return (write_asize);
+}
+
+/*
+ * This thread feeds the L2ARC at regular intervals.  This is the beating
+ * heart of the L2ARC.
+ */
+/* ARGSUSED */
+static void
+l2arc_feed_thread(void *unused)
+{
+	callb_cpr_t cpr;
+	l2arc_dev_t *dev;
+	spa_t *spa;
+	uint64_t size, wrote;
+	clock_t begin, next = ddi_get_lbolt();
+	fstrans_cookie_t cookie;
+
+	CALLB_CPR_INIT(&cpr, &l2arc_feed_thr_lock, callb_generic_cpr, FTAG);
+
+	mutex_enter(&l2arc_feed_thr_lock);
+
+	cookie = spl_fstrans_mark();
+	while (l2arc_thread_exit == 0) {
+		CALLB_CPR_SAFE_BEGIN(&cpr);
+		(void) cv_timedwait_sig(&l2arc_feed_thr_cv,
+		    &l2arc_feed_thr_lock, next);
+		CALLB_CPR_SAFE_END(&cpr, &l2arc_feed_thr_lock);
+		next = ddi_get_lbolt() + hz;
+
+		/*
+		 * Quick check for L2ARC devices.
+		 */
+		mutex_enter(&l2arc_dev_mtx);
+		if (l2arc_ndev == 0) {
+			mutex_exit(&l2arc_dev_mtx);
+			continue;
+		}
+		mutex_exit(&l2arc_dev_mtx);
+		begin = ddi_get_lbolt();
+
+		/*
+		 * This selects the next l2arc device to write to, and in
+		 * doing so the next spa to feed from: dev->l2ad_spa.   This
+		 * will return NULL if there are now no l2arc devices or if
+		 * they are all faulted.
+		 *
+		 * If a device is returned, its spa's config lock is also
+		 * held to prevent device removal.  l2arc_dev_get_next()
+		 * will grab and release l2arc_dev_mtx.
+		 */
+		if ((dev = l2arc_dev_get_next()) == NULL)
+			continue;
+
+		spa = dev->l2ad_spa;
+		ASSERT3P(spa, !=, NULL);
+
+		/*
+		 * If the pool is read-only then force the feed thread to
+		 * sleep a little longer.
+		 */
+		if (!spa_writeable(spa)) {
+			next = ddi_get_lbolt() + 5 * l2arc_feed_secs * hz;
+			spa_config_exit(spa, SCL_L2ARC, dev);
+			continue;
+		}
+
+		/*
+		 * Avoid contributing to memory pressure.
+		 */
+		if (arc_reclaim_needed()) {
+			ARCSTAT_BUMP(arcstat_l2_abort_lowmem);
+			spa_config_exit(spa, SCL_L2ARC, dev);
+			continue;
+		}
+
+		ARCSTAT_BUMP(arcstat_l2_feeds);
+
+		size = l2arc_write_size(dev);
+
+		/*
+		 * Evict L2ARC buffers that will be overwritten.
+		 */
+		l2arc_evict(dev, size, B_FALSE);
+
+		/*
+		 * Write ARC buffers.
+		 */
+		wrote = l2arc_write_buffers(spa, dev, size);
+
+		/*
+		 * Calculate interval between writes.
+		 */
+		next = l2arc_write_interval(begin, size, wrote);
+		spa_config_exit(spa, SCL_L2ARC, dev);
+	}
+	spl_fstrans_unmark(cookie);
+
+	l2arc_thread_exit = 0;
+	cv_broadcast(&l2arc_feed_thr_cv);
+	CALLB_CPR_EXIT(&cpr);		/* drops l2arc_feed_thr_lock */
+	thread_exit();
+}
+
+boolean_t
+l2arc_vdev_present(vdev_t *vd)
+{
+	return (l2arc_vdev_get(vd) != NULL);
+}
+
+/*
+ * Returns the l2arc_dev_t associated with a particular vdev_t or NULL if
+ * the vdev_t isn't an L2ARC device.
+ */
+l2arc_dev_t *
+l2arc_vdev_get(vdev_t *vd)
+{
+	l2arc_dev_t	*dev;
+
+	mutex_enter(&l2arc_dev_mtx);
+	for (dev = list_head(l2arc_dev_list); dev != NULL;
+	    dev = list_next(l2arc_dev_list, dev)) {
+		if (dev->l2ad_vdev == vd)
+			break;
+	}
+	mutex_exit(&l2arc_dev_mtx);
+
+	return (dev);
+}
+
+/*
+ * Add a vdev for use by the L2ARC.  By this point the spa has already
+ * validated the vdev and opened it.
+ */
+void
+l2arc_add_vdev(spa_t *spa, vdev_t *vd)
+{
+	l2arc_dev_t		*adddev;
+	uint64_t		l2dhdr_asize;
+
+	ASSERT(!l2arc_vdev_present(vd));
+
+	vdev_ashift_optimize(vd);
+
+	/*
+	 * Create a new l2arc device entry.
+	 */
+	adddev = vmem_zalloc(sizeof (l2arc_dev_t), KM_SLEEP);
+	adddev->l2ad_spa = spa;
+	adddev->l2ad_vdev = vd;
+	/* leave extra size for an l2arc device header */
+	l2dhdr_asize = adddev->l2ad_dev_hdr_asize =
+	    MAX(sizeof (*adddev->l2ad_dev_hdr), 1 << vd->vdev_ashift);
+	adddev->l2ad_start = VDEV_LABEL_START_SIZE + l2dhdr_asize;
+	adddev->l2ad_end = VDEV_LABEL_START_SIZE + vdev_get_min_asize(vd);
+	ASSERT3U(adddev->l2ad_start, <, adddev->l2ad_end);
+	adddev->l2ad_hand = adddev->l2ad_start;
+	adddev->l2ad_evict = adddev->l2ad_start;
+	adddev->l2ad_first = B_TRUE;
+	adddev->l2ad_writing = B_FALSE;
+	adddev->l2ad_trim_all = B_FALSE;
+	list_link_init(&adddev->l2ad_node);
+	adddev->l2ad_dev_hdr = kmem_zalloc(l2dhdr_asize, KM_SLEEP);
+
+	mutex_init(&adddev->l2ad_mtx, NULL, MUTEX_DEFAULT, NULL);
+	/*
+	 * This is a list of all ARC buffers that are still valid on the
+	 * device.
+	 */
+	list_create(&adddev->l2ad_buflist, sizeof (arc_buf_hdr_t),
+	    offsetof(arc_buf_hdr_t, b_l2hdr.b_l2node));
+
+	/*
+	 * This is a list of pointers to log blocks that are still present
+	 * on the device.
+	 */
+	list_create(&adddev->l2ad_lbptr_list, sizeof (l2arc_lb_ptr_buf_t),
+	    offsetof(l2arc_lb_ptr_buf_t, node));
+
+	vdev_space_update(vd, 0, 0, adddev->l2ad_end - adddev->l2ad_hand);
+	zfs_refcount_create(&adddev->l2ad_alloc);
+	zfs_refcount_create(&adddev->l2ad_lb_asize);
+	zfs_refcount_create(&adddev->l2ad_lb_count);
+
+	/*
+	 * Add device to global list
+	 */
+	mutex_enter(&l2arc_dev_mtx);
+	list_insert_head(l2arc_dev_list, adddev);
+	atomic_inc_64(&l2arc_ndev);
+	mutex_exit(&l2arc_dev_mtx);
+
+	/*
+	 * Decide if vdev is eligible for L2ARC rebuild
+	 */
+	l2arc_rebuild_vdev(adddev->l2ad_vdev, B_FALSE);
+}
+
+void
+l2arc_rebuild_vdev(vdev_t *vd, boolean_t reopen)
+{
+	l2arc_dev_t		*dev = NULL;
+	l2arc_dev_hdr_phys_t	*l2dhdr;
+	uint64_t		l2dhdr_asize;
+	spa_t			*spa;
+	int			err;
+	boolean_t		l2dhdr_valid = B_TRUE;
+
+	dev = l2arc_vdev_get(vd);
+	ASSERT3P(dev, !=, NULL);
+	spa = dev->l2ad_spa;
+	l2dhdr = dev->l2ad_dev_hdr;
+	l2dhdr_asize = dev->l2ad_dev_hdr_asize;
+
+	/*
+	 * The L2ARC has to hold at least the payload of one log block for
+	 * them to be restored (persistent L2ARC). The payload of a log block
+	 * depends on the amount of its log entries. We always write log blocks
+	 * with 1022 entries. How many of them are committed or restored depends
+	 * on the size of the L2ARC device. Thus the maximum payload of
+	 * one log block is 1022 * SPA_MAXBLOCKSIZE = 16GB. If the L2ARC device
+	 * is less than that, we reduce the amount of committed and restored
+	 * log entries per block so as to enable persistence.
+	 */
+	if (dev->l2ad_end < l2arc_rebuild_blocks_min_l2size) {
+		dev->l2ad_log_entries = 0;
+	} else {
+		dev->l2ad_log_entries = MIN((dev->l2ad_end -
+		    dev->l2ad_start) >> SPA_MAXBLOCKSHIFT,
+		    L2ARC_LOG_BLK_MAX_ENTRIES);
+	}
+
+	/*
+	 * Read the device header, if an error is returned do not rebuild L2ARC.
+	 */
+	if ((err = l2arc_dev_hdr_read(dev)) != 0)
+		l2dhdr_valid = B_FALSE;
+
+	if (l2dhdr_valid && dev->l2ad_log_entries > 0) {
+		/*
+		 * If we are onlining a cache device (vdev_reopen) that was
+		 * still present (l2arc_vdev_present()) and rebuild is enabled,
+		 * we should evict all ARC buffers and pointers to log blocks
+		 * and reclaim their space before restoring its contents to
+		 * L2ARC.
+		 */
+		if (reopen) {
+			if (!l2arc_rebuild_enabled) {
+				return;
+			} else {
+				l2arc_evict(dev, 0, B_TRUE);
+				/* start a new log block */
+				dev->l2ad_log_ent_idx = 0;
+				dev->l2ad_log_blk_payload_asize = 0;
+				dev->l2ad_log_blk_payload_start = 0;
+			}
+		}
+		/*
+		 * Just mark the device as pending for a rebuild. We won't
+		 * be starting a rebuild in line here as it would block pool
+		 * import. Instead spa_load_impl will hand that off to an
+		 * async task which will call l2arc_spa_rebuild_start.
+		 */
+		dev->l2ad_rebuild = B_TRUE;
+	} else if (spa_writeable(spa)) {
+		/*
+		 * In this case TRIM the whole device if l2arc_trim_ahead > 0,
+		 * otherwise create a new header. We zero out the memory holding
+		 * the header to reset dh_start_lbps. If we TRIM the whole
+		 * device the new header will be written by
+		 * vdev_trim_l2arc_thread() at the end of the TRIM to update the
+		 * trim_state in the header too. When reading the header, if
+		 * trim_state is not VDEV_TRIM_COMPLETE and l2arc_trim_ahead > 0
+		 * we opt to TRIM the whole device again.
+		 */
+		if (l2arc_trim_ahead > 0) {
+			dev->l2ad_trim_all = B_TRUE;
+		} else {
+			bzero(l2dhdr, l2dhdr_asize);
+			l2arc_dev_hdr_update(dev);
+		}
+	}
+}
+
+/*
+ * Remove a vdev from the L2ARC.
+ */
+void
+l2arc_remove_vdev(vdev_t *vd)
+{
+	l2arc_dev_t *remdev = NULL;
+
+	/*
+	 * Find the device by vdev
+	 */
+	remdev = l2arc_vdev_get(vd);
+	ASSERT3P(remdev, !=, NULL);
+
+	/*
+	 * Cancel any ongoing or scheduled rebuild.
+	 */
+	mutex_enter(&l2arc_rebuild_thr_lock);
+	if (remdev->l2ad_rebuild_began == B_TRUE) {
+		remdev->l2ad_rebuild_cancel = B_TRUE;
+		while (remdev->l2ad_rebuild == B_TRUE)
+			cv_wait(&l2arc_rebuild_thr_cv, &l2arc_rebuild_thr_lock);
+	}
+	mutex_exit(&l2arc_rebuild_thr_lock);
+
+	/*
+	 * Remove device from global list
+	 */
+	mutex_enter(&l2arc_dev_mtx);
+	list_remove(l2arc_dev_list, remdev);
+	l2arc_dev_last = NULL;		/* may have been invalidated */
+	atomic_dec_64(&l2arc_ndev);
+	mutex_exit(&l2arc_dev_mtx);
+
+	/*
+	 * Clear all buflists and ARC references.  L2ARC device flush.
+	 */
+	l2arc_evict(remdev, 0, B_TRUE);
+	list_destroy(&remdev->l2ad_buflist);
+	ASSERT(list_is_empty(&remdev->l2ad_lbptr_list));
+	list_destroy(&remdev->l2ad_lbptr_list);
+	mutex_destroy(&remdev->l2ad_mtx);
+	zfs_refcount_destroy(&remdev->l2ad_alloc);
+	zfs_refcount_destroy(&remdev->l2ad_lb_asize);
+	zfs_refcount_destroy(&remdev->l2ad_lb_count);
+	kmem_free(remdev->l2ad_dev_hdr, remdev->l2ad_dev_hdr_asize);
+	vmem_free(remdev, sizeof (l2arc_dev_t));
+}
+
+void
+l2arc_init(void)
+{
+	l2arc_thread_exit = 0;
+	l2arc_ndev = 0;
+	l2arc_writes_sent = 0;
+	l2arc_writes_done = 0;
+
+	mutex_init(&l2arc_feed_thr_lock, NULL, MUTEX_DEFAULT, NULL);
+	cv_init(&l2arc_feed_thr_cv, NULL, CV_DEFAULT, NULL);
+	mutex_init(&l2arc_rebuild_thr_lock, NULL, MUTEX_DEFAULT, NULL);
+	cv_init(&l2arc_rebuild_thr_cv, NULL, CV_DEFAULT, NULL);
+	mutex_init(&l2arc_dev_mtx, NULL, MUTEX_DEFAULT, NULL);
+	mutex_init(&l2arc_free_on_write_mtx, NULL, MUTEX_DEFAULT, NULL);
+
+	l2arc_dev_list = &L2ARC_dev_list;
+	l2arc_free_on_write = &L2ARC_free_on_write;
+	list_create(l2arc_dev_list, sizeof (l2arc_dev_t),
+	    offsetof(l2arc_dev_t, l2ad_node));
+	list_create(l2arc_free_on_write, sizeof (l2arc_data_free_t),
+	    offsetof(l2arc_data_free_t, l2df_list_node));
+}
+
+void
+l2arc_fini(void)
+{
+	mutex_destroy(&l2arc_feed_thr_lock);
+	cv_destroy(&l2arc_feed_thr_cv);
+	mutex_destroy(&l2arc_rebuild_thr_lock);
+	cv_destroy(&l2arc_rebuild_thr_cv);
+	mutex_destroy(&l2arc_dev_mtx);
+	mutex_destroy(&l2arc_free_on_write_mtx);
+
+	list_destroy(l2arc_dev_list);
+	list_destroy(l2arc_free_on_write);
+}
+
+void
+l2arc_start(void)
+{
+	if (!(spa_mode_global & SPA_MODE_WRITE))
+		return;
+
+	(void) thread_create(NULL, 0, l2arc_feed_thread, NULL, 0, &p0,
+	    TS_RUN, defclsyspri);
+}
+
+void
+l2arc_stop(void)
+{
+	if (!(spa_mode_global & SPA_MODE_WRITE))
+		return;
+
+	mutex_enter(&l2arc_feed_thr_lock);
+	cv_signal(&l2arc_feed_thr_cv);	/* kick thread out of startup */
+	l2arc_thread_exit = 1;
+	while (l2arc_thread_exit != 0)
+		cv_wait(&l2arc_feed_thr_cv, &l2arc_feed_thr_lock);
+	mutex_exit(&l2arc_feed_thr_lock);
+}
+
+/*
+ * Punches out rebuild threads for the L2ARC devices in a spa. This should
+ * be called after pool import from the spa async thread, since starting
+ * these threads directly from spa_import() will make them part of the
+ * "zpool import" context and delay process exit (and thus pool import).
+ */
+void
+l2arc_spa_rebuild_start(spa_t *spa)
+{
+	ASSERT(MUTEX_HELD(&spa_namespace_lock));
+
+	/*
+	 * Locate the spa's l2arc devices and kick off rebuild threads.
+	 */
+	for (int i = 0; i < spa->spa_l2cache.sav_count; i++) {
+		l2arc_dev_t *dev =
+		    l2arc_vdev_get(spa->spa_l2cache.sav_vdevs[i]);
+		if (dev == NULL) {
+			/* Don't attempt a rebuild if the vdev is UNAVAIL */
+			continue;
+		}
+		mutex_enter(&l2arc_rebuild_thr_lock);
+		if (dev->l2ad_rebuild && !dev->l2ad_rebuild_cancel) {
+			dev->l2ad_rebuild_began = B_TRUE;
+			(void) thread_create(NULL, 0, l2arc_dev_rebuild_thread,
+			    dev, 0, &p0, TS_RUN, minclsyspri);
+		}
+		mutex_exit(&l2arc_rebuild_thr_lock);
+	}
+}
+
+/*
+ * Main entry point for L2ARC rebuilding.
+ */
+static void
+l2arc_dev_rebuild_thread(void *arg)
+{
+	l2arc_dev_t *dev = arg;
+
+	VERIFY(!dev->l2ad_rebuild_cancel);
+	VERIFY(dev->l2ad_rebuild);
+	(void) l2arc_rebuild(dev);
+	mutex_enter(&l2arc_rebuild_thr_lock);
+	dev->l2ad_rebuild_began = B_FALSE;
+	dev->l2ad_rebuild = B_FALSE;
+	mutex_exit(&l2arc_rebuild_thr_lock);
+
+	thread_exit();
+}
+
+/*
+ * This function implements the actual L2ARC metadata rebuild. It:
+ * starts reading the log block chain and restores each block's contents
+ * to memory (reconstructing arc_buf_hdr_t's).
+ *
+ * Operation stops under any of the following conditions:
+ *
+ * 1) We reach the end of the log block chain.
+ * 2) We encounter *any* error condition (cksum errors, io errors)
+ */
+static int
+l2arc_rebuild(l2arc_dev_t *dev)
+{
+	vdev_t			*vd = dev->l2ad_vdev;
+	spa_t			*spa = vd->vdev_spa;
+	int			err = 0;
+	l2arc_dev_hdr_phys_t	*l2dhdr = dev->l2ad_dev_hdr;
+	l2arc_log_blk_phys_t	*this_lb, *next_lb;
+	zio_t			*this_io = NULL, *next_io = NULL;
+	l2arc_log_blkptr_t	lbps[2];
+	l2arc_lb_ptr_buf_t	*lb_ptr_buf;
+	boolean_t		lock_held;
+
+	this_lb = vmem_zalloc(sizeof (*this_lb), KM_SLEEP);
+	next_lb = vmem_zalloc(sizeof (*next_lb), KM_SLEEP);
+
+	/*
+	 * We prevent device removal while issuing reads to the device,
+	 * then during the rebuilding phases we drop this lock again so
+	 * that a spa_unload or device remove can be initiated - this is
+	 * safe, because the spa will signal us to stop before removing
+	 * our device and wait for us to stop.
+	 */
+	spa_config_enter(spa, SCL_L2ARC, vd, RW_READER);
+	lock_held = B_TRUE;
+
+	/*
+	 * Retrieve the persistent L2ARC device state.
+	 * L2BLK_GET_PSIZE returns aligned size for log blocks.
+	 */
+	dev->l2ad_evict = MAX(l2dhdr->dh_evict, dev->l2ad_start);
+	dev->l2ad_hand = MAX(l2dhdr->dh_start_lbps[0].lbp_daddr +
+	    L2BLK_GET_PSIZE((&l2dhdr->dh_start_lbps[0])->lbp_prop),
+	    dev->l2ad_start);
+	dev->l2ad_first = !!(l2dhdr->dh_flags & L2ARC_DEV_HDR_EVICT_FIRST);
+
+	vd->vdev_trim_action_time = l2dhdr->dh_trim_action_time;
+	vd->vdev_trim_state = l2dhdr->dh_trim_state;
+
+	/*
+	 * In case the zfs module parameter l2arc_rebuild_enabled is false
+	 * we do not start the rebuild process.
+	 */
+	if (!l2arc_rebuild_enabled)
+		goto out;
+
+	/* Prepare the rebuild process */
+	bcopy(l2dhdr->dh_start_lbps, lbps, sizeof (lbps));
+
+	/* Start the rebuild process */
+	for (;;) {
+		if (!l2arc_log_blkptr_valid(dev, &lbps[0]))
+			break;
+
+		if ((err = l2arc_log_blk_read(dev, &lbps[0], &lbps[1],
+		    this_lb, next_lb, this_io, &next_io)) != 0)
+			goto out;
+
+		/*
+		 * Our memory pressure valve. If the system is running low
+		 * on memory, rather than swamping memory with new ARC buf
+		 * hdrs, we opt not to rebuild the L2ARC. At this point,
+		 * however, we have already set up our L2ARC dev to chain in
+		 * new metadata log blocks, so the user may choose to offline/
+		 * online the L2ARC dev at a later time (or re-import the pool)
+		 * to reconstruct it (when there's less memory pressure).
+		 */
+		if (arc_reclaim_needed()) {
+			ARCSTAT_BUMP(arcstat_l2_rebuild_abort_lowmem);
+			cmn_err(CE_NOTE, "System running low on memory, "
+			    "aborting L2ARC rebuild.");
+			err = SET_ERROR(ENOMEM);
+			goto out;
+		}
+
+		spa_config_exit(spa, SCL_L2ARC, vd);
+		lock_held = B_FALSE;
+
+		/*
+		 * Now that we know that the next_lb checks out alright, we
+		 * can start reconstruction from this log block.
+		 * L2BLK_GET_PSIZE returns aligned size for log blocks.
+		 */
+		uint64_t asize = L2BLK_GET_PSIZE((&lbps[0])->lbp_prop);
+		l2arc_log_blk_restore(dev, this_lb, asize, lbps[0].lbp_daddr);
+
+		/*
+		 * log block restored, include its pointer in the list of
+		 * pointers to log blocks present in the L2ARC device.
+		 */
+		lb_ptr_buf = kmem_zalloc(sizeof (l2arc_lb_ptr_buf_t), KM_SLEEP);
+		lb_ptr_buf->lb_ptr = kmem_zalloc(sizeof (l2arc_log_blkptr_t),
+		    KM_SLEEP);
+		bcopy(&lbps[0], lb_ptr_buf->lb_ptr,
+		    sizeof (l2arc_log_blkptr_t));
+		mutex_enter(&dev->l2ad_mtx);
+		list_insert_tail(&dev->l2ad_lbptr_list, lb_ptr_buf);
+		ARCSTAT_INCR(arcstat_l2_log_blk_asize, asize);
+		ARCSTAT_BUMP(arcstat_l2_log_blk_count);
+		zfs_refcount_add_many(&dev->l2ad_lb_asize, asize, lb_ptr_buf);
+		zfs_refcount_add(&dev->l2ad_lb_count, lb_ptr_buf);
+		mutex_exit(&dev->l2ad_mtx);
+		vdev_space_update(vd, asize, 0, 0);
+
+		/*
+		 * Protection against loops of log blocks:
+		 *
+		 *				       l2ad_hand  l2ad_evict
+		 *                                         V	      V
+		 * l2ad_start |=======================================| l2ad_end
+		 *             -----|||----|||---|||----|||
+		 *                  (3)    (2)   (1)    (0)
+		 *             ---|||---|||----|||---|||
+		 *		  (7)   (6)    (5)   (4)
+		 *
+		 * In this situation the pointer of log block (4) passes
+		 * l2arc_log_blkptr_valid() but the log block should not be
+		 * restored as it is overwritten by the payload of log block
+		 * (0). Only log blocks (0)-(3) should be restored. We check
+		 * whether l2ad_evict lies in between the payload starting
+		 * offset of the next log block (lbps[1].lbp_payload_start)
+		 * and the payload starting offset of the present log block
+		 * (lbps[0].lbp_payload_start). If true and this isn't the
+		 * first pass, we are looping from the beginning and we should
+		 * stop.
+		 */
+		if (l2arc_range_check_overlap(lbps[1].lbp_payload_start,
+		    lbps[0].lbp_payload_start, dev->l2ad_evict) &&
+		    !dev->l2ad_first)
+			goto out;
+
+		for (;;) {
+			mutex_enter(&l2arc_rebuild_thr_lock);
+			if (dev->l2ad_rebuild_cancel) {
+				dev->l2ad_rebuild = B_FALSE;
+				cv_signal(&l2arc_rebuild_thr_cv);
+				mutex_exit(&l2arc_rebuild_thr_lock);
+				err = SET_ERROR(ECANCELED);
+				goto out;
+			}
+			mutex_exit(&l2arc_rebuild_thr_lock);
+			if (spa_config_tryenter(spa, SCL_L2ARC, vd,
+			    RW_READER)) {
+				lock_held = B_TRUE;
+				break;
+			}
+			/*
+			 * L2ARC config lock held by somebody in writer,
+			 * possibly due to them trying to remove us. They'll
+			 * likely to want us to shut down, so after a little
+			 * delay, we check l2ad_rebuild_cancel and retry
+			 * the lock again.
+			 */
+			delay(1);
+		}
+
+		/*
+		 * Continue with the next log block.
+		 */
+		lbps[0] = lbps[1];
+		lbps[1] = this_lb->lb_prev_lbp;
+		PTR_SWAP(this_lb, next_lb);
+		this_io = next_io;
+		next_io = NULL;
+		}
+
+	if (this_io != NULL)
+		l2arc_log_blk_fetch_abort(this_io);
+out:
+	if (next_io != NULL)
+		l2arc_log_blk_fetch_abort(next_io);
+	vmem_free(this_lb, sizeof (*this_lb));
+	vmem_free(next_lb, sizeof (*next_lb));
+
+	if (!l2arc_rebuild_enabled) {
+		spa_history_log_internal(spa, "L2ARC rebuild", NULL,
+		    "disabled");
+	} else if (err == 0 && zfs_refcount_count(&dev->l2ad_lb_count) > 0) {
+		ARCSTAT_BUMP(arcstat_l2_rebuild_success);
+		spa_history_log_internal(spa, "L2ARC rebuild", NULL,
+		    "successful, restored %llu blocks",
+		    (u_longlong_t)zfs_refcount_count(&dev->l2ad_lb_count));
+	} else if (err == 0 && zfs_refcount_count(&dev->l2ad_lb_count) == 0) {
+		/*
+		 * No error but also nothing restored, meaning the lbps array
+		 * in the device header points to invalid/non-present log
+		 * blocks. Reset the header.
+		 */
+		spa_history_log_internal(spa, "L2ARC rebuild", NULL,
+		    "no valid log blocks");
+		bzero(l2dhdr, dev->l2ad_dev_hdr_asize);
+		l2arc_dev_hdr_update(dev);
+	} else if (err == ECANCELED) {
+		/*
+		 * In case the rebuild was canceled do not log to spa history
+		 * log as the pool may be in the process of being removed.
+		 */
+		zfs_dbgmsg("L2ARC rebuild aborted, restored %llu blocks",
+		    zfs_refcount_count(&dev->l2ad_lb_count));
+	} else if (err != 0) {
+		spa_history_log_internal(spa, "L2ARC rebuild", NULL,
+		    "aborted, restored %llu blocks",
+		    (u_longlong_t)zfs_refcount_count(&dev->l2ad_lb_count));
+	}
+
+	if (lock_held)
+		spa_config_exit(spa, SCL_L2ARC, vd);
+
+	return (err);
+}
+
+/*
+ * Attempts to read the device header on the provided L2ARC device and writes
+ * it to `hdr'. On success, this function returns 0, otherwise the appropriate
+ * error code is returned.
+ */
+static int
+l2arc_dev_hdr_read(l2arc_dev_t *dev)
+{
+	int			err;
+	uint64_t		guid;
+	l2arc_dev_hdr_phys_t	*l2dhdr = dev->l2ad_dev_hdr;
+	const uint64_t		l2dhdr_asize = dev->l2ad_dev_hdr_asize;
+	abd_t 			*abd;
+
+	guid = spa_guid(dev->l2ad_vdev->vdev_spa);
+
+	abd = abd_get_from_buf(l2dhdr, l2dhdr_asize);
+
+	err = zio_wait(zio_read_phys(NULL, dev->l2ad_vdev,
+	    VDEV_LABEL_START_SIZE, l2dhdr_asize, abd,
+	    ZIO_CHECKSUM_LABEL, NULL, NULL, ZIO_PRIORITY_ASYNC_READ,
+	    ZIO_FLAG_DONT_CACHE | ZIO_FLAG_CANFAIL |
+	    ZIO_FLAG_DONT_PROPAGATE | ZIO_FLAG_DONT_RETRY |
+	    ZIO_FLAG_SPECULATIVE, B_FALSE));
+
+	abd_put(abd);
+
+	if (err != 0) {
+		ARCSTAT_BUMP(arcstat_l2_rebuild_abort_dh_errors);
+		zfs_dbgmsg("L2ARC IO error (%d) while reading device header, "
+		    "vdev guid: %llu", err, dev->l2ad_vdev->vdev_guid);
+		return (err);
+	}
+
+	if (l2dhdr->dh_magic == BSWAP_64(L2ARC_DEV_HDR_MAGIC))
+		byteswap_uint64_array(l2dhdr, sizeof (*l2dhdr));
+
+	if (l2dhdr->dh_magic != L2ARC_DEV_HDR_MAGIC ||
+	    l2dhdr->dh_spa_guid != guid ||
+	    l2dhdr->dh_vdev_guid != dev->l2ad_vdev->vdev_guid ||
+	    l2dhdr->dh_version != L2ARC_PERSISTENT_VERSION ||
+	    l2dhdr->dh_log_entries != dev->l2ad_log_entries ||
+	    l2dhdr->dh_end != dev->l2ad_end ||
+	    !l2arc_range_check_overlap(dev->l2ad_start, dev->l2ad_end,
+	    l2dhdr->dh_evict) ||
+	    (l2dhdr->dh_trim_state != VDEV_TRIM_COMPLETE &&
+	    l2arc_trim_ahead > 0)) {
+		/*
+		 * Attempt to rebuild a device containing no actual dev hdr
+		 * or containing a header from some other pool or from another
+		 * version of persistent L2ARC.
+		 */
+		ARCSTAT_BUMP(arcstat_l2_rebuild_abort_unsupported);
+		return (SET_ERROR(ENOTSUP));
+	}
+
+	return (0);
+}
+
+/*
+ * Reads L2ARC log blocks from storage and validates their contents.
+ *
+ * This function implements a simple fetcher to make sure that while
+ * we're processing one buffer the L2ARC is already fetching the next
+ * one in the chain.
+ *
+ * The arguments this_lp and next_lp point to the current and next log block
+ * address in the block chain. Similarly, this_lb and next_lb hold the
+ * l2arc_log_blk_phys_t's of the current and next L2ARC blk.
+ *
+ * The `this_io' and `next_io' arguments are used for block fetching.
+ * When issuing the first blk IO during rebuild, you should pass NULL for
+ * `this_io'. This function will then issue a sync IO to read the block and
+ * also issue an async IO to fetch the next block in the block chain. The
+ * fetched IO is returned in `next_io'. On subsequent calls to this
+ * function, pass the value returned in `next_io' from the previous call
+ * as `this_io' and a fresh `next_io' pointer to hold the next fetch IO.
+ * Prior to the call, you should initialize your `next_io' pointer to be
+ * NULL. If no fetch IO was issued, the pointer is left set at NULL.
+ *
+ * On success, this function returns 0, otherwise it returns an appropriate
+ * error code. On error the fetching IO is aborted and cleared before
+ * returning from this function. Therefore, if we return `success', the
+ * caller can assume that we have taken care of cleanup of fetch IOs.
+ */
+static int
+l2arc_log_blk_read(l2arc_dev_t *dev,
+    const l2arc_log_blkptr_t *this_lbp, const l2arc_log_blkptr_t *next_lbp,
+    l2arc_log_blk_phys_t *this_lb, l2arc_log_blk_phys_t *next_lb,
+    zio_t *this_io, zio_t **next_io)
+{
+	int		err = 0;
+	zio_cksum_t	cksum;
+	abd_t		*abd = NULL;
+	uint64_t	asize;
+
+	ASSERT(this_lbp != NULL && next_lbp != NULL);
+	ASSERT(this_lb != NULL && next_lb != NULL);
+	ASSERT(next_io != NULL && *next_io == NULL);
+	ASSERT(l2arc_log_blkptr_valid(dev, this_lbp));
+
+	/*
+	 * Check to see if we have issued the IO for this log block in a
+	 * previous run. If not, this is the first call, so issue it now.
+	 */
+	if (this_io == NULL) {
+		this_io = l2arc_log_blk_fetch(dev->l2ad_vdev, this_lbp,
+		    this_lb);
+	}
+
+	/*
+	 * Peek to see if we can start issuing the next IO immediately.
+	 */
+	if (l2arc_log_blkptr_valid(dev, next_lbp)) {
+		/*
+		 * Start issuing IO for the next log block early - this
+		 * should help keep the L2ARC device busy while we
+		 * decompress and restore this log block.
+		 */
+		*next_io = l2arc_log_blk_fetch(dev->l2ad_vdev, next_lbp,
+		    next_lb);
+	}
+
+	/* Wait for the IO to read this log block to complete */
+	if ((err = zio_wait(this_io)) != 0) {
+		ARCSTAT_BUMP(arcstat_l2_rebuild_abort_io_errors);
+		zfs_dbgmsg("L2ARC IO error (%d) while reading log block, "
+		    "offset: %llu, vdev guid: %llu", err, this_lbp->lbp_daddr,
+		    dev->l2ad_vdev->vdev_guid);
+		goto cleanup;
+	}
+
+	/*
+	 * Make sure the buffer checks out.
+	 * L2BLK_GET_PSIZE returns aligned size for log blocks.
+	 */
+	asize = L2BLK_GET_PSIZE((this_lbp)->lbp_prop);
+	fletcher_4_native(this_lb, asize, NULL, &cksum);
+	if (!ZIO_CHECKSUM_EQUAL(cksum, this_lbp->lbp_cksum)) {
+		ARCSTAT_BUMP(arcstat_l2_rebuild_abort_cksum_lb_errors);
+		zfs_dbgmsg("L2ARC log block cksum failed, offset: %llu, "
+		    "vdev guid: %llu, l2ad_hand: %llu, l2ad_evict: %llu",
+		    this_lbp->lbp_daddr, dev->l2ad_vdev->vdev_guid,
+		    dev->l2ad_hand, dev->l2ad_evict);
+		err = SET_ERROR(ECKSUM);
+		goto cleanup;
+	}
+
+	/* Now we can take our time decoding this buffer */
+	switch (L2BLK_GET_COMPRESS((this_lbp)->lbp_prop)) {
+	case ZIO_COMPRESS_OFF:
+		break;
+	case ZIO_COMPRESS_LZ4:
+		abd = abd_alloc_for_io(asize, B_TRUE);
+		abd_copy_from_buf_off(abd, this_lb, 0, asize);
+		if ((err = zio_decompress_data(
+		    L2BLK_GET_COMPRESS((this_lbp)->lbp_prop),
+		    abd, this_lb, asize, sizeof (*this_lb), NULL)) != 0) {
+			err = SET_ERROR(EINVAL);
+			goto cleanup;
+		}
+		break;
+	default:
+		err = SET_ERROR(EINVAL);
+		goto cleanup;
+	}
+	if (this_lb->lb_magic == BSWAP_64(L2ARC_LOG_BLK_MAGIC))
+		byteswap_uint64_array(this_lb, sizeof (*this_lb));
+	if (this_lb->lb_magic != L2ARC_LOG_BLK_MAGIC) {
+		err = SET_ERROR(EINVAL);
+		goto cleanup;
+	}
+cleanup:
+	/* Abort an in-flight fetch I/O in case of error */
+	if (err != 0 && *next_io != NULL) {
+		l2arc_log_blk_fetch_abort(*next_io);
+		*next_io = NULL;
+	}
+	if (abd != NULL)
+		abd_free(abd);
+	return (err);
+}
+
+/*
+ * Restores the payload of a log block to ARC. This creates empty ARC hdr
+ * entries which only contain an l2arc hdr, essentially restoring the
+ * buffers to their L2ARC evicted state. This function also updates space
+ * usage on the L2ARC vdev to make sure it tracks restored buffers.
+ */
+static void
+l2arc_log_blk_restore(l2arc_dev_t *dev, const l2arc_log_blk_phys_t *lb,
+    uint64_t lb_asize, uint64_t lb_daddr)
+{
+	uint64_t	size = 0, asize = 0;
+	uint64_t	log_entries = dev->l2ad_log_entries;
+
+	for (int i = log_entries - 1; i >= 0; i--) {
+		/*
+		 * Restore goes in the reverse temporal direction to preserve
+		 * correct temporal ordering of buffers in the l2ad_buflist.
+		 * l2arc_hdr_restore also does a list_insert_tail instead of
+		 * list_insert_head on the l2ad_buflist:
+		 *
+		 *		LIST	l2ad_buflist		LIST
+		 *		HEAD  <------ (time) ------	TAIL
+		 * direction	+-----+-----+-----+-----+-----+    direction
+		 * of l2arc <== | buf | buf | buf | buf | buf | ===> of rebuild
+		 * fill		+-----+-----+-----+-----+-----+
+		 *		^				^
+		 *		|				|
+		 *		|				|
+		 *	l2arc_feed_thread		l2arc_rebuild
+		 *	will place new bufs here	restores bufs here
+		 *
+		 * During l2arc_rebuild() the device is not used by
+		 * l2arc_feed_thread() as dev->l2ad_rebuild is set to true.
+		 */
+		size += L2BLK_GET_LSIZE((&lb->lb_entries[i])->le_prop);
+		asize += vdev_psize_to_asize(dev->l2ad_vdev,
+		    L2BLK_GET_PSIZE((&lb->lb_entries[i])->le_prop));
+		l2arc_hdr_restore(&lb->lb_entries[i], dev);
+	}
+
+	/*
+	 * Record rebuild stats:
+	 *	size		Logical size of restored buffers in the L2ARC
+	 *	asize		Aligned size of restored buffers in the L2ARC
+	 */
+	ARCSTAT_INCR(arcstat_l2_rebuild_size, size);
+	ARCSTAT_INCR(arcstat_l2_rebuild_asize, asize);
+	ARCSTAT_INCR(arcstat_l2_rebuild_bufs, log_entries);
+	ARCSTAT_F_AVG(arcstat_l2_log_blk_avg_asize, lb_asize);
+	ARCSTAT_F_AVG(arcstat_l2_data_to_meta_ratio, asize / lb_asize);
+	ARCSTAT_BUMP(arcstat_l2_rebuild_log_blks);
+}
+
+/*
+ * Restores a single ARC buf hdr from a log entry. The ARC buffer is put
+ * into a state indicating that it has been evicted to L2ARC.
+ */
+static void
+l2arc_hdr_restore(const l2arc_log_ent_phys_t *le, l2arc_dev_t *dev)
+{
+	arc_buf_hdr_t		*hdr, *exists;
+	kmutex_t		*hash_lock;
+	arc_buf_contents_t	type = L2BLK_GET_TYPE((le)->le_prop);
+	uint64_t		asize;
+
+	/*
+	 * Do all the allocation before grabbing any locks, this lets us
+	 * sleep if memory is full and we don't have to deal with failed
+	 * allocations.
+	 */
+	hdr = arc_buf_alloc_l2only(L2BLK_GET_LSIZE((le)->le_prop), type,
+	    dev, le->le_dva, le->le_daddr,
+	    L2BLK_GET_PSIZE((le)->le_prop), le->le_birth,
+	    L2BLK_GET_COMPRESS((le)->le_prop), le->le_complevel,
+	    L2BLK_GET_PROTECTED((le)->le_prop),
+	    L2BLK_GET_PREFETCH((le)->le_prop));
+	asize = vdev_psize_to_asize(dev->l2ad_vdev,
+	    L2BLK_GET_PSIZE((le)->le_prop));
+
+	/*
+	 * vdev_space_update() has to be called before arc_hdr_destroy() to
+	 * avoid underflow since the latter also calls the former.
+	 */
+	vdev_space_update(dev->l2ad_vdev, asize, 0, 0);
+
+	ARCSTAT_INCR(arcstat_l2_lsize, HDR_GET_LSIZE(hdr));
+	ARCSTAT_INCR(arcstat_l2_psize, HDR_GET_PSIZE(hdr));
+
+	mutex_enter(&dev->l2ad_mtx);
+	list_insert_tail(&dev->l2ad_buflist, hdr);
+	(void) zfs_refcount_add_many(&dev->l2ad_alloc, arc_hdr_size(hdr), hdr);
+	mutex_exit(&dev->l2ad_mtx);
+
+	exists = buf_hash_insert(hdr, &hash_lock);
+	if (exists) {
+		/* Buffer was already cached, no need to restore it. */
+		arc_hdr_destroy(hdr);
+		/*
+		 * If the buffer is already cached, check whether it has
+		 * L2ARC metadata. If not, enter them and update the flag.
+		 * This is important is case of onlining a cache device, since
+		 * we previously evicted all L2ARC metadata from ARC.
+		 */
+		if (!HDR_HAS_L2HDR(exists)) {
+			arc_hdr_set_flags(exists, ARC_FLAG_HAS_L2HDR);
+			exists->b_l2hdr.b_dev = dev;
+			exists->b_l2hdr.b_daddr = le->le_daddr;
+			mutex_enter(&dev->l2ad_mtx);
+			list_insert_tail(&dev->l2ad_buflist, exists);
+			(void) zfs_refcount_add_many(&dev->l2ad_alloc,
+			    arc_hdr_size(exists), exists);
+			mutex_exit(&dev->l2ad_mtx);
+			vdev_space_update(dev->l2ad_vdev, asize, 0, 0);
+			ARCSTAT_INCR(arcstat_l2_lsize, HDR_GET_LSIZE(exists));
+			ARCSTAT_INCR(arcstat_l2_psize, HDR_GET_PSIZE(exists));
+		}
+		ARCSTAT_BUMP(arcstat_l2_rebuild_bufs_precached);
+	}
+
+	mutex_exit(hash_lock);
+}
+
+/*
+ * Starts an asynchronous read IO to read a log block. This is used in log
+ * block reconstruction to start reading the next block before we are done
+ * decoding and reconstructing the current block, to keep the l2arc device
+ * nice and hot with read IO to process.
+ * The returned zio will contain a newly allocated memory buffers for the IO
+ * data which should then be freed by the caller once the zio is no longer
+ * needed (i.e. due to it having completed). If you wish to abort this
+ * zio, you should do so using l2arc_log_blk_fetch_abort, which takes
+ * care of disposing of the allocated buffers correctly.
+ */
+static zio_t *
+l2arc_log_blk_fetch(vdev_t *vd, const l2arc_log_blkptr_t *lbp,
+    l2arc_log_blk_phys_t *lb)
+{
+	uint32_t		asize;
+	zio_t			*pio;
+	l2arc_read_callback_t	*cb;
+
+	/* L2BLK_GET_PSIZE returns aligned size for log blocks */
+	asize = L2BLK_GET_PSIZE((lbp)->lbp_prop);
+	ASSERT(asize <= sizeof (l2arc_log_blk_phys_t));
+
+	cb = kmem_zalloc(sizeof (l2arc_read_callback_t), KM_SLEEP);
+	cb->l2rcb_abd = abd_get_from_buf(lb, asize);
+	pio = zio_root(vd->vdev_spa, l2arc_blk_fetch_done, cb,
+	    ZIO_FLAG_DONT_CACHE | ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_PROPAGATE |
+	    ZIO_FLAG_DONT_RETRY);
+	(void) zio_nowait(zio_read_phys(pio, vd, lbp->lbp_daddr, asize,
+	    cb->l2rcb_abd, ZIO_CHECKSUM_OFF, NULL, NULL,
+	    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_DONT_CACHE | ZIO_FLAG_CANFAIL |
+	    ZIO_FLAG_DONT_PROPAGATE | ZIO_FLAG_DONT_RETRY, B_FALSE));
+
+	return (pio);
+}
+
+/*
+ * Aborts a zio returned from l2arc_log_blk_fetch and frees the data
+ * buffers allocated for it.
+ */
+static void
+l2arc_log_blk_fetch_abort(zio_t *zio)
+{
+	(void) zio_wait(zio);
+}
+
+/*
+ * Creates a zio to update the device header on an l2arc device.
+ */
+void
+l2arc_dev_hdr_update(l2arc_dev_t *dev)
+{
+	l2arc_dev_hdr_phys_t	*l2dhdr = dev->l2ad_dev_hdr;
+	const uint64_t		l2dhdr_asize = dev->l2ad_dev_hdr_asize;
+	abd_t			*abd;
+	int			err;
+
+	VERIFY(spa_config_held(dev->l2ad_spa, SCL_STATE_ALL, RW_READER));
+
+	l2dhdr->dh_magic = L2ARC_DEV_HDR_MAGIC;
+	l2dhdr->dh_version = L2ARC_PERSISTENT_VERSION;
+	l2dhdr->dh_spa_guid = spa_guid(dev->l2ad_vdev->vdev_spa);
+	l2dhdr->dh_vdev_guid = dev->l2ad_vdev->vdev_guid;
+	l2dhdr->dh_log_entries = dev->l2ad_log_entries;
+	l2dhdr->dh_evict = dev->l2ad_evict;
+	l2dhdr->dh_start = dev->l2ad_start;
+	l2dhdr->dh_end = dev->l2ad_end;
+	l2dhdr->dh_lb_asize = zfs_refcount_count(&dev->l2ad_lb_asize);
+	l2dhdr->dh_lb_count = zfs_refcount_count(&dev->l2ad_lb_count);
+	l2dhdr->dh_flags = 0;
+	l2dhdr->dh_trim_action_time = dev->l2ad_vdev->vdev_trim_action_time;
+	l2dhdr->dh_trim_state = dev->l2ad_vdev->vdev_trim_state;
+	if (dev->l2ad_first)
+		l2dhdr->dh_flags |= L2ARC_DEV_HDR_EVICT_FIRST;
+
+	abd = abd_get_from_buf(l2dhdr, l2dhdr_asize);
+
+	err = zio_wait(zio_write_phys(NULL, dev->l2ad_vdev,
+	    VDEV_LABEL_START_SIZE, l2dhdr_asize, abd, ZIO_CHECKSUM_LABEL, NULL,
+	    NULL, ZIO_PRIORITY_ASYNC_WRITE, ZIO_FLAG_CANFAIL, B_FALSE));
+
+	abd_put(abd);
+
+	if (err != 0) {
+		zfs_dbgmsg("L2ARC IO error (%d) while writing device header, "
+		    "vdev guid: %llu", err, dev->l2ad_vdev->vdev_guid);
+	}
+}
+
+/*
+ * Commits a log block to the L2ARC device. This routine is invoked from
+ * l2arc_write_buffers when the log block fills up.
+ * This function allocates some memory to temporarily hold the serialized
+ * buffer to be written. This is then released in l2arc_write_done.
+ */
+static void
+l2arc_log_blk_commit(l2arc_dev_t *dev, zio_t *pio, l2arc_write_callback_t *cb)
+{
+	l2arc_log_blk_phys_t	*lb = &dev->l2ad_log_blk;
+	l2arc_dev_hdr_phys_t	*l2dhdr = dev->l2ad_dev_hdr;
+	uint64_t		psize, asize;
+	zio_t			*wzio;
+	l2arc_lb_abd_buf_t	*abd_buf;
+	uint8_t			*tmpbuf;
+	l2arc_lb_ptr_buf_t	*lb_ptr_buf;
+
+	VERIFY3S(dev->l2ad_log_ent_idx, ==, dev->l2ad_log_entries);
+
+	tmpbuf = zio_buf_alloc(sizeof (*lb));
+	abd_buf = zio_buf_alloc(sizeof (*abd_buf));
+	abd_buf->abd = abd_get_from_buf(lb, sizeof (*lb));
+	lb_ptr_buf = kmem_zalloc(sizeof (l2arc_lb_ptr_buf_t), KM_SLEEP);
+	lb_ptr_buf->lb_ptr = kmem_zalloc(sizeof (l2arc_log_blkptr_t), KM_SLEEP);
+
+	/* link the buffer into the block chain */
+	lb->lb_prev_lbp = l2dhdr->dh_start_lbps[1];
+	lb->lb_magic = L2ARC_LOG_BLK_MAGIC;
+
+	/*
+	 * l2arc_log_blk_commit() may be called multiple times during a single
+	 * l2arc_write_buffers() call. Save the allocated abd buffers in a list
+	 * so we can free them in l2arc_write_done() later on.
+	 */
+	list_insert_tail(&cb->l2wcb_abd_list, abd_buf);
+
+	/* try to compress the buffer */
+	psize = zio_compress_data(ZIO_COMPRESS_LZ4,
+	    abd_buf->abd, tmpbuf, sizeof (*lb), 0);
+
+	/* a log block is never entirely zero */
+	ASSERT(psize != 0);
+	asize = vdev_psize_to_asize(dev->l2ad_vdev, psize);
+	ASSERT(asize <= sizeof (*lb));
+
+	/*
+	 * Update the start log block pointer in the device header to point
+	 * to the log block we're about to write.
+	 */
+	l2dhdr->dh_start_lbps[1] = l2dhdr->dh_start_lbps[0];
+	l2dhdr->dh_start_lbps[0].lbp_daddr = dev->l2ad_hand;
+	l2dhdr->dh_start_lbps[0].lbp_payload_asize =
+	    dev->l2ad_log_blk_payload_asize;
+	l2dhdr->dh_start_lbps[0].lbp_payload_start =
+	    dev->l2ad_log_blk_payload_start;
+	_NOTE(CONSTCOND)
+	L2BLK_SET_LSIZE(
+	    (&l2dhdr->dh_start_lbps[0])->lbp_prop, sizeof (*lb));
+	L2BLK_SET_PSIZE(
+	    (&l2dhdr->dh_start_lbps[0])->lbp_prop, asize);
+	L2BLK_SET_CHECKSUM(
+	    (&l2dhdr->dh_start_lbps[0])->lbp_prop,
+	    ZIO_CHECKSUM_FLETCHER_4);
+	if (asize < sizeof (*lb)) {
+		/* compression succeeded */
+		bzero(tmpbuf + psize, asize - psize);
+		L2BLK_SET_COMPRESS(
+		    (&l2dhdr->dh_start_lbps[0])->lbp_prop,
+		    ZIO_COMPRESS_LZ4);
+	} else {
+		/* compression failed */
+		bcopy(lb, tmpbuf, sizeof (*lb));
+		L2BLK_SET_COMPRESS(
+		    (&l2dhdr->dh_start_lbps[0])->lbp_prop,
+		    ZIO_COMPRESS_OFF);
+	}
+
+	/* checksum what we're about to write */
+	fletcher_4_native(tmpbuf, asize, NULL,
+	    &l2dhdr->dh_start_lbps[0].lbp_cksum);
+
+	abd_put(abd_buf->abd);
+
+	/* perform the write itself */
+	abd_buf->abd = abd_get_from_buf(tmpbuf, sizeof (*lb));
+	abd_take_ownership_of_buf(abd_buf->abd, B_TRUE);
+	wzio = zio_write_phys(pio, dev->l2ad_vdev, dev->l2ad_hand,
+	    asize, abd_buf->abd, ZIO_CHECKSUM_OFF, NULL, NULL,
+	    ZIO_PRIORITY_ASYNC_WRITE, ZIO_FLAG_CANFAIL, B_FALSE);
+	DTRACE_PROBE2(l2arc__write, vdev_t *, dev->l2ad_vdev, zio_t *, wzio);
+	(void) zio_nowait(wzio);
+
+	dev->l2ad_hand += asize;
+	/*
+	 * Include the committed log block's pointer  in the list of pointers
+	 * to log blocks present in the L2ARC device.
+	 */
+	bcopy(&l2dhdr->dh_start_lbps[0], lb_ptr_buf->lb_ptr,
+	    sizeof (l2arc_log_blkptr_t));
+	mutex_enter(&dev->l2ad_mtx);
+	list_insert_head(&dev->l2ad_lbptr_list, lb_ptr_buf);
+	ARCSTAT_INCR(arcstat_l2_log_blk_asize, asize);
+	ARCSTAT_BUMP(arcstat_l2_log_blk_count);
+	zfs_refcount_add_many(&dev->l2ad_lb_asize, asize, lb_ptr_buf);
+	zfs_refcount_add(&dev->l2ad_lb_count, lb_ptr_buf);
+	mutex_exit(&dev->l2ad_mtx);
+	vdev_space_update(dev->l2ad_vdev, asize, 0, 0);
+
+	/* bump the kstats */
+	ARCSTAT_INCR(arcstat_l2_write_bytes, asize);
+	ARCSTAT_BUMP(arcstat_l2_log_blk_writes);
+	ARCSTAT_F_AVG(arcstat_l2_log_blk_avg_asize, asize);
+	ARCSTAT_F_AVG(arcstat_l2_data_to_meta_ratio,
+	    dev->l2ad_log_blk_payload_asize / asize);
+
+	/* start a new log block */
+	dev->l2ad_log_ent_idx = 0;
+	dev->l2ad_log_blk_payload_asize = 0;
+	dev->l2ad_log_blk_payload_start = 0;
+}
+
+/*
+ * Validates an L2ARC log block address to make sure that it can be read
+ * from the provided L2ARC device.
+ */
+boolean_t
+l2arc_log_blkptr_valid(l2arc_dev_t *dev, const l2arc_log_blkptr_t *lbp)
+{
+	/* L2BLK_GET_PSIZE returns aligned size for log blocks */
+	uint64_t asize = L2BLK_GET_PSIZE((lbp)->lbp_prop);
+	uint64_t end = lbp->lbp_daddr + asize - 1;
+	uint64_t start = lbp->lbp_payload_start;
+	boolean_t evicted = B_FALSE;
+
+	/*
+	 * A log block is valid if all of the following conditions are true:
+	 * - it fits entirely (including its payload) between l2ad_start and
+	 *   l2ad_end
+	 * - it has a valid size
+	 * - neither the log block itself nor part of its payload was evicted
+	 *   by l2arc_evict():
+	 *
+	 *		l2ad_hand          l2ad_evict
+	 *		|			 |	lbp_daddr
+	 *		|     start		 |	|  end
+	 *		|     |			 |	|  |
+	 *		V     V		         V	V  V
+	 *   l2ad_start ============================================ l2ad_end
+	 *                    --------------------------||||
+	 *				^		 ^
+	 *				|		log block
+	 *				payload
+	 */
+
+	evicted =
+	    l2arc_range_check_overlap(start, end, dev->l2ad_hand) ||
+	    l2arc_range_check_overlap(start, end, dev->l2ad_evict) ||
+	    l2arc_range_check_overlap(dev->l2ad_hand, dev->l2ad_evict, start) ||
+	    l2arc_range_check_overlap(dev->l2ad_hand, dev->l2ad_evict, end);
+
+	return (start >= dev->l2ad_start && end <= dev->l2ad_end &&
+	    asize > 0 && asize <= sizeof (l2arc_log_blk_phys_t) &&
+	    (!evicted || dev->l2ad_first));
+}
+
+/*
+ * Inserts ARC buffer header `hdr' into the current L2ARC log block on
+ * the device. The buffer being inserted must be present in L2ARC.
+ * Returns B_TRUE if the L2ARC log block is full and needs to be committed
+ * to L2ARC, or B_FALSE if it still has room for more ARC buffers.
+ */
+static boolean_t
+l2arc_log_blk_insert(l2arc_dev_t *dev, const arc_buf_hdr_t *hdr)
+{
+	l2arc_log_blk_phys_t	*lb = &dev->l2ad_log_blk;
+	l2arc_log_ent_phys_t	*le;
+
+	if (dev->l2ad_log_entries == 0)
+		return (B_FALSE);
+
+	int index = dev->l2ad_log_ent_idx++;
+
+	ASSERT3S(index, <, dev->l2ad_log_entries);
+	ASSERT(HDR_HAS_L2HDR(hdr));
+
+	le = &lb->lb_entries[index];
+	bzero(le, sizeof (*le));
+	le->le_dva = hdr->b_dva;
+	le->le_birth = hdr->b_birth;
+	le->le_daddr = hdr->b_l2hdr.b_daddr;
+	if (index == 0)
+		dev->l2ad_log_blk_payload_start = le->le_daddr;
+	L2BLK_SET_LSIZE((le)->le_prop, HDR_GET_LSIZE(hdr));
+	L2BLK_SET_PSIZE((le)->le_prop, HDR_GET_PSIZE(hdr));
+	L2BLK_SET_COMPRESS((le)->le_prop, HDR_GET_COMPRESS(hdr));
+	le->le_complevel = hdr->b_complevel;
+	L2BLK_SET_TYPE((le)->le_prop, hdr->b_type);
+	L2BLK_SET_PROTECTED((le)->le_prop, !!(HDR_PROTECTED(hdr)));
+	L2BLK_SET_PREFETCH((le)->le_prop, !!(HDR_PREFETCH(hdr)));
+
+	dev->l2ad_log_blk_payload_asize += vdev_psize_to_asize(dev->l2ad_vdev,
+	    HDR_GET_PSIZE(hdr));
+
+	return (dev->l2ad_log_ent_idx == dev->l2ad_log_entries);
+}
+
+/*
+ * Checks whether a given L2ARC device address sits in a time-sequential
+ * range. The trick here is that the L2ARC is a rotary buffer, so we can't
+ * just do a range comparison, we need to handle the situation in which the
+ * range wraps around the end of the L2ARC device. Arguments:
+ *	bottom -- Lower end of the range to check (written to earlier).
+ *	top    -- Upper end of the range to check (written to later).
+ *	check  -- The address for which we want to determine if it sits in
+ *		  between the top and bottom.
+ *
+ * The 3-way conditional below represents the following cases:
+ *
+ *	bottom < top : Sequentially ordered case:
+ *	  <check>--------+-------------------+
+ *	                 |  (overlap here?)  |
+ *	 L2ARC dev       V                   V
+ *	 |---------------<bottom>============<top>--------------|
+ *
+ *	bottom > top: Looped-around case:
+ *	                      <check>--------+------------------+
+ *	                                     |  (overlap here?) |
+ *	 L2ARC dev                           V                  V
+ *	 |===============<top>---------------<bottom>===========|
+ *	 ^               ^
+ *	 |  (or here?)   |
+ *	 +---------------+---------<check>
+ *
+ *	top == bottom : Just a single address comparison.
+ */
+boolean_t
+l2arc_range_check_overlap(uint64_t bottom, uint64_t top, uint64_t check)
+{
+	if (bottom < top)
+		return (bottom <= check && check <= top);
+	else if (bottom > top)
+		return (check <= top || bottom <= check);
+	else
+		return (check == top);
+}
+
+EXPORT_SYMBOL(arc_buf_size);
+EXPORT_SYMBOL(arc_write);
+EXPORT_SYMBOL(arc_read);
+EXPORT_SYMBOL(arc_buf_info);
+EXPORT_SYMBOL(arc_getbuf_func);
+EXPORT_SYMBOL(arc_add_prune_callback);
+EXPORT_SYMBOL(arc_remove_prune_callback);
+
+/* BEGIN CSTYLED */
+ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, min, param_set_arc_long,
+	param_get_long, ZMOD_RW, "Min arc size");
+
+ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, max, param_set_arc_long,
+	param_get_long, ZMOD_RW, "Max arc size");
+
+ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, meta_limit, param_set_arc_long,
+	param_get_long, ZMOD_RW, "Metadata limit for arc size");
+
+ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, meta_limit_percent,
+	param_set_arc_long, param_get_long, ZMOD_RW,
+	"Percent of arc size for arc meta limit");
+
+ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, meta_min, param_set_arc_long,
+	param_get_long, ZMOD_RW, "Min arc metadata");
+
+ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, meta_prune, INT, ZMOD_RW,
+	"Meta objects to scan for prune");
+
+ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, meta_adjust_restarts, INT, ZMOD_RW,
+	"Limit number of restarts in arc_evict_meta");
+
+ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, meta_strategy, INT, ZMOD_RW,
+	"Meta reclaim strategy");
+
+ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, grow_retry, param_set_arc_int,
+	param_get_int, ZMOD_RW, "Seconds before growing arc size");
+
+ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, p_dampener_disable, INT, ZMOD_RW,
+	"Disable arc_p adapt dampener");
+
+ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, shrink_shift, param_set_arc_int,
+	param_get_int, ZMOD_RW, "log2(fraction of arc to reclaim)");
+
+ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, pc_percent, UINT, ZMOD_RW,
+	"Percent of pagecache to reclaim arc to");
+
+ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, p_min_shift, param_set_arc_int,
+	param_get_int, ZMOD_RW, "arc_c shift to calc min/max arc_p");
+
+ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, average_blocksize, INT, ZMOD_RD,
+	"Target average block size");
+
+ZFS_MODULE_PARAM(zfs, zfs_, compressed_arc_enabled, INT, ZMOD_RW,
+	"Disable compressed arc buffers");
+
+ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, min_prefetch_ms, param_set_arc_int,
+	param_get_int, ZMOD_RW, "Min life of prefetch block in ms");
+
+ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, min_prescient_prefetch_ms,
+	param_set_arc_int, param_get_int, ZMOD_RW,
+	"Min life of prescient prefetched block in ms");
+
+ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, write_max, ULONG, ZMOD_RW,
+	"Max write bytes per interval");
+
+ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, write_boost, ULONG, ZMOD_RW,
+	"Extra write bytes during device warmup");
+
+ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, headroom, ULONG, ZMOD_RW,
+	"Number of max device writes to precache");
+
+ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, headroom_boost, ULONG, ZMOD_RW,
+	"Compressed l2arc_headroom multiplier");
+
+ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, trim_ahead, ULONG, ZMOD_RW,
+	"TRIM ahead L2ARC write size multiplier");
+
+ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, feed_secs, ULONG, ZMOD_RW,
+	"Seconds between L2ARC writing");
+
+ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, feed_min_ms, ULONG, ZMOD_RW,
+	"Min feed interval in milliseconds");
+
+ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, noprefetch, INT, ZMOD_RW,
+	"Skip caching prefetched buffers");
+
+ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, feed_again, INT, ZMOD_RW,
+	"Turbo L2ARC warmup");
+
+ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, norw, INT, ZMOD_RW,
+	"No reads during writes");
+
+ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, rebuild_enabled, INT, ZMOD_RW,
+	"Rebuild the L2ARC when importing a pool");
+
+ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, rebuild_blocks_min_l2size, ULONG, ZMOD_RW,
+	"Min size in bytes to write rebuild log blocks in L2ARC");
+
+ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, lotsfree_percent, param_set_arc_int,
+	param_get_int, ZMOD_RW, "System free memory I/O throttle in bytes");
+
+ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, sys_free, param_set_arc_long,
+	param_get_long, ZMOD_RW, "System free memory target size in bytes");
+
+ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, dnode_limit, param_set_arc_long,
+	param_get_long, ZMOD_RW, "Minimum bytes of dnodes in arc");
+
+ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, dnode_limit_percent,
+	param_set_arc_long, param_get_long, ZMOD_RW,
+	"Percent of ARC meta buffers for dnodes");
+
+ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, dnode_reduce_percent, ULONG, ZMOD_RW,
+	"Percentage of excess dnodes to try to unpin");
+
+ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, eviction_pct, INT, ZMOD_RW,
+       "When full, ARC allocation waits for eviction of this % of alloc size");
+/* END CSTYLED */
diff --git a/sys/contrib/openzfs/module/zfs/blkptr.c b/sys/contrib/openzfs/module/zfs/blkptr.c
new file mode 100644
index 000000000000..aa09ded8dba3
--- /dev/null
+++ b/sys/contrib/openzfs/module/zfs/blkptr.c
@@ -0,0 +1,153 @@
+/*
+ * CDDL HEADER START
+ *
+ * This file and its contents are supplied under the terms of the
+ * Common Development and Distribution License ("CDDL"), version 1.0.
+ * You may only use this file in accordance with the terms of version
+ * 1.0 of the CDDL.
+ *
+ * A full copy of the text of the CDDL should have accompanied this
+ * source.  A copy of the CDDL is also available via the Internet at
+ * http://www.illumos.org/license/CDDL.
+ *
+ * CDDL HEADER END
+ */
+
+/*
+ * Copyright (c) 2013, 2016 by Delphix. All rights reserved.
+ */
+
+#include <sys/blkptr.h>
+#include <sys/zfs_context.h>
+#include <sys/zio.h>
+#include <sys/zio_compress.h>
+
+/*
+ * Embedded-data Block Pointers
+ *
+ * Normally, block pointers point (via their DVAs) to a block which holds data.
+ * If the data that we need to store is very small, this is an inefficient
+ * use of space, because a block must be at minimum 1 sector (typically 512
+ * bytes or 4KB).  Additionally, reading these small blocks tends to generate
+ * more random reads.
+ *
+ * Embedded-data Block Pointers allow small pieces of data (the "payload",
+ * up to 112 bytes) to be stored in the block pointer itself, instead of
+ * being pointed to.  The "Pointer" part of this name is a bit of a
+ * misnomer, as nothing is pointed to.
+ *
+ * BP_EMBEDDED_TYPE_DATA block pointers allow highly-compressible data to
+ * be embedded in the block pointer.  The logic for this is handled in
+ * the SPA, by the zio pipeline.  Therefore most code outside the zio
+ * pipeline doesn't need special-cases to handle these block pointers.
+ *
+ * See spa.h for details on the exact layout of embedded block pointers.
+ */
+
+void
+encode_embedded_bp_compressed(blkptr_t *bp, void *data,
+    enum zio_compress comp, int uncompressed_size, int compressed_size)
+{
+	uint64_t *bp64 = (uint64_t *)bp;
+	uint64_t w = 0;
+	uint8_t *data8 = data;
+
+	ASSERT3U(compressed_size, <=, BPE_PAYLOAD_SIZE);
+	ASSERT(uncompressed_size == compressed_size ||
+	    comp != ZIO_COMPRESS_OFF);
+	ASSERT3U(comp, >=, ZIO_COMPRESS_OFF);
+	ASSERT3U(comp, <, ZIO_COMPRESS_FUNCTIONS);
+
+	bzero(bp, sizeof (*bp));
+	BP_SET_EMBEDDED(bp, B_TRUE);
+	BP_SET_COMPRESS(bp, comp);
+	BP_SET_BYTEORDER(bp, ZFS_HOST_BYTEORDER);
+	BPE_SET_LSIZE(bp, uncompressed_size);
+	BPE_SET_PSIZE(bp, compressed_size);
+
+	/*
+	 * Encode the byte array into the words of the block pointer.
+	 * First byte goes into low bits of first word (little endian).
+	 */
+	for (int i = 0; i < compressed_size; i++) {
+		BF64_SET(w, (i % sizeof (w)) * NBBY, NBBY, data8[i]);
+		if (i % sizeof (w) == sizeof (w) - 1) {
+			/* we've reached the end of a word */
+			ASSERT3P(bp64, <, bp + 1);
+			*bp64 = w;
+			bp64++;
+			if (!BPE_IS_PAYLOADWORD(bp, bp64))
+				bp64++;
+			w = 0;
+		}
+	}
+	/* write last partial word */
+	if (bp64 < (uint64_t *)(bp + 1))
+		*bp64 = w;
+}
+
+/*
+ * buf must be at least BPE_GET_PSIZE(bp) bytes long (which will never be
+ * more than BPE_PAYLOAD_SIZE bytes).
+ */
+void
+decode_embedded_bp_compressed(const blkptr_t *bp, void *buf)
+{
+	int psize;
+	uint8_t *buf8 = buf;
+	uint64_t w = 0;
+	const uint64_t *bp64 = (const uint64_t *)bp;
+
+	ASSERT(BP_IS_EMBEDDED(bp));
+
+	psize = BPE_GET_PSIZE(bp);
+
+	/*
+	 * Decode the words of the block pointer into the byte array.
+	 * Low bits of first word are the first byte (little endian).
+	 */
+	for (int i = 0; i < psize; i++) {
+		if (i % sizeof (w) == 0) {
+			/* beginning of a word */
+			ASSERT3P(bp64, <, bp + 1);
+			w = *bp64;
+			bp64++;
+			if (!BPE_IS_PAYLOADWORD(bp, bp64))
+				bp64++;
+		}
+		buf8[i] = BF64_GET(w, (i % sizeof (w)) * NBBY, NBBY);
+	}
+}
+
+/*
+ * Fill in the buffer with the (decompressed) payload of the embedded
+ * blkptr_t.  Takes into account compression and byteorder (the payload is
+ * treated as a stream of bytes).
+ * Return 0 on success, or ENOSPC if it won't fit in the buffer.
+ */
+int
+decode_embedded_bp(const blkptr_t *bp, void *buf, int buflen)
+{
+	int lsize, psize;
+
+	ASSERT(BP_IS_EMBEDDED(bp));
+
+	lsize = BPE_GET_LSIZE(bp);
+	psize = BPE_GET_PSIZE(bp);
+
+	if (lsize > buflen)
+		return (SET_ERROR(ENOSPC));
+	ASSERT3U(lsize, ==, buflen);
+
+	if (BP_GET_COMPRESS(bp) != ZIO_COMPRESS_OFF) {
+		uint8_t dstbuf[BPE_PAYLOAD_SIZE];
+		decode_embedded_bp_compressed(bp, dstbuf);
+		VERIFY0(zio_decompress_data_buf(BP_GET_COMPRESS(bp),
+		    dstbuf, buf, psize, buflen, NULL));
+	} else {
+		ASSERT3U(lsize, ==, psize);
+		decode_embedded_bp_compressed(bp, buf);
+	}
+
+	return (0);
+}
diff --git a/sys/contrib/openzfs/module/zfs/bplist.c b/sys/contrib/openzfs/module/zfs/bplist.c
new file mode 100644
index 000000000000..47ea364ef26f
--- /dev/null
+++ b/sys/contrib/openzfs/module/zfs/bplist.c
@@ -0,0 +1,91 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012, 2018 by Delphix. All rights reserved.
+ */
+
+#include <sys/bplist.h>
+#include <sys/zfs_context.h>
+
+
+void
+bplist_create(bplist_t *bpl)
+{
+	mutex_init(&bpl->bpl_lock, NULL, MUTEX_DEFAULT, NULL);
+	list_create(&bpl->bpl_list, sizeof (bplist_entry_t),
+	    offsetof(bplist_entry_t, bpe_node));
+}
+
+void
+bplist_destroy(bplist_t *bpl)
+{
+	list_destroy(&bpl->bpl_list);
+	mutex_destroy(&bpl->bpl_lock);
+}
+
+void
+bplist_append(bplist_t *bpl, const blkptr_t *bp)
+{
+	bplist_entry_t *bpe = kmem_alloc(sizeof (*bpe), KM_SLEEP);
+
+	mutex_enter(&bpl->bpl_lock);
+	bpe->bpe_blk = *bp;
+	list_insert_tail(&bpl->bpl_list, bpe);
+	mutex_exit(&bpl->bpl_lock);
+}
+
+/*
+ * To aid debugging, we keep the most recently removed entry.  This way if
+ * we are in the callback, we can easily locate the entry.
+ */
+static bplist_entry_t *bplist_iterate_last_removed;
+
+void
+bplist_iterate(bplist_t *bpl, bplist_itor_t *func, void *arg, dmu_tx_t *tx)
+{
+	bplist_entry_t *bpe;
+
+	mutex_enter(&bpl->bpl_lock);
+	while ((bpe = list_head(&bpl->bpl_list))) {
+		bplist_iterate_last_removed = bpe;
+		list_remove(&bpl->bpl_list, bpe);
+		mutex_exit(&bpl->bpl_lock);
+		func(arg, &bpe->bpe_blk, tx);
+		kmem_free(bpe, sizeof (*bpe));
+		mutex_enter(&bpl->bpl_lock);
+	}
+	mutex_exit(&bpl->bpl_lock);
+}
+
+void
+bplist_clear(bplist_t *bpl)
+{
+	bplist_entry_t *bpe;
+
+	mutex_enter(&bpl->bpl_lock);
+	while ((bpe = list_head(&bpl->bpl_list))) {
+		bplist_iterate_last_removed = bpe;
+		list_remove(&bpl->bpl_list, bpe);
+		kmem_free(bpe, sizeof (*bpe));
+	}
+	mutex_exit(&bpl->bpl_lock);
+}
diff --git a/sys/contrib/openzfs/module/zfs/bpobj.c b/sys/contrib/openzfs/module/zfs/bpobj.c
new file mode 100644
index 000000000000..e75ba5cccde6
--- /dev/null
+++ b/sys/contrib/openzfs/module/zfs/bpobj.c
@@ -0,0 +1,943 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2011, 2018 by Delphix. All rights reserved.
+ * Copyright (c) 2017 Datto Inc.
+ */
+
+#include <sys/bpobj.h>
+#include <sys/zfs_context.h>
+#include <sys/zfs_refcount.h>
+#include <sys/dsl_pool.h>
+#include <sys/zfeature.h>
+#include <sys/zap.h>
+
+/*
+ * Return an empty bpobj, preferably the empty dummy one (dp_empty_bpobj).
+ */
+uint64_t
+bpobj_alloc_empty(objset_t *os, int blocksize, dmu_tx_t *tx)
+{
+	spa_t *spa = dmu_objset_spa(os);
+	dsl_pool_t *dp = dmu_objset_pool(os);
+
+	if (spa_feature_is_enabled(spa, SPA_FEATURE_EMPTY_BPOBJ)) {
+		if (!spa_feature_is_active(spa, SPA_FEATURE_EMPTY_BPOBJ)) {
+			ASSERT0(dp->dp_empty_bpobj);
+			dp->dp_empty_bpobj =
+			    bpobj_alloc(os, SPA_OLD_MAXBLOCKSIZE, tx);
+			VERIFY(zap_add(os,
+			    DMU_POOL_DIRECTORY_OBJECT,
+			    DMU_POOL_EMPTY_BPOBJ, sizeof (uint64_t), 1,
+			    &dp->dp_empty_bpobj, tx) == 0);
+		}
+		spa_feature_incr(spa, SPA_FEATURE_EMPTY_BPOBJ, tx);
+		ASSERT(dp->dp_empty_bpobj != 0);
+		return (dp->dp_empty_bpobj);
+	} else {
+		return (bpobj_alloc(os, blocksize, tx));
+	}
+}
+
+void
+bpobj_decr_empty(objset_t *os, dmu_tx_t *tx)
+{
+	dsl_pool_t *dp = dmu_objset_pool(os);
+
+	spa_feature_decr(dmu_objset_spa(os), SPA_FEATURE_EMPTY_BPOBJ, tx);
+	if (!spa_feature_is_active(dmu_objset_spa(os),
+	    SPA_FEATURE_EMPTY_BPOBJ)) {
+		VERIFY3U(0, ==, zap_remove(dp->dp_meta_objset,
+		    DMU_POOL_DIRECTORY_OBJECT,
+		    DMU_POOL_EMPTY_BPOBJ, tx));
+		VERIFY3U(0, ==, dmu_object_free(os, dp->dp_empty_bpobj, tx));
+		dp->dp_empty_bpobj = 0;
+	}
+}
+
+uint64_t
+bpobj_alloc(objset_t *os, int blocksize, dmu_tx_t *tx)
+{
+	int size;
+
+	if (spa_version(dmu_objset_spa(os)) < SPA_VERSION_BPOBJ_ACCOUNT)
+		size = BPOBJ_SIZE_V0;
+	else if (spa_version(dmu_objset_spa(os)) < SPA_VERSION_DEADLISTS)
+		size = BPOBJ_SIZE_V1;
+	else if (!spa_feature_is_active(dmu_objset_spa(os),
+	    SPA_FEATURE_LIVELIST))
+		size = BPOBJ_SIZE_V2;
+	else
+		size = sizeof (bpobj_phys_t);
+
+	return (dmu_object_alloc(os, DMU_OT_BPOBJ, blocksize,
+	    DMU_OT_BPOBJ_HDR, size, tx));
+}
+
+void
+bpobj_free(objset_t *os, uint64_t obj, dmu_tx_t *tx)
+{
+	int64_t i;
+	bpobj_t bpo;
+	dmu_object_info_t doi;
+	int epb;
+	dmu_buf_t *dbuf = NULL;
+
+	ASSERT(obj != dmu_objset_pool(os)->dp_empty_bpobj);
+	VERIFY3U(0, ==, bpobj_open(&bpo, os, obj));
+
+	mutex_enter(&bpo.bpo_lock);
+
+	if (!bpo.bpo_havesubobj || bpo.bpo_phys->bpo_subobjs == 0)
+		goto out;
+
+	VERIFY3U(0, ==, dmu_object_info(os, bpo.bpo_phys->bpo_subobjs, &doi));
+	epb = doi.doi_data_block_size / sizeof (uint64_t);
+
+	for (i = bpo.bpo_phys->bpo_num_subobjs - 1; i >= 0; i--) {
+		uint64_t *objarray;
+		uint64_t offset, blkoff;
+
+		offset = i * sizeof (uint64_t);
+		blkoff = P2PHASE(i, epb);
+
+		if (dbuf == NULL || dbuf->db_offset > offset) {
+			if (dbuf)
+				dmu_buf_rele(dbuf, FTAG);
+			VERIFY3U(0, ==, dmu_buf_hold(os,
+			    bpo.bpo_phys->bpo_subobjs, offset, FTAG, &dbuf, 0));
+		}
+
+		ASSERT3U(offset, >=, dbuf->db_offset);
+		ASSERT3U(offset, <, dbuf->db_offset + dbuf->db_size);
+
+		objarray = dbuf->db_data;
+		bpobj_free(os, objarray[blkoff], tx);
+	}
+	if (dbuf) {
+		dmu_buf_rele(dbuf, FTAG);
+		dbuf = NULL;
+	}
+	VERIFY3U(0, ==, dmu_object_free(os, bpo.bpo_phys->bpo_subobjs, tx));
+
+out:
+	mutex_exit(&bpo.bpo_lock);
+	bpobj_close(&bpo);
+
+	VERIFY3U(0, ==, dmu_object_free(os, obj, tx));
+}
+
+int
+bpobj_open(bpobj_t *bpo, objset_t *os, uint64_t object)
+{
+	dmu_object_info_t doi;
+	int err;
+
+	err = dmu_object_info(os, object, &doi);
+	if (err)
+		return (err);
+
+	bzero(bpo, sizeof (*bpo));
+	mutex_init(&bpo->bpo_lock, NULL, MUTEX_DEFAULT, NULL);
+
+	ASSERT(bpo->bpo_dbuf == NULL);
+	ASSERT(bpo->bpo_phys == NULL);
+	ASSERT(object != 0);
+	ASSERT3U(doi.doi_type, ==, DMU_OT_BPOBJ);
+	ASSERT3U(doi.doi_bonus_type, ==, DMU_OT_BPOBJ_HDR);
+
+	err = dmu_bonus_hold(os, object, bpo, &bpo->bpo_dbuf);
+	if (err)
+		return (err);
+
+	bpo->bpo_os = os;
+	bpo->bpo_object = object;
+	bpo->bpo_epb = doi.doi_data_block_size >> SPA_BLKPTRSHIFT;
+	bpo->bpo_havecomp = (doi.doi_bonus_size > BPOBJ_SIZE_V0);
+	bpo->bpo_havesubobj = (doi.doi_bonus_size > BPOBJ_SIZE_V1);
+	bpo->bpo_havefreed = (doi.doi_bonus_size > BPOBJ_SIZE_V2);
+	bpo->bpo_phys = bpo->bpo_dbuf->db_data;
+	return (0);
+}
+
+boolean_t
+bpobj_is_open(const bpobj_t *bpo)
+{
+	return (bpo->bpo_object != 0);
+}
+
+void
+bpobj_close(bpobj_t *bpo)
+{
+	/* Lame workaround for closing a bpobj that was never opened. */
+	if (bpo->bpo_object == 0)
+		return;
+
+	dmu_buf_rele(bpo->bpo_dbuf, bpo);
+	if (bpo->bpo_cached_dbuf != NULL)
+		dmu_buf_rele(bpo->bpo_cached_dbuf, bpo);
+	bpo->bpo_dbuf = NULL;
+	bpo->bpo_phys = NULL;
+	bpo->bpo_cached_dbuf = NULL;
+	bpo->bpo_object = 0;
+
+	mutex_destroy(&bpo->bpo_lock);
+}
+
+static boolean_t
+bpobj_is_empty_impl(bpobj_t *bpo)
+{
+	ASSERT(MUTEX_HELD(&bpo->bpo_lock));
+	return (bpo->bpo_phys->bpo_num_blkptrs == 0 &&
+	    (!bpo->bpo_havesubobj || bpo->bpo_phys->bpo_num_subobjs == 0));
+}
+
+boolean_t
+bpobj_is_empty(bpobj_t *bpo)
+{
+	mutex_enter(&bpo->bpo_lock);
+	boolean_t is_empty = bpobj_is_empty_impl(bpo);
+	mutex_exit(&bpo->bpo_lock);
+	return (is_empty);
+}
+
+/*
+ * A recursive iteration of the bpobjs would be nice here but we run the risk
+ * of overflowing function stack space.  Instead, find each subobj and add it
+ * to the head of our list so it can be scanned for subjobjs.  Like a
+ * recursive implementation, the "deepest" subobjs will be freed first.
+ * When a subobj is found to have no additional subojs, free it.
+ */
+typedef struct bpobj_info {
+	bpobj_t *bpi_bpo;
+	/*
+	 * This object is a subobj of bpi_parent,
+	 * at bpi_index in its subobj array.
+	 */
+	struct bpobj_info *bpi_parent;
+	uint64_t bpi_index;
+	/* How many of our subobj's are left to process. */
+	uint64_t bpi_unprocessed_subobjs;
+	/* True after having visited this bpo's directly referenced BPs. */
+	boolean_t bpi_visited;
+	list_node_t bpi_node;
+} bpobj_info_t;
+
+static bpobj_info_t *
+bpi_alloc(bpobj_t *bpo, bpobj_info_t *parent, uint64_t index)
+{
+	bpobj_info_t *bpi = kmem_zalloc(sizeof (bpobj_info_t), KM_SLEEP);
+	bpi->bpi_bpo = bpo;
+	bpi->bpi_parent = parent;
+	bpi->bpi_index = index;
+	if (bpo->bpo_havesubobj && bpo->bpo_phys->bpo_subobjs != 0) {
+		bpi->bpi_unprocessed_subobjs = bpo->bpo_phys->bpo_num_subobjs;
+	}
+	return (bpi);
+}
+
+/*
+ * Update bpobj and all of its parents with new space accounting.
+ */
+static void
+propagate_space_reduction(bpobj_info_t *bpi, int64_t freed,
+    int64_t comp_freed, int64_t uncomp_freed, dmu_tx_t *tx)
+{
+
+	for (; bpi != NULL; bpi = bpi->bpi_parent) {
+		bpobj_t *p = bpi->bpi_bpo;
+		ASSERT(dmu_buf_is_dirty(p->bpo_dbuf, tx));
+		p->bpo_phys->bpo_bytes -= freed;
+		ASSERT3S(p->bpo_phys->bpo_bytes, >=, 0);
+		if (p->bpo_havecomp) {
+			p->bpo_phys->bpo_comp -= comp_freed;
+			p->bpo_phys->bpo_uncomp -= uncomp_freed;
+		}
+	}
+}
+
+static int
+bpobj_iterate_blkptrs(bpobj_info_t *bpi, bpobj_itor_t func, void *arg,
+    int64_t start, dmu_tx_t *tx, boolean_t free)
+{
+	int err = 0;
+	int64_t freed = 0, comp_freed = 0, uncomp_freed = 0;
+	dmu_buf_t *dbuf = NULL;
+	bpobj_t *bpo = bpi->bpi_bpo;
+
+	for (int64_t i = bpo->bpo_phys->bpo_num_blkptrs - 1; i >= start; i--) {
+		uint64_t offset = i * sizeof (blkptr_t);
+		uint64_t blkoff = P2PHASE(i, bpo->bpo_epb);
+
+		if (dbuf == NULL || dbuf->db_offset > offset) {
+			if (dbuf)
+				dmu_buf_rele(dbuf, FTAG);
+			err = dmu_buf_hold(bpo->bpo_os, bpo->bpo_object,
+			    offset, FTAG, &dbuf, 0);
+			if (err)
+				break;
+		}
+
+		ASSERT3U(offset, >=, dbuf->db_offset);
+		ASSERT3U(offset, <, dbuf->db_offset + dbuf->db_size);
+
+		blkptr_t *bparray = dbuf->db_data;
+		blkptr_t *bp = &bparray[blkoff];
+
+		boolean_t bp_freed = BP_GET_FREE(bp);
+		err = func(arg, bp, bp_freed, tx);
+		if (err)
+			break;
+
+		if (free) {
+			int sign = bp_freed ? -1 : +1;
+			spa_t *spa = dmu_objset_spa(bpo->bpo_os);
+			freed += sign * bp_get_dsize_sync(spa, bp);
+			comp_freed += sign * BP_GET_PSIZE(bp);
+			uncomp_freed += sign * BP_GET_UCSIZE(bp);
+			ASSERT(dmu_buf_is_dirty(bpo->bpo_dbuf, tx));
+			bpo->bpo_phys->bpo_num_blkptrs--;
+			ASSERT3S(bpo->bpo_phys->bpo_num_blkptrs, >=, 0);
+			if (bp_freed) {
+				ASSERT(bpo->bpo_havefreed);
+				bpo->bpo_phys->bpo_num_freed--;
+				ASSERT3S(bpo->bpo_phys->bpo_num_freed, >=, 0);
+			}
+		}
+	}
+	if (free) {
+		propagate_space_reduction(bpi, freed, comp_freed,
+		    uncomp_freed, tx);
+		VERIFY0(dmu_free_range(bpo->bpo_os,
+		    bpo->bpo_object,
+		    bpo->bpo_phys->bpo_num_blkptrs * sizeof (blkptr_t),
+		    DMU_OBJECT_END, tx));
+	}
+	if (dbuf) {
+		dmu_buf_rele(dbuf, FTAG);
+		dbuf = NULL;
+	}
+	return (err);
+}
+
+/*
+ * Given an initial bpo, start by freeing the BPs that are directly referenced
+ * by that bpo. If the bpo has subobjs, read in its last subobj and push the
+ * subobj to our stack. By popping items off our stack, eventually we will
+ * encounter a bpo that has no subobjs.  We can free its bpobj_info_t, and if
+ * requested also free the now-empty bpo from disk and decrement
+ * its parent's subobj count. We continue popping each subobj from our stack,
+ * visiting its last subobj until they too have no more subobjs, and so on.
+ */
+static int
+bpobj_iterate_impl(bpobj_t *initial_bpo, bpobj_itor_t func, void *arg,
+    dmu_tx_t *tx, boolean_t free, uint64_t *bpobj_size)
+{
+	list_t stack;
+	bpobj_info_t *bpi;
+	int err = 0;
+
+	/*
+	 * Create a "stack" for us to work with without worrying about
+	 * stack overflows. Initialize it with the initial_bpo.
+	 */
+	list_create(&stack, sizeof (bpobj_info_t),
+	    offsetof(bpobj_info_t, bpi_node));
+	mutex_enter(&initial_bpo->bpo_lock);
+
+	if (bpobj_size != NULL)
+		*bpobj_size = initial_bpo->bpo_phys->bpo_num_blkptrs;
+
+	list_insert_head(&stack, bpi_alloc(initial_bpo, NULL, 0));
+
+	while ((bpi = list_head(&stack)) != NULL) {
+		bpobj_t *bpo = bpi->bpi_bpo;
+
+		ASSERT3P(bpo, !=, NULL);
+		ASSERT(MUTEX_HELD(&bpo->bpo_lock));
+		ASSERT(bpobj_is_open(bpo));
+
+		if (free)
+			dmu_buf_will_dirty(bpo->bpo_dbuf, tx);
+
+		if (bpi->bpi_visited == B_FALSE) {
+			err = bpobj_iterate_blkptrs(bpi, func, arg, 0, tx,
+			    free);
+			bpi->bpi_visited = B_TRUE;
+			if (err != 0)
+				break;
+		}
+		/*
+		 * We've finished with this bpo's directly-referenced BP's and
+		 * it has no more unprocessed subobjs. We can free its
+		 * bpobj_info_t (unless it is the topmost, initial_bpo).
+		 * If we are freeing from disk, we can also do that.
+		 */
+		if (bpi->bpi_unprocessed_subobjs == 0) {
+			/*
+			 * If there are no entries, there should
+			 * be no bytes.
+			 */
+			if (bpobj_is_empty_impl(bpo)) {
+				ASSERT0(bpo->bpo_phys->bpo_bytes);
+				ASSERT0(bpo->bpo_phys->bpo_comp);
+				ASSERT0(bpo->bpo_phys->bpo_uncomp);
+			}
+
+			/* The initial_bpo has no parent and is not closed. */
+			if (bpi->bpi_parent != NULL) {
+				if (free) {
+					bpobj_t *p = bpi->bpi_parent->bpi_bpo;
+
+					ASSERT0(bpo->bpo_phys->bpo_num_blkptrs);
+					ASSERT3U(p->bpo_phys->bpo_num_subobjs,
+					    >, 0);
+					ASSERT3U(bpi->bpi_index, ==,
+					    p->bpo_phys->bpo_num_subobjs - 1);
+					ASSERT(dmu_buf_is_dirty(bpo->bpo_dbuf,
+					    tx));
+
+					p->bpo_phys->bpo_num_subobjs--;
+
+					VERIFY0(dmu_free_range(p->bpo_os,
+					    p->bpo_phys->bpo_subobjs,
+					    bpi->bpi_index * sizeof (uint64_t),
+					    sizeof (uint64_t), tx));
+
+					/* eliminate the empty subobj list */
+					if (bpo->bpo_havesubobj &&
+					    bpo->bpo_phys->bpo_subobjs != 0) {
+						ASSERT0(bpo->bpo_phys->
+						    bpo_num_subobjs);
+						err = dmu_object_free(
+						    bpo->bpo_os,
+						    bpo->bpo_phys->bpo_subobjs,
+						    tx);
+						if (err)
+							break;
+						bpo->bpo_phys->bpo_subobjs = 0;
+					}
+					err = dmu_object_free(p->bpo_os,
+					    bpo->bpo_object, tx);
+					if (err)
+						break;
+				}
+
+				mutex_exit(&bpo->bpo_lock);
+				bpobj_close(bpo);
+				kmem_free(bpo, sizeof (bpobj_t));
+			} else {
+				mutex_exit(&bpo->bpo_lock);
+			}
+
+			/*
+			 * Finished processing this bpo. Unlock, and free
+			 * our "stack" info.
+			 */
+			list_remove_head(&stack);
+			kmem_free(bpi, sizeof (bpobj_info_t));
+		} else {
+			/*
+			 * We have unprocessed subobjs. Process the next one.
+			 */
+			ASSERT(bpo->bpo_havecomp);
+			ASSERT3P(bpobj_size, ==, NULL);
+
+			/* Add the last subobj to stack. */
+			int64_t i = bpi->bpi_unprocessed_subobjs - 1;
+			uint64_t offset = i * sizeof (uint64_t);
+
+			uint64_t obj_from_sublist;
+			err = dmu_read(bpo->bpo_os, bpo->bpo_phys->bpo_subobjs,
+			    offset, sizeof (uint64_t), &obj_from_sublist,
+			    DMU_READ_PREFETCH);
+			if (err)
+				break;
+			bpobj_t *sublist = kmem_alloc(sizeof (bpobj_t),
+			    KM_SLEEP);
+
+			err = bpobj_open(sublist, bpo->bpo_os,
+			    obj_from_sublist);
+			if (err)
+				break;
+
+			list_insert_head(&stack, bpi_alloc(sublist, bpi, i));
+			mutex_enter(&sublist->bpo_lock);
+			bpi->bpi_unprocessed_subobjs--;
+		}
+	}
+	/*
+	 * Cleanup anything left on the "stack" after we left the loop.
+	 * Every bpo on the stack is locked so we must remember to undo
+	 * that now (in LIFO order).
+	 */
+	while ((bpi = list_remove_head(&stack)) != NULL) {
+		bpobj_t *bpo = bpi->bpi_bpo;
+		ASSERT(err != 0);
+		ASSERT3P(bpo, !=, NULL);
+
+		mutex_exit(&bpo->bpo_lock);
+
+		/* do not free the initial_bpo */
+		if (bpi->bpi_parent != NULL) {
+			bpobj_close(bpi->bpi_bpo);
+			kmem_free(bpi->bpi_bpo, sizeof (bpobj_t));
+		}
+		kmem_free(bpi, sizeof (bpobj_info_t));
+	}
+
+	list_destroy(&stack);
+
+	return (err);
+}
+
+/*
+ * Iterate and remove the entries.  If func returns nonzero, iteration
+ * will stop and that entry will not be removed.
+ */
+int
+bpobj_iterate(bpobj_t *bpo, bpobj_itor_t func, void *arg, dmu_tx_t *tx)
+{
+	return (bpobj_iterate_impl(bpo, func, arg, tx, B_TRUE, NULL));
+}
+
+/*
+ * Iterate the entries.  If func returns nonzero, iteration will stop.
+ *
+ * If there are no subobjs:
+ *
+ * *bpobj_size can be used to return the number of block pointers in the
+ * bpobj.  Note that this may be different from the number of block pointers
+ * that are iterated over, if iteration is terminated early (e.g. by the func
+ * returning nonzero).
+ *
+ * If there are concurrent (or subsequent) modifications to the bpobj then the
+ * returned *bpobj_size can be passed as "start" to
+ * livelist_bpobj_iterate_from_nofree() to iterate the newly added entries.
+ */
+int
+bpobj_iterate_nofree(bpobj_t *bpo, bpobj_itor_t func, void *arg,
+    uint64_t *bpobj_size)
+{
+	return (bpobj_iterate_impl(bpo, func, arg, NULL, B_FALSE, bpobj_size));
+}
+
+/*
+ * Iterate over the blkptrs in the bpobj beginning at index start. If func
+ * returns nonzero, iteration will stop. This is a livelist specific function
+ * since it assumes that there are no subobjs present.
+ */
+int
+livelist_bpobj_iterate_from_nofree(bpobj_t *bpo, bpobj_itor_t func, void *arg,
+    int64_t start)
+{
+	if (bpo->bpo_havesubobj)
+		VERIFY0(bpo->bpo_phys->bpo_subobjs);
+	bpobj_info_t *bpi = bpi_alloc(bpo, NULL, 0);
+	int err = bpobj_iterate_blkptrs(bpi, func, arg, start, NULL, B_FALSE);
+	kmem_free(bpi, sizeof (bpobj_info_t));
+	return (err);
+}
+
+/*
+ * Logically add subobj's contents to the parent bpobj.
+ *
+ * In the most general case, this is accomplished in constant time by adding
+ * a reference to subobj.  This case is used when enqueuing a large subobj:
+ * +--------------+                        +--------------+
+ * | bpobj        |----------------------->| subobj list  |
+ * +----+----+----+----+----+              +-----+-----+--+--+
+ * | bp | bp | bp | bp | bp |              | obj | obj | obj |
+ * +----+----+----+----+----+              +-----+-----+-----+
+ *
+ * +--------------+                        +--------------+
+ * | sub-bpobj    |----------------------> | subsubobj    |
+ * +----+----+----+----+---------+----+    +-----+-----+--+--------+-----+
+ * | bp | bp | bp | bp |   ...   | bp |    | obj | obj |    ...    | obj |
+ * +----+----+----+----+---------+----+    +-----+-----+-----------+-----+
+ *
+ * Result: sub-bpobj added to parent's subobj list.
+ * +--------------+                        +--------------+
+ * | bpobj        |----------------------->| subobj list  |
+ * +----+----+----+----+----+              +-----+-----+--+--+-----+
+ * | bp | bp | bp | bp | bp |              | obj | obj | obj | OBJ |
+ * +----+----+----+----+----+              +-----+-----+-----+--|--+
+ *                                                              |
+ *       /-----------------------------------------------------/
+ *       v
+ * +--------------+                        +--------------+
+ * | sub-bpobj    |----------------------> | subsubobj    |
+ * +----+----+----+----+---------+----+    +-----+-----+--+--------+-----+
+ * | bp | bp | bp | bp |   ...   | bp |    | obj | obj |    ...    | obj |
+ * +----+----+----+----+---------+----+    +-----+-----+-----------+-----+
+ *
+ *
+ * In a common case, the subobj is small: its bp's and its list of subobj's
+ * are each stored in a single block.  In this case we copy the subobj's
+ * contents to the parent:
+ * +--------------+                        +--------------+
+ * | bpobj        |----------------------->| subobj list  |
+ * +----+----+----+----+----+              +-----+-----+--+--+
+ * | bp | bp | bp | bp | bp |              | obj | obj | obj |
+ * +----+----+----+----+----+              +-----+-----+-----+
+ *                          ^                                ^
+ * +--------------+         |              +--------------+  |
+ * | sub-bpobj    |---------^------------> | subsubobj    |  ^
+ * +----+----+----+         |              +-----+-----+--+  |
+ * | BP | BP |-->-->-->-->-/               | OBJ | OBJ |-->-/
+ * +----+----+                             +-----+-----+
+ *
+ * Result: subobj destroyed, contents copied to parent:
+ * +--------------+                        +--------------+
+ * | bpobj        |----------------------->| subobj list  |
+ * +----+----+----+----+----+----+----+    +-----+-----+--+--+-----+-----+
+ * | bp | bp | bp | bp | bp | BP | BP |    | obj | obj | obj | OBJ | OBJ |
+ * +----+----+----+----+----+----+----+    +-----+-----+-----+-----+-----+
+ *
+ *
+ * If the subobj has many BP's but few subobj's, we can copy the sub-subobj's
+ * but retain the sub-bpobj:
+ * +--------------+                        +--------------+
+ * | bpobj        |----------------------->| subobj list  |
+ * +----+----+----+----+----+              +-----+-----+--+--+
+ * | bp | bp | bp | bp | bp |              | obj | obj | obj |
+ * +----+----+----+----+----+              +-----+-----+-----+
+ *                                                           ^
+ * +--------------+                        +--------------+  |
+ * | sub-bpobj    |----------------------> | subsubobj    |  ^
+ * +----+----+----+----+---------+----+    +-----+-----+--+  |
+ * | bp | bp | bp | bp |   ...   | bp |    | OBJ | OBJ |-->-/
+ * +----+----+----+----+---------+----+    +-----+-----+
+ *
+ * Result: sub-sub-bpobjs and subobj added to parent's subobj list.
+ * +--------------+                     +--------------+
+ * | bpobj        |-------------------->| subobj list  |
+ * +----+----+----+----+----+           +-----+-----+--+--+-----+-----+------+
+ * | bp | bp | bp | bp | bp |           | obj | obj | obj | OBJ | OBJ | OBJ* |
+ * +----+----+----+----+----+           +-----+-----+-----+-----+-----+--|---+
+ *                                                                       |
+ *       /--------------------------------------------------------------/
+ *       v
+ * +--------------+
+ * | sub-bpobj    |
+ * +----+----+----+----+---------+----+
+ * | bp | bp | bp | bp |   ...   | bp |
+ * +----+----+----+----+---------+----+
+ */
+void
+bpobj_enqueue_subobj(bpobj_t *bpo, uint64_t subobj, dmu_tx_t *tx)
+{
+	bpobj_t subbpo;
+	uint64_t used, comp, uncomp, subsubobjs;
+	boolean_t copy_subsub = B_TRUE;
+	boolean_t copy_bps = B_TRUE;
+
+	ASSERT(bpobj_is_open(bpo));
+	ASSERT(subobj != 0);
+	ASSERT(bpo->bpo_havesubobj);
+	ASSERT(bpo->bpo_havecomp);
+	ASSERT(bpo->bpo_object != dmu_objset_pool(bpo->bpo_os)->dp_empty_bpobj);
+
+	if (subobj == dmu_objset_pool(bpo->bpo_os)->dp_empty_bpobj) {
+		bpobj_decr_empty(bpo->bpo_os, tx);
+		return;
+	}
+
+	VERIFY3U(0, ==, bpobj_open(&subbpo, bpo->bpo_os, subobj));
+	VERIFY3U(0, ==, bpobj_space(&subbpo, &used, &comp, &uncomp));
+
+	if (bpobj_is_empty(&subbpo)) {
+		/* No point in having an empty subobj. */
+		bpobj_close(&subbpo);
+		bpobj_free(bpo->bpo_os, subobj, tx);
+		return;
+	}
+
+	mutex_enter(&bpo->bpo_lock);
+	dmu_buf_will_dirty(bpo->bpo_dbuf, tx);
+
+	dmu_object_info_t doi;
+
+	if (bpo->bpo_phys->bpo_subobjs != 0) {
+		ASSERT0(dmu_object_info(bpo->bpo_os, bpo->bpo_phys->bpo_subobjs,
+		    &doi));
+		ASSERT3U(doi.doi_type, ==, DMU_OT_BPOBJ_SUBOBJ);
+	}
+
+	/*
+	 * If subobj has only one block of subobjs, then move subobj's
+	 * subobjs to bpo's subobj list directly.  This reduces recursion in
+	 * bpobj_iterate due to nested subobjs.
+	 */
+	subsubobjs = subbpo.bpo_phys->bpo_subobjs;
+	if (subsubobjs != 0) {
+		VERIFY0(dmu_object_info(bpo->bpo_os, subsubobjs, &doi));
+		if (doi.doi_max_offset > doi.doi_data_block_size) {
+			copy_subsub = B_FALSE;
+		}
+	}
+
+	/*
+	 * If, in addition to having only one block of subobj's, subobj has
+	 * only one block of bp's, then move subobj's bp's to bpo's bp list
+	 * directly. This reduces recursion in bpobj_iterate due to nested
+	 * subobjs.
+	 */
+	VERIFY3U(0, ==, dmu_object_info(bpo->bpo_os, subobj, &doi));
+	if (doi.doi_max_offset > doi.doi_data_block_size || !copy_subsub) {
+		copy_bps = B_FALSE;
+	}
+
+	if (copy_subsub && subsubobjs != 0) {
+		dmu_buf_t *subdb;
+		uint64_t numsubsub = subbpo.bpo_phys->bpo_num_subobjs;
+
+		VERIFY0(dmu_buf_hold(bpo->bpo_os, subsubobjs,
+		    0, FTAG, &subdb, 0));
+		/*
+		 * Make sure that we are not asking dmu_write()
+		 * to write more data than we have in our buffer.
+		 */
+		VERIFY3U(subdb->db_size, >=,
+		    numsubsub * sizeof (subobj));
+		if (bpo->bpo_phys->bpo_subobjs == 0) {
+			bpo->bpo_phys->bpo_subobjs =
+			    dmu_object_alloc(bpo->bpo_os,
+			    DMU_OT_BPOBJ_SUBOBJ, SPA_OLD_MAXBLOCKSIZE,
+			    DMU_OT_NONE, 0, tx);
+		}
+		dmu_write(bpo->bpo_os, bpo->bpo_phys->bpo_subobjs,
+		    bpo->bpo_phys->bpo_num_subobjs * sizeof (subobj),
+		    numsubsub * sizeof (subobj), subdb->db_data, tx);
+		dmu_buf_rele(subdb, FTAG);
+		bpo->bpo_phys->bpo_num_subobjs += numsubsub;
+
+		dmu_buf_will_dirty(subbpo.bpo_dbuf, tx);
+		subbpo.bpo_phys->bpo_subobjs = 0;
+		VERIFY0(dmu_object_free(bpo->bpo_os, subsubobjs, tx));
+	}
+
+	if (copy_bps) {
+		dmu_buf_t *bps;
+		uint64_t numbps = subbpo.bpo_phys->bpo_num_blkptrs;
+
+		ASSERT(copy_subsub);
+		VERIFY0(dmu_buf_hold(bpo->bpo_os, subobj,
+		    0, FTAG, &bps, 0));
+
+		/*
+		 * Make sure that we are not asking dmu_write()
+		 * to write more data than we have in our buffer.
+		 */
+		VERIFY3U(bps->db_size, >=, numbps * sizeof (blkptr_t));
+		dmu_write(bpo->bpo_os, bpo->bpo_object,
+		    bpo->bpo_phys->bpo_num_blkptrs * sizeof (blkptr_t),
+		    numbps * sizeof (blkptr_t),
+		    bps->db_data, tx);
+		dmu_buf_rele(bps, FTAG);
+		bpo->bpo_phys->bpo_num_blkptrs += numbps;
+
+		bpobj_close(&subbpo);
+		VERIFY0(dmu_object_free(bpo->bpo_os, subobj, tx));
+	} else {
+		bpobj_close(&subbpo);
+		if (bpo->bpo_phys->bpo_subobjs == 0) {
+			bpo->bpo_phys->bpo_subobjs =
+			    dmu_object_alloc(bpo->bpo_os,
+			    DMU_OT_BPOBJ_SUBOBJ, SPA_OLD_MAXBLOCKSIZE,
+			    DMU_OT_NONE, 0, tx);
+		}
+
+		dmu_write(bpo->bpo_os, bpo->bpo_phys->bpo_subobjs,
+		    bpo->bpo_phys->bpo_num_subobjs * sizeof (subobj),
+		    sizeof (subobj), &subobj, tx);
+		bpo->bpo_phys->bpo_num_subobjs++;
+	}
+
+	bpo->bpo_phys->bpo_bytes += used;
+	bpo->bpo_phys->bpo_comp += comp;
+	bpo->bpo_phys->bpo_uncomp += uncomp;
+	mutex_exit(&bpo->bpo_lock);
+
+}
+
+void
+bpobj_enqueue(bpobj_t *bpo, const blkptr_t *bp, boolean_t bp_freed,
+    dmu_tx_t *tx)
+{
+	blkptr_t stored_bp = *bp;
+	uint64_t offset;
+	int blkoff;
+	blkptr_t *bparray;
+
+	ASSERT(bpobj_is_open(bpo));
+	ASSERT(!BP_IS_HOLE(bp));
+	ASSERT(bpo->bpo_object != dmu_objset_pool(bpo->bpo_os)->dp_empty_bpobj);
+
+	if (BP_IS_EMBEDDED(bp)) {
+		/*
+		 * The bpobj will compress better without the payload.
+		 *
+		 * Note that we store EMBEDDED bp's because they have an
+		 * uncompressed size, which must be accounted for.  An
+		 * alternative would be to add their size to bpo_uncomp
+		 * without storing the bp, but that would create additional
+		 * complications: bpo_uncomp would be inconsistent with the
+		 * set of BP's stored, and bpobj_iterate() wouldn't visit
+		 * all the space accounted for in the bpobj.
+		 */
+		bzero(&stored_bp, sizeof (stored_bp));
+		stored_bp.blk_prop = bp->blk_prop;
+		stored_bp.blk_birth = bp->blk_birth;
+	} else if (!BP_GET_DEDUP(bp)) {
+		/* The bpobj will compress better without the checksum */
+		bzero(&stored_bp.blk_cksum, sizeof (stored_bp.blk_cksum));
+	}
+
+	stored_bp.blk_fill = 0;
+	BP_SET_FREE(&stored_bp, bp_freed);
+
+	mutex_enter(&bpo->bpo_lock);
+
+	offset = bpo->bpo_phys->bpo_num_blkptrs * sizeof (stored_bp);
+	blkoff = P2PHASE(bpo->bpo_phys->bpo_num_blkptrs, bpo->bpo_epb);
+
+	if (bpo->bpo_cached_dbuf == NULL ||
+	    offset < bpo->bpo_cached_dbuf->db_offset ||
+	    offset >= bpo->bpo_cached_dbuf->db_offset +
+	    bpo->bpo_cached_dbuf->db_size) {
+		if (bpo->bpo_cached_dbuf)
+			dmu_buf_rele(bpo->bpo_cached_dbuf, bpo);
+		VERIFY3U(0, ==, dmu_buf_hold(bpo->bpo_os, bpo->bpo_object,
+		    offset, bpo, &bpo->bpo_cached_dbuf, 0));
+	}
+
+	dmu_buf_will_dirty(bpo->bpo_cached_dbuf, tx);
+	bparray = bpo->bpo_cached_dbuf->db_data;
+	bparray[blkoff] = stored_bp;
+
+	dmu_buf_will_dirty(bpo->bpo_dbuf, tx);
+	bpo->bpo_phys->bpo_num_blkptrs++;
+	int sign = bp_freed ? -1 : +1;
+	bpo->bpo_phys->bpo_bytes += sign *
+	    bp_get_dsize_sync(dmu_objset_spa(bpo->bpo_os), bp);
+	if (bpo->bpo_havecomp) {
+		bpo->bpo_phys->bpo_comp += sign * BP_GET_PSIZE(bp);
+		bpo->bpo_phys->bpo_uncomp += sign * BP_GET_UCSIZE(bp);
+	}
+	if (bp_freed) {
+		ASSERT(bpo->bpo_havefreed);
+		bpo->bpo_phys->bpo_num_freed++;
+	}
+	mutex_exit(&bpo->bpo_lock);
+}
+
+struct space_range_arg {
+	spa_t *spa;
+	uint64_t mintxg;
+	uint64_t maxtxg;
+	uint64_t used;
+	uint64_t comp;
+	uint64_t uncomp;
+};
+
+/* ARGSUSED */
+static int
+space_range_cb(void *arg, const blkptr_t *bp, boolean_t bp_freed, dmu_tx_t *tx)
+{
+	struct space_range_arg *sra = arg;
+
+	if (bp->blk_birth > sra->mintxg && bp->blk_birth <= sra->maxtxg) {
+		if (dsl_pool_sync_context(spa_get_dsl(sra->spa)))
+			sra->used += bp_get_dsize_sync(sra->spa, bp);
+		else
+			sra->used += bp_get_dsize(sra->spa, bp);
+		sra->comp += BP_GET_PSIZE(bp);
+		sra->uncomp += BP_GET_UCSIZE(bp);
+	}
+	return (0);
+}
+
+int
+bpobj_space(bpobj_t *bpo, uint64_t *usedp, uint64_t *compp, uint64_t *uncompp)
+{
+	ASSERT(bpobj_is_open(bpo));
+	mutex_enter(&bpo->bpo_lock);
+
+	*usedp = bpo->bpo_phys->bpo_bytes;
+	if (bpo->bpo_havecomp) {
+		*compp = bpo->bpo_phys->bpo_comp;
+		*uncompp = bpo->bpo_phys->bpo_uncomp;
+		mutex_exit(&bpo->bpo_lock);
+		return (0);
+	} else {
+		mutex_exit(&bpo->bpo_lock);
+		return (bpobj_space_range(bpo, 0, UINT64_MAX,
+		    usedp, compp, uncompp));
+	}
+}
+
+/*
+ * Return the amount of space in the bpobj which is:
+ * mintxg < blk_birth <= maxtxg
+ */
+int
+bpobj_space_range(bpobj_t *bpo, uint64_t mintxg, uint64_t maxtxg,
+    uint64_t *usedp, uint64_t *compp, uint64_t *uncompp)
+{
+	struct space_range_arg sra = { 0 };
+	int err;
+
+	ASSERT(bpobj_is_open(bpo));
+
+	/*
+	 * As an optimization, if they want the whole txg range, just
+	 * get bpo_bytes rather than iterating over the bps.
+	 */
+	if (mintxg < TXG_INITIAL && maxtxg == UINT64_MAX && bpo->bpo_havecomp)
+		return (bpobj_space(bpo, usedp, compp, uncompp));
+
+	sra.spa = dmu_objset_spa(bpo->bpo_os);
+	sra.mintxg = mintxg;
+	sra.maxtxg = maxtxg;
+
+	err = bpobj_iterate_nofree(bpo, space_range_cb, &sra, NULL);
+	*usedp = sra.used;
+	*compp = sra.comp;
+	*uncompp = sra.uncomp;
+	return (err);
+}
+
+/*
+ * A bpobj_itor_t to append blkptrs to a bplist. Note that while blkptrs in a
+ * bpobj are designated as free or allocated that information is not preserved
+ * in bplists.
+ */
+/* ARGSUSED */
+int
+bplist_append_cb(void *arg, const blkptr_t *bp, boolean_t bp_freed,
+    dmu_tx_t *tx)
+{
+	bplist_t *bpl = arg;
+	bplist_append(bpl, bp);
+	return (0);
+}
diff --git a/sys/contrib/openzfs/module/zfs/bptree.c b/sys/contrib/openzfs/module/zfs/bptree.c
new file mode 100644
index 000000000000..1827a3c4e326
--- /dev/null
+++ b/sys/contrib/openzfs/module/zfs/bptree.c
@@ -0,0 +1,303 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+
+/*
+ * Copyright (c) 2011, 2018 by Delphix. All rights reserved.
+ */
+
+#include <sys/arc.h>
+#include <sys/bptree.h>
+#include <sys/dmu.h>
+#include <sys/dmu_objset.h>
+#include <sys/dmu_tx.h>
+#include <sys/dmu_traverse.h>
+#include <sys/dsl_dataset.h>
+#include <sys/dsl_dir.h>
+#include <sys/dsl_pool.h>
+#include <sys/dnode.h>
+#include <sys/spa.h>
+
+/*
+ * A bptree is a queue of root block pointers from destroyed datasets. When a
+ * dataset is destroyed its root block pointer is put on the end of the pool's
+ * bptree queue so the dataset's blocks can be freed asynchronously by
+ * dsl_scan_sync. This allows the delete operation to finish without traversing
+ * all the dataset's blocks.
+ *
+ * Note that while bt_begin and bt_end are only ever incremented in this code,
+ * they are effectively reset to 0 every time the entire bptree is freed because
+ * the bptree's object is destroyed and re-created.
+ */
+
+struct bptree_args {
+	bptree_phys_t *ba_phys;	/* data in bonus buffer, dirtied if freeing */
+	boolean_t ba_free;	/* true if freeing during traversal */
+
+	bptree_itor_t *ba_func;	/* function to call for each blockpointer */
+	void *ba_arg;		/* caller supplied argument to ba_func */
+	dmu_tx_t *ba_tx;	/* caller supplied tx, NULL if not freeing */
+} bptree_args_t;
+
+uint64_t
+bptree_alloc(objset_t *os, dmu_tx_t *tx)
+{
+	uint64_t obj;
+	dmu_buf_t *db;
+	bptree_phys_t *bt;
+
+	obj = dmu_object_alloc(os, DMU_OTN_UINT64_METADATA,
+	    SPA_OLD_MAXBLOCKSIZE, DMU_OTN_UINT64_METADATA,
+	    sizeof (bptree_phys_t), tx);
+
+	/*
+	 * Bonus buffer contents are already initialized to 0, but for
+	 * readability we make it explicit.
+	 */
+	VERIFY3U(0, ==, dmu_bonus_hold(os, obj, FTAG, &db));
+	dmu_buf_will_dirty(db, tx);
+	bt = db->db_data;
+	bt->bt_begin = 0;
+	bt->bt_end = 0;
+	bt->bt_bytes = 0;
+	bt->bt_comp = 0;
+	bt->bt_uncomp = 0;
+	dmu_buf_rele(db, FTAG);
+
+	return (obj);
+}
+
+int
+bptree_free(objset_t *os, uint64_t obj, dmu_tx_t *tx)
+{
+	dmu_buf_t *db;
+	bptree_phys_t *bt;
+
+	VERIFY3U(0, ==, dmu_bonus_hold(os, obj, FTAG, &db));
+	bt = db->db_data;
+	ASSERT3U(bt->bt_begin, ==, bt->bt_end);
+	ASSERT0(bt->bt_bytes);
+	ASSERT0(bt->bt_comp);
+	ASSERT0(bt->bt_uncomp);
+	dmu_buf_rele(db, FTAG);
+
+	return (dmu_object_free(os, obj, tx));
+}
+
+boolean_t
+bptree_is_empty(objset_t *os, uint64_t obj)
+{
+	dmu_buf_t *db;
+	bptree_phys_t *bt;
+	boolean_t rv;
+
+	VERIFY0(dmu_bonus_hold(os, obj, FTAG, &db));
+	bt = db->db_data;
+	rv = (bt->bt_begin == bt->bt_end);
+	dmu_buf_rele(db, FTAG);
+	return (rv);
+}
+
+void
+bptree_add(objset_t *os, uint64_t obj, blkptr_t *bp, uint64_t birth_txg,
+    uint64_t bytes, uint64_t comp, uint64_t uncomp, dmu_tx_t *tx)
+{
+	dmu_buf_t *db;
+	bptree_phys_t *bt;
+	bptree_entry_phys_t *bte;
+
+	/*
+	 * bptree objects are in the pool mos, therefore they can only be
+	 * modified in syncing context. Furthermore, this is only modified
+	 * by the sync thread, so no locking is necessary.
+	 */
+	ASSERT(dmu_tx_is_syncing(tx));
+
+	VERIFY3U(0, ==, dmu_bonus_hold(os, obj, FTAG, &db));
+	bt = db->db_data;
+
+	bte = kmem_zalloc(sizeof (*bte), KM_SLEEP);
+	bte->be_birth_txg = birth_txg;
+	bte->be_bp = *bp;
+	dmu_write(os, obj, bt->bt_end * sizeof (*bte), sizeof (*bte), bte, tx);
+	kmem_free(bte, sizeof (*bte));
+
+	dmu_buf_will_dirty(db, tx);
+	bt->bt_end++;
+	bt->bt_bytes += bytes;
+	bt->bt_comp += comp;
+	bt->bt_uncomp += uncomp;
+	dmu_buf_rele(db, FTAG);
+}
+
+/* ARGSUSED */
+static int
+bptree_visit_cb(spa_t *spa, zilog_t *zilog, const blkptr_t *bp,
+    const zbookmark_phys_t *zb, const dnode_phys_t *dnp, void *arg)
+{
+	int err;
+	struct bptree_args *ba = arg;
+
+	if (zb->zb_level == ZB_DNODE_LEVEL || BP_IS_HOLE(bp) ||
+	    BP_IS_REDACTED(bp))
+		return (0);
+
+	err = ba->ba_func(ba->ba_arg, bp, ba->ba_tx);
+	if (err == 0 && ba->ba_free) {
+		ba->ba_phys->bt_bytes -= bp_get_dsize_sync(spa, bp);
+		ba->ba_phys->bt_comp -= BP_GET_PSIZE(bp);
+		ba->ba_phys->bt_uncomp -= BP_GET_UCSIZE(bp);
+	}
+	return (err);
+}
+
+/*
+ * If "free" is set:
+ *  - It is assumed that "func" will be freeing the block pointers.
+ *  - If "func" returns nonzero, the bookmark will be remembered and
+ *    iteration will be restarted from this point on next invocation.
+ *  - If an i/o error is encountered (e.g. "func" returns EIO or ECKSUM),
+ *    bptree_iterate will remember the bookmark, continue traversing
+ *    any additional entries, and return 0.
+ *
+ * If "free" is not set, traversal will stop and return an error if
+ * an i/o error is encountered.
+ *
+ * In either case, if zfs_free_leak_on_eio is set, i/o errors will be
+ * ignored and traversal will continue (i.e. TRAVERSE_HARD will be passed to
+ * traverse_dataset_destroyed()).
+ */
+int
+bptree_iterate(objset_t *os, uint64_t obj, boolean_t free, bptree_itor_t func,
+    void *arg, dmu_tx_t *tx)
+{
+	boolean_t ioerr = B_FALSE;
+	int err;
+	uint64_t i;
+	dmu_buf_t *db;
+	struct bptree_args ba;
+
+	ASSERT(!free || dmu_tx_is_syncing(tx));
+
+	err = dmu_bonus_hold(os, obj, FTAG, &db);
+	if (err != 0)
+		return (err);
+
+	if (free)
+		dmu_buf_will_dirty(db, tx);
+
+	ba.ba_phys = db->db_data;
+	ba.ba_free = free;
+	ba.ba_func = func;
+	ba.ba_arg = arg;
+	ba.ba_tx = tx;
+
+	err = 0;
+	for (i = ba.ba_phys->bt_begin; i < ba.ba_phys->bt_end; i++) {
+		bptree_entry_phys_t bte;
+		int flags = TRAVERSE_PREFETCH_METADATA | TRAVERSE_POST |
+		    TRAVERSE_NO_DECRYPT;
+
+		err = dmu_read(os, obj, i * sizeof (bte), sizeof (bte),
+		    &bte, DMU_READ_NO_PREFETCH);
+		if (err != 0)
+			break;
+
+		if (zfs_free_leak_on_eio)
+			flags |= TRAVERSE_HARD;
+		zfs_dbgmsg("bptree index %lld: traversing from min_txg=%lld "
+		    "bookmark %lld/%lld/%lld/%lld",
+		    (longlong_t)i,
+		    (longlong_t)bte.be_birth_txg,
+		    (longlong_t)bte.be_zb.zb_objset,
+		    (longlong_t)bte.be_zb.zb_object,
+		    (longlong_t)bte.be_zb.zb_level,
+		    (longlong_t)bte.be_zb.zb_blkid);
+		err = traverse_dataset_destroyed(os->os_spa, &bte.be_bp,
+		    bte.be_birth_txg, &bte.be_zb, flags,
+		    bptree_visit_cb, &ba);
+		if (free) {
+			/*
+			 * The callback has freed the visited block pointers.
+			 * Record our traversal progress on disk, either by
+			 * updating this record's bookmark, or by logically
+			 * removing this record by advancing bt_begin.
+			 */
+			if (err != 0) {
+				/* save bookmark for future resume */
+				ASSERT3U(bte.be_zb.zb_objset, ==,
+				    ZB_DESTROYED_OBJSET);
+				ASSERT0(bte.be_zb.zb_level);
+				dmu_write(os, obj, i * sizeof (bte),
+				    sizeof (bte), &bte, tx);
+				if (err == EIO || err == ECKSUM ||
+				    err == ENXIO) {
+					/*
+					 * Skip the rest of this tree and
+					 * continue on to the next entry.
+					 */
+					err = 0;
+					ioerr = B_TRUE;
+				} else {
+					break;
+				}
+			} else if (ioerr) {
+				/*
+				 * This entry is finished, but there were
+				 * i/o errors on previous entries, so we
+				 * can't adjust bt_begin.  Set this entry's
+				 * be_birth_txg such that it will be
+				 * treated as a no-op in future traversals.
+				 */
+				bte.be_birth_txg = UINT64_MAX;
+				dmu_write(os, obj, i * sizeof (bte),
+				    sizeof (bte), &bte, tx);
+			}
+
+			if (!ioerr) {
+				ba.ba_phys->bt_begin++;
+				(void) dmu_free_range(os, obj,
+				    i * sizeof (bte), sizeof (bte), tx);
+			}
+		} else if (err != 0) {
+			break;
+		}
+	}
+
+	ASSERT(!free || err != 0 || ioerr ||
+	    ba.ba_phys->bt_begin == ba.ba_phys->bt_end);
+
+	/* if all blocks are free there should be no used space */
+	if (ba.ba_phys->bt_begin == ba.ba_phys->bt_end) {
+		if (zfs_free_leak_on_eio) {
+			ba.ba_phys->bt_bytes = 0;
+			ba.ba_phys->bt_comp = 0;
+			ba.ba_phys->bt_uncomp = 0;
+		}
+
+		ASSERT0(ba.ba_phys->bt_bytes);
+		ASSERT0(ba.ba_phys->bt_comp);
+		ASSERT0(ba.ba_phys->bt_uncomp);
+	}
+
+	dmu_buf_rele(db, FTAG);
+
+	return (err);
+}
diff --git a/sys/contrib/openzfs/module/zfs/bqueue.c b/sys/contrib/openzfs/module/zfs/bqueue.c
new file mode 100644
index 000000000000..22539efc4e23
--- /dev/null
+++ b/sys/contrib/openzfs/module/zfs/bqueue.c
@@ -0,0 +1,155 @@
+/*
+ * CDDL HEADER START
+ *
+ * This file and its contents are supplied under the terms of the
+ * Common Development and Distribution License ("CDDL"), version 1.0.
+ * You may only use this file in accordance with the terms of version
+ * 1.0 of the CDDL.
+ *
+ * A full copy of the text of the CDDL should have accompanied this
+ * source.  A copy of the CDDL is also available via the Internet at
+ * http://www.illumos.org/license/CDDL.
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright (c) 2014, 2018 by Delphix. All rights reserved.
+ */
+
+#include	<sys/bqueue.h>
+#include	<sys/zfs_context.h>
+
+static inline bqueue_node_t *
+obj2node(bqueue_t *q, void *data)
+{
+	return ((bqueue_node_t *)((char *)data + q->bq_node_offset));
+}
+
+/*
+ * Initialize a blocking queue  The maximum capacity of the queue is set to
+ * size.  Types that are stored in a bqueue must contain a bqueue_node_t,
+ * and node_offset must be its offset from the start of the struct.
+ * fill_fraction is a performance tuning value; when the queue is full, any
+ * threads attempting to enqueue records will block.  They will block until
+ * they're signaled, which will occur when the queue is at least 1/fill_fraction
+ * empty.  Similar behavior occurs on dequeue; if the queue is empty, threads
+ * block.  They will be signalled when the queue has 1/fill_fraction full, or
+ * when bqueue_flush is called.  As a result, you must call bqueue_flush when
+ * you enqueue your final record on a thread, in case the dequeueing threads are
+ * currently blocked and that enqueue does not cause them to be awoken.
+ * Alternatively, this behavior can be disabled (causing signaling to happen
+ * immediately) by setting fill_fraction to any value larger than size.
+ * Return 0 on success, or -1 on failure.
+ */
+int
+bqueue_init(bqueue_t *q, uint64_t fill_fraction, uint64_t size,
+    size_t node_offset)
+{
+	if (fill_fraction == 0) {
+		return (-1);
+	}
+	list_create(&q->bq_list, node_offset + sizeof (bqueue_node_t),
+	    node_offset + offsetof(bqueue_node_t, bqn_node));
+	cv_init(&q->bq_add_cv, NULL, CV_DEFAULT, NULL);
+	cv_init(&q->bq_pop_cv, NULL, CV_DEFAULT, NULL);
+	mutex_init(&q->bq_lock, NULL, MUTEX_DEFAULT, NULL);
+	q->bq_node_offset = node_offset;
+	q->bq_size = 0;
+	q->bq_maxsize = size;
+	q->bq_fill_fraction = fill_fraction;
+	return (0);
+}
+
+/*
+ * Destroy a blocking queue.  This function asserts that there are no
+ * elements in the queue, and no one is blocked on the condition
+ * variables.
+ */
+void
+bqueue_destroy(bqueue_t *q)
+{
+	mutex_enter(&q->bq_lock);
+	ASSERT0(q->bq_size);
+	cv_destroy(&q->bq_add_cv);
+	cv_destroy(&q->bq_pop_cv);
+	list_destroy(&q->bq_list);
+	mutex_exit(&q->bq_lock);
+	mutex_destroy(&q->bq_lock);
+}
+
+static void
+bqueue_enqueue_impl(bqueue_t *q, void *data, uint64_t item_size,
+    boolean_t flush)
+{
+	ASSERT3U(item_size, >, 0);
+	ASSERT3U(item_size, <=, q->bq_maxsize);
+	mutex_enter(&q->bq_lock);
+	obj2node(q, data)->bqn_size = item_size;
+	while (q->bq_size + item_size > q->bq_maxsize) {
+		cv_wait_sig(&q->bq_add_cv, &q->bq_lock);
+	}
+	q->bq_size += item_size;
+	list_insert_tail(&q->bq_list, data);
+	if (q->bq_size >= q->bq_maxsize / q->bq_fill_fraction)
+		cv_signal(&q->bq_pop_cv);
+	if (flush)
+		cv_broadcast(&q->bq_pop_cv);
+	mutex_exit(&q->bq_lock);
+}
+
+/*
+ * Add data to q, consuming size units of capacity.  If there is insufficient
+ * capacity to consume size units, block until capacity exists.  Asserts size is
+ * > 0.
+ */
+void
+bqueue_enqueue(bqueue_t *q, void *data, uint64_t item_size)
+{
+	bqueue_enqueue_impl(q, data, item_size, B_FALSE);
+}
+
+/*
+ * Enqueue an entry, and then flush the queue.  This forces the popping threads
+ * to wake up, even if we're below the fill fraction.  We have this in a single
+ * function, rather than having a separate call, because it prevents race
+ * conditions between the enqueuing thread and the dequeueing thread, where the
+ * enqueueing thread will wake up the dequeueing thread, that thread will
+ * destroy the condvar before the enqueuing thread is done.
+ */
+void
+bqueue_enqueue_flush(bqueue_t *q, void *data, uint64_t item_size)
+{
+	bqueue_enqueue_impl(q, data, item_size, B_TRUE);
+}
+
+/*
+ * Take the first element off of q.  If there are no elements on the queue, wait
+ * until one is put there.  Return the removed element.
+ */
+void *
+bqueue_dequeue(bqueue_t *q)
+{
+	void *ret = NULL;
+	uint64_t item_size;
+	mutex_enter(&q->bq_lock);
+	while (q->bq_size == 0) {
+		cv_wait_sig(&q->bq_pop_cv, &q->bq_lock);
+	}
+	ret = list_remove_head(&q->bq_list);
+	ASSERT3P(ret, !=, NULL);
+	item_size = obj2node(q, ret)->bqn_size;
+	q->bq_size -= item_size;
+	if (q->bq_size <= q->bq_maxsize - (q->bq_maxsize / q->bq_fill_fraction))
+		cv_signal(&q->bq_add_cv);
+	mutex_exit(&q->bq_lock);
+	return (ret);
+}
+
+/*
+ * Returns true if the space used is 0.
+ */
+boolean_t
+bqueue_empty(bqueue_t *q)
+{
+	return (q->bq_size == 0);
+}
diff --git a/sys/contrib/openzfs/module/zfs/btree.c b/sys/contrib/openzfs/module/zfs/btree.c
new file mode 100644
index 000000000000..57b9dbbb2b50
--- /dev/null
+++ b/sys/contrib/openzfs/module/zfs/btree.c
@@ -0,0 +1,2124 @@
+/*
+ * CDDL HEADER START
+ *
+ * This file and its contents are supplied under the terms of the
+ * Common Development and Distribution License ("CDDL"), version 1.0.
+ * You may only use this file in accordance with the terms of version
+ * 1.0 of the CDDL.
+ *
+ * A full copy of the text of the CDDL should have accompanied this
+ * source.  A copy of the CDDL is also available via the Internet at
+ * http://www.illumos.org/license/CDDL.
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright (c) 2019 by Delphix. All rights reserved.
+ */
+
+#include	<sys/btree.h>
+#include	<sys/bitops.h>
+#include	<sys/zfs_context.h>
+
+kmem_cache_t *zfs_btree_leaf_cache;
+
+/*
+ * Control the extent of the verification that occurs when zfs_btree_verify is
+ * called. Primarily used for debugging when extending the btree logic and
+ * functionality. As the intensity is increased, new verification steps are
+ * added. These steps are cumulative; intensity = 3 includes the intensity = 1
+ * and intensity = 2 steps as well.
+ *
+ * Intensity 1: Verify that the tree's height is consistent throughout.
+ * Intensity 2: Verify that a core node's children's parent pointers point
+ * to the core node.
+ * Intensity 3: Verify that the total number of elements in the tree matches the
+ * sum of the number of elements in each node. Also verifies that each node's
+ * count obeys the invariants (less than or equal to maximum value, greater than
+ * or equal to half the maximum minus one).
+ * Intensity 4: Verify that each element compares less than the element
+ * immediately after it and greater than the one immediately before it using the
+ * comparator function. For core nodes, also checks that each element is greater
+ * than the last element in the first of the two nodes it separates, and less
+ * than the first element in the second of the two nodes.
+ * Intensity 5: Verifies, if ZFS_DEBUG is defined, that all unused memory inside
+ * of each node is poisoned appropriately. Note that poisoning always occurs if
+ * ZFS_DEBUG is set, so it is safe to set the intensity to 5 during normal
+ * operation.
+ *
+ * Intensity 4 and 5 are particularly expensive to perform; the previous levels
+ * are a few memory operations per node, while these levels require multiple
+ * operations per element. In addition, when creating large btrees, these
+ * operations are called at every step, resulting in extremely slow operation
+ * (while the asymptotic complexity of the other steps is the same, the
+ * importance of the constant factors cannot be denied).
+ */
+int zfs_btree_verify_intensity = 0;
+
+/*
+ * A convenience function to silence warnings from memmove's return value and
+ * change argument order to src, dest.
+ */
+static void
+bmov(const void *src, void *dest, size_t size)
+{
+	(void) memmove(dest, src, size);
+}
+
+#ifdef _ILP32
+#define	BTREE_POISON 0xabadb10c
+#else
+#define	BTREE_POISON 0xabadb10cdeadbeef
+#endif
+
+static void
+zfs_btree_poison_node(zfs_btree_t *tree, zfs_btree_hdr_t *hdr)
+{
+#ifdef ZFS_DEBUG
+	size_t size = tree->bt_elem_size;
+	if (!hdr->bth_core) {
+		zfs_btree_leaf_t *leaf = (zfs_btree_leaf_t *)hdr;
+		(void) memset(leaf->btl_elems + hdr->bth_count * size, 0x0f,
+		    BTREE_LEAF_SIZE - sizeof (zfs_btree_hdr_t) -
+		    hdr->bth_count * size);
+	} else {
+		zfs_btree_core_t *node = (zfs_btree_core_t *)hdr;
+		for (int i = hdr->bth_count + 1; i <= BTREE_CORE_ELEMS; i++) {
+			node->btc_children[i] =
+			    (zfs_btree_hdr_t *)BTREE_POISON;
+		}
+		(void) memset(node->btc_elems + hdr->bth_count * size, 0x0f,
+		    (BTREE_CORE_ELEMS - hdr->bth_count) * size);
+	}
+#endif
+}
+
+static inline void
+zfs_btree_poison_node_at(zfs_btree_t *tree, zfs_btree_hdr_t *hdr,
+    uint64_t offset)
+{
+#ifdef ZFS_DEBUG
+	size_t size = tree->bt_elem_size;
+	ASSERT3U(offset, >=, hdr->bth_count);
+	if (!hdr->bth_core) {
+		zfs_btree_leaf_t *leaf = (zfs_btree_leaf_t *)hdr;
+		(void) memset(leaf->btl_elems + offset * size, 0x0f, size);
+	} else {
+		zfs_btree_core_t *node = (zfs_btree_core_t *)hdr;
+		node->btc_children[offset + 1] =
+		    (zfs_btree_hdr_t *)BTREE_POISON;
+		(void) memset(node->btc_elems + offset * size, 0x0f, size);
+	}
+#endif
+}
+
+static inline void
+zfs_btree_verify_poison_at(zfs_btree_t *tree, zfs_btree_hdr_t *hdr,
+    uint64_t offset)
+{
+#ifdef ZFS_DEBUG
+	size_t size = tree->bt_elem_size;
+	uint8_t eval = 0x0f;
+	if (hdr->bth_core) {
+		zfs_btree_core_t *node = (zfs_btree_core_t *)hdr;
+		zfs_btree_hdr_t *cval = (zfs_btree_hdr_t *)BTREE_POISON;
+		VERIFY3P(node->btc_children[offset + 1], ==, cval);
+		for (int i = 0; i < size; i++)
+			VERIFY3U(node->btc_elems[offset * size + i], ==, eval);
+	} else  {
+		zfs_btree_leaf_t *leaf = (zfs_btree_leaf_t *)hdr;
+		for (int i = 0; i < size; i++)
+			VERIFY3U(leaf->btl_elems[offset * size + i], ==, eval);
+	}
+#endif
+}
+
+void
+zfs_btree_init(void)
+{
+	zfs_btree_leaf_cache = kmem_cache_create("zfs_btree_leaf_cache",
+	    BTREE_LEAF_SIZE, 0, NULL, NULL, NULL, NULL,
+	    NULL, 0);
+}
+
+void
+zfs_btree_fini(void)
+{
+	kmem_cache_destroy(zfs_btree_leaf_cache);
+}
+
+void
+zfs_btree_create(zfs_btree_t *tree, int (*compar) (const void *, const void *),
+    size_t size)
+{
+	/*
+	 * We need a minimmum of 4 elements so that when we split a node we
+	 * always have at least two elements in each node. This simplifies the
+	 * logic in zfs_btree_bulk_finish, since it means the last leaf will
+	 * always have a left sibling to share with (unless it's the root).
+	 */
+	ASSERT3U(size, <=, (BTREE_LEAF_SIZE - sizeof (zfs_btree_hdr_t)) / 4);
+
+	bzero(tree, sizeof (*tree));
+	tree->bt_compar = compar;
+	tree->bt_elem_size = size;
+	tree->bt_height = -1;
+	tree->bt_bulk = NULL;
+}
+
+/*
+ * Find value in the array of elements provided. Uses a simple binary search.
+ */
+static void *
+zfs_btree_find_in_buf(zfs_btree_t *tree, uint8_t *buf, uint64_t nelems,
+    const void *value, zfs_btree_index_t *where)
+{
+	uint64_t max = nelems;
+	uint64_t min = 0;
+	while (max > min) {
+		uint64_t idx = (min + max) / 2;
+		uint8_t *cur = buf + idx * tree->bt_elem_size;
+		int comp = tree->bt_compar(cur, value);
+		if (comp == -1) {
+			min = idx + 1;
+		} else if (comp == 1) {
+			max = idx;
+		} else {
+			ASSERT0(comp);
+			where->bti_offset = idx;
+			where->bti_before = B_FALSE;
+			return (cur);
+		}
+	}
+
+	where->bti_offset = max;
+	where->bti_before = B_TRUE;
+	return (NULL);
+}
+
+/*
+ * Find the given value in the tree. where may be passed as null to use as a
+ * membership test or if the btree is being used as a map.
+ */
+void *
+zfs_btree_find(zfs_btree_t *tree, const void *value, zfs_btree_index_t *where)
+{
+	if (tree->bt_height == -1) {
+		if (where != NULL) {
+			where->bti_node = NULL;
+			where->bti_offset = 0;
+		}
+		ASSERT0(tree->bt_num_elems);
+		return (NULL);
+	}
+
+	/*
+	 * If we're in bulk-insert mode, we check the last spot in the tree
+	 * and the last leaf in the tree before doing the normal search,
+	 * because for most workloads the vast majority of finds in
+	 * bulk-insert mode are to insert new elements.
+	 */
+	zfs_btree_index_t idx;
+	if (tree->bt_bulk != NULL) {
+		zfs_btree_leaf_t *last_leaf = tree->bt_bulk;
+		int compar = tree->bt_compar(last_leaf->btl_elems +
+		    ((last_leaf->btl_hdr.bth_count - 1) * tree->bt_elem_size),
+		    value);
+		if (compar < 0) {
+			/*
+			 * If what they're looking for is after the last
+			 * element, it's not in the tree.
+			 */
+			if (where != NULL) {
+				where->bti_node = (zfs_btree_hdr_t *)last_leaf;
+				where->bti_offset =
+				    last_leaf->btl_hdr.bth_count;
+				where->bti_before = B_TRUE;
+			}
+			return (NULL);
+		} else if (compar == 0) {
+			if (where != NULL) {
+				where->bti_node = (zfs_btree_hdr_t *)last_leaf;
+				where->bti_offset =
+				    last_leaf->btl_hdr.bth_count - 1;
+				where->bti_before = B_FALSE;
+			}
+			return (last_leaf->btl_elems +
+			    ((last_leaf->btl_hdr.bth_count - 1) *
+			    tree->bt_elem_size));
+		}
+		if (tree->bt_compar(last_leaf->btl_elems, value) <= 0) {
+			/*
+			 * If what they're looking for is after the first
+			 * element in the last leaf, it's in the last leaf or
+			 * it's not in the tree.
+			 */
+			void *d = zfs_btree_find_in_buf(tree,
+			    last_leaf->btl_elems, last_leaf->btl_hdr.bth_count,
+			    value, &idx);
+
+			if (where != NULL) {
+				idx.bti_node = (zfs_btree_hdr_t *)last_leaf;
+				*where = idx;
+			}
+			return (d);
+		}
+	}
+
+	zfs_btree_core_t *node = NULL;
+	uint64_t child = 0;
+	uint64_t depth = 0;
+
+	/*
+	 * Iterate down the tree, finding which child the value should be in
+	 * by comparing with the separators.
+	 */
+	for (node = (zfs_btree_core_t *)tree->bt_root; depth < tree->bt_height;
+	    node = (zfs_btree_core_t *)node->btc_children[child], depth++) {
+		ASSERT3P(node, !=, NULL);
+		void *d = zfs_btree_find_in_buf(tree, node->btc_elems,
+		    node->btc_hdr.bth_count, value, &idx);
+		EQUIV(d != NULL, !idx.bti_before);
+		if (d != NULL) {
+			if (where != NULL) {
+				idx.bti_node = (zfs_btree_hdr_t *)node;
+				*where = idx;
+			}
+			return (d);
+		}
+		ASSERT(idx.bti_before);
+		child = idx.bti_offset;
+	}
+
+	/*
+	 * The value is in this leaf, or it would be if it were in the
+	 * tree. Find its proper location and return it.
+	 */
+	zfs_btree_leaf_t *leaf = (depth == 0 ?
+	    (zfs_btree_leaf_t *)tree->bt_root : (zfs_btree_leaf_t *)node);
+	void *d = zfs_btree_find_in_buf(tree, leaf->btl_elems,
+	    leaf->btl_hdr.bth_count, value, &idx);
+
+	if (where != NULL) {
+		idx.bti_node = (zfs_btree_hdr_t *)leaf;
+		*where = idx;
+	}
+
+	return (d);
+}
+
+/*
+ * To explain the following functions, it is useful to understand the four
+ * kinds of shifts used in btree operation. First, a shift is a movement of
+ * elements within a node. It is used to create gaps for inserting new
+ * elements and children, or cover gaps created when things are removed. A
+ * shift has two fundamental properties, each of which can be one of two
+ * values, making four types of shifts.  There is the direction of the shift
+ * (left or right) and the shape of the shift (parallelogram or isoceles
+ * trapezoid (shortened to trapezoid hereafter)). The shape distinction only
+ * applies to shifts of core nodes.
+ *
+ * The names derive from the following imagining of the layout of a node:
+ *
+ *  Elements:       *   *   *   *   *   *   *   ...   *   *   *
+ *  Children:     *   *   *   *   *   *   *   *   ...   *   *   *
+ *
+ * This layout follows from the fact that the elements act as separators
+ * between pairs of children, and that children root subtrees "below" the
+ * current node. A left and right shift are fairly self-explanatory; a left
+ * shift moves things to the left, while a right shift moves things to the
+ * right. A parallelogram shift is a shift with the same number of elements
+ * and children being moved, while a trapezoid shift is a shift that moves one
+ * more children than elements. An example follows:
+ *
+ * A parallelogram shift could contain the following:
+ *      _______________
+ *      \*   *   *   * \ *   *   *   ...   *   *   *
+ *     * \ *   *   *   *\  *   *   *   ...   *   *   *
+ *        ---------------
+ * A trapezoid shift could contain the following:
+ *          ___________
+ *       * / *   *   * \ *   *   *   ...   *   *   *
+ *     *  / *  *   *   *\  *   *   *   ...   *   *   *
+ *        ---------------
+ *
+ * Note that a parallelogram shift is always shaped like a "left-leaning"
+ * parallelogram, where the starting index of the children being moved is
+ * always one higher than the starting index of the elements being moved. No
+ * "right-leaning" parallelogram shifts are needed (shifts where the starting
+ * element index and starting child index being moved are the same) to achieve
+ * any btree operations, so we ignore them.
+ */
+
+enum bt_shift_shape {
+	BSS_TRAPEZOID,
+	BSS_PARALLELOGRAM
+};
+
+enum bt_shift_direction {
+	BSD_LEFT,
+	BSD_RIGHT
+};
+
+/*
+ * Shift elements and children in the provided core node by off spots.  The
+ * first element moved is idx, and count elements are moved. The shape of the
+ * shift is determined by shape. The direction is determined by dir.
+ */
+static inline void
+bt_shift_core(zfs_btree_t *tree, zfs_btree_core_t *node, uint64_t idx,
+    uint64_t count, uint64_t off, enum bt_shift_shape shape,
+    enum bt_shift_direction dir)
+{
+	size_t size = tree->bt_elem_size;
+	ASSERT(node->btc_hdr.bth_core);
+
+	uint8_t *e_start = node->btc_elems + idx * size;
+	int sign = (dir == BSD_LEFT ? -1 : +1);
+	uint8_t *e_out = e_start + sign * off * size;
+	uint64_t e_count = count;
+	bmov(e_start, e_out, e_count * size);
+
+	zfs_btree_hdr_t **c_start = node->btc_children + idx +
+	    (shape == BSS_TRAPEZOID ? 0 : 1);
+	zfs_btree_hdr_t **c_out = (dir == BSD_LEFT ? c_start - off :
+	    c_start + off);
+	uint64_t c_count = count + (shape == BSS_TRAPEZOID ? 1 : 0);
+	bmov(c_start, c_out, c_count * sizeof (*c_start));
+}
+
+/*
+ * Shift elements and children in the provided core node left by one spot.
+ * The first element moved is idx, and count elements are moved. The
+ * shape of the shift is determined by trap; true if the shift is a trapezoid,
+ * false if it is a parallelogram.
+ */
+static inline void
+bt_shift_core_left(zfs_btree_t *tree, zfs_btree_core_t *node, uint64_t idx,
+    uint64_t count, enum bt_shift_shape shape)
+{
+	bt_shift_core(tree, node, idx, count, 1, shape, BSD_LEFT);
+}
+
+/*
+ * Shift elements and children in the provided core node right by one spot.
+ * Starts with elements[idx] and children[idx] and one more child than element.
+ */
+static inline void
+bt_shift_core_right(zfs_btree_t *tree, zfs_btree_core_t *node, uint64_t idx,
+    uint64_t count, enum bt_shift_shape shape)
+{
+	bt_shift_core(tree, node, idx, count, 1, shape, BSD_RIGHT);
+}
+
+/*
+ * Shift elements and children in the provided leaf node by off spots.
+ * The first element moved is idx, and count elements are moved. The direction
+ * is determined by left.
+ */
+static inline void
+bt_shift_leaf(zfs_btree_t *tree, zfs_btree_leaf_t *node, uint64_t idx,
+    uint64_t count, uint64_t off, enum bt_shift_direction dir)
+{
+	size_t size = tree->bt_elem_size;
+	ASSERT(!node->btl_hdr.bth_core);
+
+	uint8_t *start = node->btl_elems + idx * size;
+	int sign = (dir == BSD_LEFT ? -1 : +1);
+	uint8_t *out = start + sign * off * size;
+	bmov(start, out, count * size);
+}
+
+static inline void
+bt_shift_leaf_right(zfs_btree_t *tree, zfs_btree_leaf_t *leaf, uint64_t idx,
+    uint64_t count)
+{
+	bt_shift_leaf(tree, leaf, idx, count, 1, BSD_RIGHT);
+}
+
+static inline void
+bt_shift_leaf_left(zfs_btree_t *tree, zfs_btree_leaf_t *leaf, uint64_t idx,
+    uint64_t count)
+{
+	bt_shift_leaf(tree, leaf, idx, count, 1, BSD_LEFT);
+}
+
+/*
+ * Move children and elements from one core node to another. The shape
+ * parameter behaves the same as it does in the shift logic.
+ */
+static inline void
+bt_transfer_core(zfs_btree_t *tree, zfs_btree_core_t *source, uint64_t sidx,
+    uint64_t count, zfs_btree_core_t *dest, uint64_t didx,
+    enum bt_shift_shape shape)
+{
+	size_t size = tree->bt_elem_size;
+	ASSERT(source->btc_hdr.bth_core);
+	ASSERT(dest->btc_hdr.bth_core);
+
+	bmov(source->btc_elems + sidx * size, dest->btc_elems + didx * size,
+	    count * size);
+
+	uint64_t c_count = count + (shape == BSS_TRAPEZOID ? 1 : 0);
+	bmov(source->btc_children + sidx + (shape == BSS_TRAPEZOID ? 0 : 1),
+	    dest->btc_children + didx + (shape == BSS_TRAPEZOID ? 0 : 1),
+	    c_count * sizeof (*source->btc_children));
+}
+
+static inline void
+bt_transfer_leaf(zfs_btree_t *tree, zfs_btree_leaf_t *source, uint64_t sidx,
+    uint64_t count, zfs_btree_leaf_t *dest, uint64_t didx)
+{
+	size_t size = tree->bt_elem_size;
+	ASSERT(!source->btl_hdr.bth_core);
+	ASSERT(!dest->btl_hdr.bth_core);
+
+	bmov(source->btl_elems + sidx * size, dest->btl_elems + didx * size,
+	    count * size);
+}
+
+/*
+ * Find the first element in the subtree rooted at hdr, return its value and
+ * put its location in where if non-null.
+ */
+static void *
+zfs_btree_first_helper(zfs_btree_hdr_t *hdr, zfs_btree_index_t *where)
+{
+	zfs_btree_hdr_t *node;
+
+	for (node = hdr; node->bth_core; node =
+	    ((zfs_btree_core_t *)node)->btc_children[0])
+		;
+
+	ASSERT(!node->bth_core);
+	zfs_btree_leaf_t *leaf = (zfs_btree_leaf_t *)node;
+	if (where != NULL) {
+		where->bti_node = node;
+		where->bti_offset = 0;
+		where->bti_before = B_FALSE;
+	}
+	return (&leaf->btl_elems[0]);
+}
+
+/* Insert an element and a child into a core node at the given offset. */
+static void
+zfs_btree_insert_core_impl(zfs_btree_t *tree, zfs_btree_core_t *parent,
+    uint64_t offset, zfs_btree_hdr_t *new_node, void *buf)
+{
+	uint64_t size = tree->bt_elem_size;
+	zfs_btree_hdr_t *par_hdr = &parent->btc_hdr;
+	ASSERT3P(par_hdr, ==, new_node->bth_parent);
+	ASSERT3U(par_hdr->bth_count, <, BTREE_CORE_ELEMS);
+
+	if (zfs_btree_verify_intensity >= 5) {
+		zfs_btree_verify_poison_at(tree, par_hdr,
+		    par_hdr->bth_count);
+	}
+	/* Shift existing elements and children */
+	uint64_t count = par_hdr->bth_count - offset;
+	bt_shift_core_right(tree, parent, offset, count,
+	    BSS_PARALLELOGRAM);
+
+	/* Insert new values */
+	parent->btc_children[offset + 1] = new_node;
+	bmov(buf, parent->btc_elems + offset * size, size);
+	par_hdr->bth_count++;
+}
+
+/*
+ * Insert new_node into the parent of old_node directly after old_node, with
+ * buf as the dividing element between the two.
+ */
+static void
+zfs_btree_insert_into_parent(zfs_btree_t *tree, zfs_btree_hdr_t *old_node,
+    zfs_btree_hdr_t *new_node, void *buf)
+{
+	ASSERT3P(old_node->bth_parent, ==, new_node->bth_parent);
+	uint64_t size = tree->bt_elem_size;
+	zfs_btree_core_t *parent = old_node->bth_parent;
+	zfs_btree_hdr_t *par_hdr = &parent->btc_hdr;
+
+	/*
+	 * If this is the root node we were splitting, we create a new root
+	 * and increase the height of the tree.
+	 */
+	if (parent == NULL) {
+		ASSERT3P(old_node, ==, tree->bt_root);
+		tree->bt_num_nodes++;
+		zfs_btree_core_t *new_root =
+		    kmem_alloc(sizeof (zfs_btree_core_t) + BTREE_CORE_ELEMS *
+		    size, KM_SLEEP);
+		zfs_btree_hdr_t *new_root_hdr = &new_root->btc_hdr;
+		new_root_hdr->bth_parent = NULL;
+		new_root_hdr->bth_core = B_TRUE;
+		new_root_hdr->bth_count = 1;
+
+		old_node->bth_parent = new_node->bth_parent = new_root;
+		new_root->btc_children[0] = old_node;
+		new_root->btc_children[1] = new_node;
+		bmov(buf, new_root->btc_elems, size);
+
+		tree->bt_height++;
+		tree->bt_root = new_root_hdr;
+		zfs_btree_poison_node(tree, new_root_hdr);
+		return;
+	}
+
+	/*
+	 * Since we have the new separator, binary search for where to put
+	 * new_node.
+	 */
+	zfs_btree_index_t idx;
+	ASSERT(par_hdr->bth_core);
+	VERIFY3P(zfs_btree_find_in_buf(tree, parent->btc_elems,
+	    par_hdr->bth_count, buf, &idx), ==, NULL);
+	ASSERT(idx.bti_before);
+	uint64_t offset = idx.bti_offset;
+	ASSERT3U(offset, <=, par_hdr->bth_count);
+	ASSERT3P(parent->btc_children[offset], ==, old_node);
+
+	/*
+	 * If the parent isn't full, shift things to accommodate our insertions
+	 * and return.
+	 */
+	if (par_hdr->bth_count != BTREE_CORE_ELEMS) {
+		zfs_btree_insert_core_impl(tree, parent, offset, new_node, buf);
+		return;
+	}
+
+	/*
+	 * We need to split this core node into two. Currently there are
+	 * BTREE_CORE_ELEMS + 1 child nodes, and we are adding one for
+	 * BTREE_CORE_ELEMS + 2. Some of the children will be part of the
+	 * current node, and the others will be moved to the new core node.
+	 * There are BTREE_CORE_ELEMS + 1 elements including the new one. One
+	 * will be used as the new separator in our parent, and the others
+	 * will be split among the two core nodes.
+	 *
+	 * Usually we will split the node in half evenly, with
+	 * BTREE_CORE_ELEMS/2 elements in each node. If we're bulk loading, we
+	 * instead move only about a quarter of the elements (and children) to
+	 * the new node. Since the average state after a long time is a 3/4
+	 * full node, shortcutting directly to that state improves efficiency.
+	 *
+	 * We do this in two stages: first we split into two nodes, and then we
+	 * reuse our existing logic to insert the new element and child.
+	 */
+	uint64_t move_count = MAX((BTREE_CORE_ELEMS / (tree->bt_bulk == NULL ?
+	    2 : 4)) - 1, 2);
+	uint64_t keep_count = BTREE_CORE_ELEMS - move_count - 1;
+	ASSERT3U(BTREE_CORE_ELEMS - move_count, >=, 2);
+	tree->bt_num_nodes++;
+	zfs_btree_core_t *new_parent = kmem_alloc(sizeof (zfs_btree_core_t) +
+	    BTREE_CORE_ELEMS * size, KM_SLEEP);
+	zfs_btree_hdr_t *new_par_hdr = &new_parent->btc_hdr;
+	new_par_hdr->bth_parent = par_hdr->bth_parent;
+	new_par_hdr->bth_core = B_TRUE;
+	new_par_hdr->bth_count = move_count;
+	zfs_btree_poison_node(tree, new_par_hdr);
+
+	par_hdr->bth_count = keep_count;
+
+	bt_transfer_core(tree, parent, keep_count + 1, move_count, new_parent,
+	    0, BSS_TRAPEZOID);
+
+	/* Store the new separator in a buffer. */
+	uint8_t *tmp_buf = kmem_alloc(size, KM_SLEEP);
+	bmov(parent->btc_elems + keep_count * size, tmp_buf,
+	    size);
+	zfs_btree_poison_node(tree, par_hdr);
+
+	if (offset < keep_count) {
+		/* Insert the new node into the left half */
+		zfs_btree_insert_core_impl(tree, parent, offset, new_node,
+		    buf);
+
+		/*
+		 * Move the new separator to the existing buffer.
+		 */
+		bmov(tmp_buf, buf, size);
+	} else if (offset > keep_count) {
+		/* Insert the new node into the right half */
+		new_node->bth_parent = new_parent;
+		zfs_btree_insert_core_impl(tree, new_parent,
+		    offset - keep_count - 1, new_node, buf);
+
+		/*
+		 * Move the new separator to the existing buffer.
+		 */
+		bmov(tmp_buf, buf, size);
+	} else {
+		/*
+		 * Move the new separator into the right half, and replace it
+		 * with buf. We also need to shift back the elements in the
+		 * right half to accommodate new_node.
+		 */
+		bt_shift_core_right(tree, new_parent, 0, move_count,
+		    BSS_TRAPEZOID);
+		new_parent->btc_children[0] = new_node;
+		bmov(tmp_buf, new_parent->btc_elems, size);
+		new_par_hdr->bth_count++;
+	}
+	kmem_free(tmp_buf, size);
+	zfs_btree_poison_node(tree, par_hdr);
+
+	for (int i = 0; i <= new_parent->btc_hdr.bth_count; i++)
+		new_parent->btc_children[i]->bth_parent = new_parent;
+
+	for (int i = 0; i <= parent->btc_hdr.bth_count; i++)
+		ASSERT3P(parent->btc_children[i]->bth_parent, ==, parent);
+
+	/*
+	 * Now that the node is split, we need to insert the new node into its
+	 * parent. This may cause further splitting.
+	 */
+	zfs_btree_insert_into_parent(tree, &parent->btc_hdr,
+	    &new_parent->btc_hdr, buf);
+}
+
+/* Insert an element into a leaf node at the given offset. */
+static void
+zfs_btree_insert_leaf_impl(zfs_btree_t *tree, zfs_btree_leaf_t *leaf,
+    uint64_t idx, const void *value)
+{
+	uint64_t size = tree->bt_elem_size;
+	uint8_t *start = leaf->btl_elems + (idx * size);
+	zfs_btree_hdr_t *hdr = &leaf->btl_hdr;
+	uint64_t capacity __maybe_unused = P2ALIGN((BTREE_LEAF_SIZE -
+	    sizeof (zfs_btree_hdr_t)) / size, 2);
+	uint64_t count = leaf->btl_hdr.bth_count - idx;
+	ASSERT3U(leaf->btl_hdr.bth_count, <, capacity);
+
+	if (zfs_btree_verify_intensity >= 5) {
+		zfs_btree_verify_poison_at(tree, &leaf->btl_hdr,
+		    leaf->btl_hdr.bth_count);
+	}
+
+	bt_shift_leaf_right(tree, leaf, idx, count);
+	bmov(value, start, size);
+	hdr->bth_count++;
+}
+
+/* Helper function for inserting a new value into leaf at the given index. */
+static void
+zfs_btree_insert_into_leaf(zfs_btree_t *tree, zfs_btree_leaf_t *leaf,
+    const void *value, uint64_t idx)
+{
+	uint64_t size = tree->bt_elem_size;
+	uint64_t capacity = P2ALIGN((BTREE_LEAF_SIZE -
+	    sizeof (zfs_btree_hdr_t)) / size, 2);
+
+	/*
+	 * If the leaf isn't full, shift the elements after idx and insert
+	 * value.
+	 */
+	if (leaf->btl_hdr.bth_count != capacity) {
+		zfs_btree_insert_leaf_impl(tree, leaf, idx, value);
+		return;
+	}
+
+	/*
+	 * Otherwise, we split the leaf node into two nodes. If we're not bulk
+	 * inserting, each is of size (capacity / 2).  If we are bulk
+	 * inserting, we move a quarter of the elements to the new node so
+	 * inserts into the old node don't cause immediate splitting but the
+	 * tree stays relatively dense. Since the average state after a long
+	 * time is a 3/4 full node, shortcutting directly to that state
+	 * improves efficiency.  At the end of the bulk insertion process
+	 * we'll need to go through and fix up any nodes (the last leaf and
+	 * its ancestors, potentially) that are below the minimum.
+	 *
+	 * In either case, we're left with one extra element. The leftover
+	 * element will become the new dividing element between the two nodes.
+	 */
+	uint64_t move_count = MAX(capacity / (tree->bt_bulk == NULL ? 2 : 4) -
+	    1, 2);
+	uint64_t keep_count = capacity - move_count - 1;
+	ASSERT3U(capacity - move_count, >=, 2);
+	tree->bt_num_nodes++;
+	zfs_btree_leaf_t *new_leaf = kmem_cache_alloc(zfs_btree_leaf_cache,
+	    KM_SLEEP);
+	zfs_btree_hdr_t *new_hdr = &new_leaf->btl_hdr;
+	new_hdr->bth_parent = leaf->btl_hdr.bth_parent;
+	new_hdr->bth_core = B_FALSE;
+	new_hdr->bth_count = move_count;
+	zfs_btree_poison_node(tree, new_hdr);
+
+	leaf->btl_hdr.bth_count = keep_count;
+
+	if (tree->bt_bulk != NULL && leaf == tree->bt_bulk)
+		tree->bt_bulk = new_leaf;
+
+	/* Copy the back part to the new leaf. */
+	bt_transfer_leaf(tree, leaf, keep_count + 1, move_count, new_leaf,
+	    0);
+
+	/* We store the new separator in a buffer we control for simplicity. */
+	uint8_t *buf = kmem_alloc(size, KM_SLEEP);
+	bmov(leaf->btl_elems + (keep_count * size), buf, size);
+	zfs_btree_poison_node(tree, &leaf->btl_hdr);
+
+	if (idx < keep_count) {
+		/* Insert into the existing leaf. */
+		zfs_btree_insert_leaf_impl(tree, leaf, idx, value);
+	} else if (idx > keep_count) {
+		/* Insert into the new leaf. */
+		zfs_btree_insert_leaf_impl(tree, new_leaf, idx - keep_count -
+		    1, value);
+	} else {
+		/*
+		 * Shift the elements in the new leaf to make room for the
+		 * separator, and use the new value as the new separator.
+		 */
+		bt_shift_leaf_right(tree, new_leaf, 0, move_count);
+		bmov(buf, new_leaf->btl_elems, size);
+		bmov(value, buf, size);
+		new_hdr->bth_count++;
+	}
+
+	/*
+	 * Now that the node is split, we need to insert the new node into its
+	 * parent. This may cause further splitting, bur only of core nodes.
+	 */
+	zfs_btree_insert_into_parent(tree, &leaf->btl_hdr, &new_leaf->btl_hdr,
+	    buf);
+	kmem_free(buf, size);
+}
+
+static uint64_t
+zfs_btree_find_parent_idx(zfs_btree_t *tree, zfs_btree_hdr_t *hdr)
+{
+	void *buf;
+	if (hdr->bth_core) {
+		buf = ((zfs_btree_core_t *)hdr)->btc_elems;
+	} else {
+		buf = ((zfs_btree_leaf_t *)hdr)->btl_elems;
+	}
+	zfs_btree_index_t idx;
+	zfs_btree_core_t *parent = hdr->bth_parent;
+	VERIFY3P(zfs_btree_find_in_buf(tree, parent->btc_elems,
+	    parent->btc_hdr.bth_count, buf, &idx), ==, NULL);
+	ASSERT(idx.bti_before);
+	ASSERT3U(idx.bti_offset, <=, parent->btc_hdr.bth_count);
+	ASSERT3P(parent->btc_children[idx.bti_offset], ==, hdr);
+	return (idx.bti_offset);
+}
+
+/*
+ * Take the b-tree out of bulk insert mode. During bulk-insert mode, some
+ * nodes may violate the invariant that non-root nodes must be at least half
+ * full. All nodes violating this invariant should be the last node in their
+ * particular level. To correct the invariant, we take values from their left
+ * neighbor until they are half full. They must have a left neighbor at their
+ * level because the last node at a level is not the first node unless it's
+ * the root.
+ */
+static void
+zfs_btree_bulk_finish(zfs_btree_t *tree)
+{
+	ASSERT3P(tree->bt_bulk, !=, NULL);
+	ASSERT3P(tree->bt_root, !=, NULL);
+	zfs_btree_leaf_t *leaf = tree->bt_bulk;
+	zfs_btree_hdr_t *hdr = &leaf->btl_hdr;
+	zfs_btree_core_t *parent = hdr->bth_parent;
+	uint64_t size = tree->bt_elem_size;
+	uint64_t capacity = P2ALIGN((BTREE_LEAF_SIZE -
+	    sizeof (zfs_btree_hdr_t)) / size, 2);
+
+	/*
+	 * The invariant doesn't apply to the root node, if that's the only
+	 * node in the tree we're done.
+	 */
+	if (parent == NULL) {
+		tree->bt_bulk = NULL;
+		return;
+	}
+
+	/* First, take elements to rebalance the leaf node. */
+	if (hdr->bth_count < capacity / 2) {
+		/*
+		 * First, find the left neighbor. The simplest way to do this
+		 * is to call zfs_btree_prev twice; the first time finds some
+		 * ancestor of this node, and the second time finds the left
+		 * neighbor. The ancestor found is the lowest common ancestor
+		 * of leaf and the neighbor.
+		 */
+		zfs_btree_index_t idx = {
+			.bti_node = hdr,
+			.bti_offset = 0
+		};
+		VERIFY3P(zfs_btree_prev(tree, &idx, &idx), !=, NULL);
+		ASSERT(idx.bti_node->bth_core);
+		zfs_btree_core_t *common = (zfs_btree_core_t *)idx.bti_node;
+		uint64_t common_idx = idx.bti_offset;
+
+		VERIFY3P(zfs_btree_prev(tree, &idx, &idx), !=, NULL);
+		ASSERT(!idx.bti_node->bth_core);
+		zfs_btree_leaf_t *l_neighbor = (zfs_btree_leaf_t *)idx.bti_node;
+		zfs_btree_hdr_t *l_hdr = idx.bti_node;
+		uint64_t move_count = (capacity / 2) - hdr->bth_count;
+		ASSERT3U(l_neighbor->btl_hdr.bth_count - move_count, >=,
+		    capacity / 2);
+
+		if (zfs_btree_verify_intensity >= 5) {
+			for (int i = 0; i < move_count; i++) {
+				zfs_btree_verify_poison_at(tree, hdr,
+				    leaf->btl_hdr.bth_count + i);
+			}
+		}
+
+		/* First, shift elements in leaf back. */
+		bt_shift_leaf(tree, leaf, 0, hdr->bth_count, move_count,
+		    BSD_RIGHT);
+
+		/* Next, move the separator from the common ancestor to leaf. */
+		uint8_t *separator = common->btc_elems + (common_idx * size);
+		uint8_t *out = leaf->btl_elems + ((move_count - 1) * size);
+		bmov(separator, out, size);
+		move_count--;
+
+		/*
+		 * Now we move elements from the tail of the left neighbor to
+		 * fill the remaining spots in leaf.
+		 */
+		bt_transfer_leaf(tree, l_neighbor, l_hdr->bth_count -
+		    move_count, move_count, leaf, 0);
+
+		/*
+		 * Finally, move the new last element in the left neighbor to
+		 * the separator.
+		 */
+		bmov(l_neighbor->btl_elems + (l_hdr->bth_count -
+		    move_count - 1) * size, separator, size);
+
+		/* Adjust the node's counts, and we're done. */
+		l_hdr->bth_count -= move_count + 1;
+		hdr->bth_count += move_count + 1;
+
+		ASSERT3U(l_hdr->bth_count, >=, capacity / 2);
+		ASSERT3U(hdr->bth_count, >=, capacity / 2);
+		zfs_btree_poison_node(tree, l_hdr);
+	}
+
+	/*
+	 * Now we have to rebalance any ancestors of leaf that may also
+	 * violate the invariant.
+	 */
+	capacity = BTREE_CORE_ELEMS;
+	while (parent->btc_hdr.bth_parent != NULL) {
+		zfs_btree_core_t *cur = parent;
+		zfs_btree_hdr_t *hdr = &cur->btc_hdr;
+		parent = hdr->bth_parent;
+		/*
+		 * If the invariant isn't violated, move on to the next
+		 * ancestor.
+		 */
+		if (hdr->bth_count >= capacity / 2)
+			continue;
+
+		/*
+		 * Because the smallest number of nodes we can move when
+		 * splitting is 2, we never need to worry about not having a
+		 * left sibling (a sibling is a neighbor with the same parent).
+		 */
+		uint64_t parent_idx = zfs_btree_find_parent_idx(tree, hdr);
+		ASSERT3U(parent_idx, >, 0);
+		zfs_btree_core_t *l_neighbor =
+		    (zfs_btree_core_t *)parent->btc_children[parent_idx - 1];
+		uint64_t move_count = (capacity / 2) - hdr->bth_count;
+		ASSERT3U(l_neighbor->btc_hdr.bth_count - move_count, >=,
+		    capacity / 2);
+
+		if (zfs_btree_verify_intensity >= 5) {
+			for (int i = 0; i < move_count; i++) {
+				zfs_btree_verify_poison_at(tree, hdr,
+				    hdr->bth_count + i);
+			}
+		}
+		/* First, shift things in the right node back. */
+		bt_shift_core(tree, cur, 0, hdr->bth_count, move_count,
+		    BSS_TRAPEZOID, BSD_RIGHT);
+
+		/* Next, move the separator to the right node. */
+		uint8_t *separator = parent->btc_elems + ((parent_idx - 1) *
+		    size);
+		uint8_t *e_out = cur->btc_elems + ((move_count - 1) * size);
+		bmov(separator, e_out, size);
+
+		/*
+		 * Now, move elements and children from the left node to the
+		 * right.  We move one more child than elements.
+		 */
+		move_count--;
+		uint64_t move_idx = l_neighbor->btc_hdr.bth_count - move_count;
+		bt_transfer_core(tree, l_neighbor, move_idx, move_count, cur, 0,
+		    BSS_TRAPEZOID);
+
+		/*
+		 * Finally, move the last element in the left node to the
+		 * separator's position.
+		 */
+		move_idx--;
+		bmov(l_neighbor->btc_elems + move_idx * size, separator, size);
+
+		l_neighbor->btc_hdr.bth_count -= move_count + 1;
+		hdr->bth_count += move_count + 1;
+
+		ASSERT3U(l_neighbor->btc_hdr.bth_count, >=, capacity / 2);
+		ASSERT3U(hdr->bth_count, >=, capacity / 2);
+
+		zfs_btree_poison_node(tree, &l_neighbor->btc_hdr);
+
+		for (int i = 0; i <= hdr->bth_count; i++)
+			cur->btc_children[i]->bth_parent = cur;
+	}
+
+	tree->bt_bulk = NULL;
+}
+
+/*
+ * Insert value into tree at the location specified by where.
+ */
+void
+zfs_btree_add_idx(zfs_btree_t *tree, const void *value,
+    const zfs_btree_index_t *where)
+{
+	zfs_btree_index_t idx = {0};
+
+	/* If we're not inserting in the last leaf, end bulk insert mode. */
+	if (tree->bt_bulk != NULL) {
+		if (where->bti_node != &tree->bt_bulk->btl_hdr) {
+			zfs_btree_bulk_finish(tree);
+			VERIFY3P(zfs_btree_find(tree, value, &idx), ==, NULL);
+			where = &idx;
+		}
+	}
+
+	tree->bt_num_elems++;
+	/*
+	 * If this is the first element in the tree, create a leaf root node
+	 * and add the value to it.
+	 */
+	if (where->bti_node == NULL) {
+		ASSERT3U(tree->bt_num_elems, ==, 1);
+		ASSERT3S(tree->bt_height, ==, -1);
+		ASSERT3P(tree->bt_root, ==, NULL);
+		ASSERT0(where->bti_offset);
+
+		tree->bt_num_nodes++;
+		zfs_btree_leaf_t *leaf = kmem_cache_alloc(zfs_btree_leaf_cache,
+		    KM_SLEEP);
+		tree->bt_root = &leaf->btl_hdr;
+		tree->bt_height++;
+
+		zfs_btree_hdr_t *hdr = &leaf->btl_hdr;
+		hdr->bth_parent = NULL;
+		hdr->bth_core = B_FALSE;
+		hdr->bth_count = 0;
+		zfs_btree_poison_node(tree, hdr);
+
+		zfs_btree_insert_into_leaf(tree, leaf, value, 0);
+		tree->bt_bulk = leaf;
+	} else if (!where->bti_node->bth_core) {
+		/*
+		 * If we're inserting into a leaf, go directly to the helper
+		 * function.
+		 */
+		zfs_btree_insert_into_leaf(tree,
+		    (zfs_btree_leaf_t *)where->bti_node, value,
+		    where->bti_offset);
+	} else {
+		/*
+		 * If we're inserting into a core node, we can't just shift
+		 * the existing element in that slot in the same node without
+		 * breaking our ordering invariants. Instead we place the new
+		 * value in the node at that spot and then insert the old
+		 * separator into the first slot in the subtree to the right.
+		 */
+		ASSERT(where->bti_node->bth_core);
+		zfs_btree_core_t *node = (zfs_btree_core_t *)where->bti_node;
+
+		/*
+		 * We can ignore bti_before, because either way the value
+		 * should end up in bti_offset.
+		 */
+		uint64_t off = where->bti_offset;
+		zfs_btree_hdr_t *subtree = node->btc_children[off + 1];
+		size_t size = tree->bt_elem_size;
+		uint8_t *buf = kmem_alloc(size, KM_SLEEP);
+		bmov(node->btc_elems + off * size, buf, size);
+		bmov(value, node->btc_elems + off * size, size);
+
+		/*
+		 * Find the first slot in the subtree to the right, insert
+		 * there.
+		 */
+		zfs_btree_index_t new_idx;
+		VERIFY3P(zfs_btree_first_helper(subtree, &new_idx), !=, NULL);
+		ASSERT0(new_idx.bti_offset);
+		ASSERT(!new_idx.bti_node->bth_core);
+		zfs_btree_insert_into_leaf(tree,
+		    (zfs_btree_leaf_t *)new_idx.bti_node, buf, 0);
+		kmem_free(buf, size);
+	}
+	zfs_btree_verify(tree);
+}
+
+/*
+ * Return the first element in the tree, and put its location in where if
+ * non-null.
+ */
+void *
+zfs_btree_first(zfs_btree_t *tree, zfs_btree_index_t *where)
+{
+	if (tree->bt_height == -1) {
+		ASSERT0(tree->bt_num_elems);
+		return (NULL);
+	}
+	return (zfs_btree_first_helper(tree->bt_root, where));
+}
+
+/*
+ * Find the last element in the subtree rooted at hdr, return its value and
+ * put its location in where if non-null.
+ */
+static void *
+zfs_btree_last_helper(zfs_btree_t *btree, zfs_btree_hdr_t *hdr,
+    zfs_btree_index_t *where)
+{
+	zfs_btree_hdr_t *node;
+
+	for (node = hdr; node->bth_core; node =
+	    ((zfs_btree_core_t *)node)->btc_children[node->bth_count])
+		;
+
+	zfs_btree_leaf_t *leaf = (zfs_btree_leaf_t *)node;
+	if (where != NULL) {
+		where->bti_node = node;
+		where->bti_offset = node->bth_count - 1;
+		where->bti_before = B_FALSE;
+	}
+	return (leaf->btl_elems + (node->bth_count - 1) * btree->bt_elem_size);
+}
+
+/*
+ * Return the last element in the tree, and put its location in where if
+ * non-null.
+ */
+void *
+zfs_btree_last(zfs_btree_t *tree, zfs_btree_index_t *where)
+{
+	if (tree->bt_height == -1) {
+		ASSERT0(tree->bt_num_elems);
+		return (NULL);
+	}
+	return (zfs_btree_last_helper(tree, tree->bt_root, where));
+}
+
+/*
+ * This function contains the logic to find the next node in the tree. A
+ * helper function is used because there are multiple internal consumemrs of
+ * this logic. The done_func is used by zfs_btree_destroy_nodes to clean up each
+ * node after we've finished with it.
+ */
+static void *
+zfs_btree_next_helper(zfs_btree_t *tree, const zfs_btree_index_t *idx,
+    zfs_btree_index_t *out_idx,
+    void (*done_func)(zfs_btree_t *, zfs_btree_hdr_t *))
+{
+	if (idx->bti_node == NULL) {
+		ASSERT3S(tree->bt_height, ==, -1);
+		return (NULL);
+	}
+
+	uint64_t offset = idx->bti_offset;
+	if (!idx->bti_node->bth_core) {
+		/*
+		 * When finding the next element of an element in a leaf,
+		 * there are two cases. If the element isn't the last one in
+		 * the leaf, in which case we just return the next element in
+		 * the leaf. Otherwise, we need to traverse up our parents
+		 * until we find one where our ancestor isn't the last child
+		 * of its parent. Once we do, the next element is the
+		 * separator after our ancestor in its parent.
+		 */
+		zfs_btree_leaf_t *leaf = (zfs_btree_leaf_t *)idx->bti_node;
+		uint64_t new_off = offset + (idx->bti_before ? 0 : 1);
+		if (leaf->btl_hdr.bth_count > new_off) {
+			out_idx->bti_node = &leaf->btl_hdr;
+			out_idx->bti_offset = new_off;
+			out_idx->bti_before = B_FALSE;
+			return (leaf->btl_elems + new_off * tree->bt_elem_size);
+		}
+
+		zfs_btree_hdr_t *prev = &leaf->btl_hdr;
+		for (zfs_btree_core_t *node = leaf->btl_hdr.bth_parent;
+		    node != NULL; node = node->btc_hdr.bth_parent) {
+			zfs_btree_hdr_t *hdr = &node->btc_hdr;
+			ASSERT(hdr->bth_core);
+			uint64_t i = zfs_btree_find_parent_idx(tree, prev);
+			if (done_func != NULL)
+				done_func(tree, prev);
+			if (i == hdr->bth_count) {
+				prev = hdr;
+				continue;
+			}
+			out_idx->bti_node = hdr;
+			out_idx->bti_offset = i;
+			out_idx->bti_before = B_FALSE;
+			return (node->btc_elems + i * tree->bt_elem_size);
+		}
+		if (done_func != NULL)
+			done_func(tree, prev);
+		/*
+		 * We've traversed all the way up and been at the end of the
+		 * node every time, so this was the last element in the tree.
+		 */
+		return (NULL);
+	}
+
+	/* If we were before an element in a core node, return that element. */
+	ASSERT(idx->bti_node->bth_core);
+	zfs_btree_core_t *node = (zfs_btree_core_t *)idx->bti_node;
+	if (idx->bti_before) {
+		out_idx->bti_before = B_FALSE;
+		return (node->btc_elems + offset * tree->bt_elem_size);
+	}
+
+	/*
+	 * The next element from one in a core node is the first element in
+	 * the subtree just to the right of the separator.
+	 */
+	zfs_btree_hdr_t *child = node->btc_children[offset + 1];
+	return (zfs_btree_first_helper(child, out_idx));
+}
+
+/*
+ * Return the next valued node in the tree.  The same address can be safely
+ * passed for idx and out_idx.
+ */
+void *
+zfs_btree_next(zfs_btree_t *tree, const zfs_btree_index_t *idx,
+    zfs_btree_index_t *out_idx)
+{
+	return (zfs_btree_next_helper(tree, idx, out_idx, NULL));
+}
+
+/*
+ * Return the previous valued node in the tree.  The same value can be safely
+ * passed for idx and out_idx.
+ */
+void *
+zfs_btree_prev(zfs_btree_t *tree, const zfs_btree_index_t *idx,
+    zfs_btree_index_t *out_idx)
+{
+	if (idx->bti_node == NULL) {
+		ASSERT3S(tree->bt_height, ==, -1);
+		return (NULL);
+	}
+
+	uint64_t offset = idx->bti_offset;
+	if (!idx->bti_node->bth_core) {
+		/*
+		 * When finding the previous element of an element in a leaf,
+		 * there are two cases. If the element isn't the first one in
+		 * the leaf, in which case we just return the previous element
+		 * in the leaf. Otherwise, we need to traverse up our parents
+		 * until we find one where our previous ancestor isn't the
+		 * first child. Once we do, the previous element is the
+		 * separator after our previous ancestor.
+		 */
+		zfs_btree_leaf_t *leaf = (zfs_btree_leaf_t *)idx->bti_node;
+		if (offset != 0) {
+			out_idx->bti_node = &leaf->btl_hdr;
+			out_idx->bti_offset = offset - 1;
+			out_idx->bti_before = B_FALSE;
+			return (leaf->btl_elems + (offset - 1) *
+			    tree->bt_elem_size);
+		}
+		zfs_btree_hdr_t *prev = &leaf->btl_hdr;
+		for (zfs_btree_core_t *node = leaf->btl_hdr.bth_parent;
+		    node != NULL; node = node->btc_hdr.bth_parent) {
+			zfs_btree_hdr_t *hdr = &node->btc_hdr;
+			ASSERT(hdr->bth_core);
+			uint64_t i = zfs_btree_find_parent_idx(tree, prev);
+			if (i == 0) {
+				prev = hdr;
+				continue;
+			}
+			out_idx->bti_node = hdr;
+			out_idx->bti_offset = i - 1;
+			out_idx->bti_before = B_FALSE;
+			return (node->btc_elems + (i - 1) * tree->bt_elem_size);
+		}
+		/*
+		 * We've traversed all the way up and been at the start of the
+		 * node every time, so this was the first node in the tree.
+		 */
+		return (NULL);
+	}
+
+	/*
+	 * The previous element from one in a core node is the last element in
+	 * the subtree just to the left of the separator.
+	 */
+	ASSERT(idx->bti_node->bth_core);
+	zfs_btree_core_t *node = (zfs_btree_core_t *)idx->bti_node;
+	zfs_btree_hdr_t *child = node->btc_children[offset];
+	return (zfs_btree_last_helper(tree, child, out_idx));
+}
+
+/*
+ * Get the value at the provided index in the tree.
+ *
+ * Note that the value returned from this function can be mutated, but only
+ * if it will not change the ordering of the element with respect to any other
+ * elements that could be in the tree.
+ */
+void *
+zfs_btree_get(zfs_btree_t *tree, zfs_btree_index_t *idx)
+{
+	ASSERT(!idx->bti_before);
+	if (!idx->bti_node->bth_core) {
+		zfs_btree_leaf_t *leaf = (zfs_btree_leaf_t *)idx->bti_node;
+		return (leaf->btl_elems + idx->bti_offset * tree->bt_elem_size);
+	}
+	ASSERT(idx->bti_node->bth_core);
+	zfs_btree_core_t *node = (zfs_btree_core_t *)idx->bti_node;
+	return (node->btc_elems + idx->bti_offset * tree->bt_elem_size);
+}
+
+/* Add the given value to the tree. Must not already be in the tree. */
+void
+zfs_btree_add(zfs_btree_t *tree, const void *node)
+{
+	zfs_btree_index_t where = {0};
+	VERIFY3P(zfs_btree_find(tree, node, &where), ==, NULL);
+	zfs_btree_add_idx(tree, node, &where);
+}
+
+/* Helper function to free a tree node. */
+static void
+zfs_btree_node_destroy(zfs_btree_t *tree, zfs_btree_hdr_t *node)
+{
+	tree->bt_num_nodes--;
+	if (!node->bth_core) {
+		kmem_cache_free(zfs_btree_leaf_cache, node);
+	} else {
+		kmem_free(node, sizeof (zfs_btree_core_t) +
+		    BTREE_CORE_ELEMS * tree->bt_elem_size);
+	}
+}
+
+/*
+ * Remove the rm_hdr and the separator to its left from the parent node. The
+ * buffer that rm_hdr was stored in may already be freed, so its contents
+ * cannot be accessed.
+ */
+static void
+zfs_btree_remove_from_node(zfs_btree_t *tree, zfs_btree_core_t *node,
+    zfs_btree_hdr_t *rm_hdr)
+{
+	size_t size = tree->bt_elem_size;
+	uint64_t min_count = (BTREE_CORE_ELEMS / 2) - 1;
+	zfs_btree_hdr_t *hdr = &node->btc_hdr;
+	/*
+	 * If the node is the root node and rm_hdr is one of two children,
+	 * promote the other child to the root.
+	 */
+	if (hdr->bth_parent == NULL && hdr->bth_count <= 1) {
+		ASSERT3U(hdr->bth_count, ==, 1);
+		ASSERT3P(tree->bt_root, ==, node);
+		ASSERT3P(node->btc_children[1], ==, rm_hdr);
+		tree->bt_root = node->btc_children[0];
+		node->btc_children[0]->bth_parent = NULL;
+		zfs_btree_node_destroy(tree, hdr);
+		tree->bt_height--;
+		return;
+	}
+
+	uint64_t idx;
+	for (idx = 0; idx <= hdr->bth_count; idx++) {
+		if (node->btc_children[idx] == rm_hdr)
+			break;
+	}
+	ASSERT3U(idx, <=, hdr->bth_count);
+
+	/*
+	 * If the node is the root or it has more than the minimum number of
+	 * children, just remove the child and separator, and return.
+	 */
+	if (hdr->bth_parent == NULL ||
+	    hdr->bth_count > min_count) {
+		/*
+		 * Shift the element and children to the right of rm_hdr to
+		 * the left by one spot.
+		 */
+		bt_shift_core_left(tree, node, idx, hdr->bth_count - idx,
+		    BSS_PARALLELOGRAM);
+		hdr->bth_count--;
+		zfs_btree_poison_node_at(tree, hdr, hdr->bth_count);
+		return;
+	}
+
+	ASSERT3U(hdr->bth_count, ==, min_count);
+
+	/*
+	 * Now we try to take a node from a neighbor. We check left, then
+	 * right. If the neighbor exists and has more than the minimum number
+	 * of elements, we move the separator between us and them to our
+	 * node, move their closest element (last for left, first for right)
+	 * to the separator, and move their closest child to our node. Along
+	 * the way we need to collapse the gap made by idx, and (for our right
+	 * neighbor) the gap made by removing their first element and child.
+	 *
+	 * Note: this logic currently doesn't support taking from a neighbor
+	 * that isn't a sibling (i.e. a neighbor with a different
+	 * parent). This isn't critical functionality, but may be worth
+	 * implementing in the future for completeness' sake.
+	 */
+	zfs_btree_core_t *parent = hdr->bth_parent;
+	uint64_t parent_idx = zfs_btree_find_parent_idx(tree, hdr);
+
+	zfs_btree_hdr_t *l_hdr = (parent_idx == 0 ? NULL :
+	    parent->btc_children[parent_idx - 1]);
+	if (l_hdr != NULL && l_hdr->bth_count > min_count) {
+		/* We can take a node from the left neighbor. */
+		ASSERT(l_hdr->bth_core);
+		zfs_btree_core_t *neighbor = (zfs_btree_core_t *)l_hdr;
+
+		/*
+		 * Start by shifting the elements and children in the current
+		 * node to the right by one spot.
+		 */
+		bt_shift_core_right(tree, node, 0, idx - 1, BSS_TRAPEZOID);
+
+		/*
+		 * Move the separator between node and neighbor to the first
+		 * element slot in the current node.
+		 */
+		uint8_t *separator = parent->btc_elems + (parent_idx - 1) *
+		    size;
+		bmov(separator, node->btc_elems, size);
+
+		/* Move the last child of neighbor to our first child slot. */
+		zfs_btree_hdr_t **take_child = neighbor->btc_children +
+		    l_hdr->bth_count;
+		bmov(take_child, node->btc_children, sizeof (*take_child));
+		node->btc_children[0]->bth_parent = node;
+
+		/* Move the last element of neighbor to the separator spot. */
+		uint8_t *take_elem = neighbor->btc_elems +
+		    (l_hdr->bth_count - 1) * size;
+		bmov(take_elem, separator, size);
+		l_hdr->bth_count--;
+		zfs_btree_poison_node_at(tree, l_hdr, l_hdr->bth_count);
+		return;
+	}
+
+	zfs_btree_hdr_t *r_hdr = (parent_idx == parent->btc_hdr.bth_count ?
+	    NULL : parent->btc_children[parent_idx + 1]);
+	if (r_hdr != NULL && r_hdr->bth_count > min_count) {
+		/* We can take a node from the right neighbor. */
+		ASSERT(r_hdr->bth_core);
+		zfs_btree_core_t *neighbor = (zfs_btree_core_t *)r_hdr;
+
+		/*
+		 * Shift elements in node left by one spot to overwrite rm_hdr
+		 * and the separator before it.
+		 */
+		bt_shift_core_left(tree, node, idx, hdr->bth_count - idx,
+		    BSS_PARALLELOGRAM);
+
+		/*
+		 * Move the separator between node and neighbor to the last
+		 * element spot in node.
+		 */
+		uint8_t *separator = parent->btc_elems + parent_idx * size;
+		bmov(separator, node->btc_elems + (hdr->bth_count - 1) * size,
+		    size);
+
+		/*
+		 * Move the first child of neighbor to the last child spot in
+		 * node.
+		 */
+		zfs_btree_hdr_t **take_child = neighbor->btc_children;
+		bmov(take_child, node->btc_children + hdr->bth_count,
+		    sizeof (*take_child));
+		node->btc_children[hdr->bth_count]->bth_parent = node;
+
+		/* Move the first element of neighbor to the separator spot. */
+		uint8_t *take_elem = neighbor->btc_elems;
+		bmov(take_elem, separator, size);
+		r_hdr->bth_count--;
+
+		/*
+		 * Shift the elements and children of neighbor to cover the
+		 * stolen elements.
+		 */
+		bt_shift_core_left(tree, neighbor, 1, r_hdr->bth_count,
+		    BSS_TRAPEZOID);
+		zfs_btree_poison_node_at(tree, r_hdr, r_hdr->bth_count);
+		return;
+	}
+
+	/*
+	 * In this case, neither of our neighbors can spare an element, so we
+	 * need to merge with one of them. We prefer the left one,
+	 * arbitrarily. Move the separator into the leftmost merging node
+	 * (which may be us or the left neighbor), and then move the right
+	 * merging node's elements. Once that's done, we go back and delete
+	 * the element we're removing. Finally, go into the parent and delete
+	 * the right merging node and the separator. This may cause further
+	 * merging.
+	 */
+	zfs_btree_hdr_t *new_rm_hdr, *keep_hdr;
+	uint64_t new_idx = idx;
+	if (l_hdr != NULL) {
+		keep_hdr = l_hdr;
+		new_rm_hdr = hdr;
+		new_idx += keep_hdr->bth_count + 1;
+	} else {
+		ASSERT3P(r_hdr, !=, NULL);
+		keep_hdr = hdr;
+		new_rm_hdr = r_hdr;
+		parent_idx++;
+	}
+
+	ASSERT(keep_hdr->bth_core);
+	ASSERT(new_rm_hdr->bth_core);
+
+	zfs_btree_core_t *keep = (zfs_btree_core_t *)keep_hdr;
+	zfs_btree_core_t *rm = (zfs_btree_core_t *)new_rm_hdr;
+
+	if (zfs_btree_verify_intensity >= 5) {
+		for (int i = 0; i < new_rm_hdr->bth_count + 1; i++) {
+			zfs_btree_verify_poison_at(tree, keep_hdr,
+			    keep_hdr->bth_count + i);
+		}
+	}
+
+	/* Move the separator into the left node. */
+	uint8_t *e_out = keep->btc_elems + keep_hdr->bth_count * size;
+	uint8_t *separator = parent->btc_elems + (parent_idx - 1) *
+	    size;
+	bmov(separator, e_out, size);
+	keep_hdr->bth_count++;
+
+	/* Move all our elements and children into the left node. */
+	bt_transfer_core(tree, rm, 0, new_rm_hdr->bth_count, keep,
+	    keep_hdr->bth_count, BSS_TRAPEZOID);
+
+	uint64_t old_count = keep_hdr->bth_count;
+
+	/* Update bookkeeping */
+	keep_hdr->bth_count += new_rm_hdr->bth_count;
+	ASSERT3U(keep_hdr->bth_count, ==, (min_count * 2) + 1);
+
+	/*
+	 * Shift the element and children to the right of rm_hdr to
+	 * the left by one spot.
+	 */
+	ASSERT3P(keep->btc_children[new_idx], ==, rm_hdr);
+	bt_shift_core_left(tree, keep, new_idx, keep_hdr->bth_count - new_idx,
+	    BSS_PARALLELOGRAM);
+	keep_hdr->bth_count--;
+
+	/* Reparent all our children to point to the left node. */
+	zfs_btree_hdr_t **new_start = keep->btc_children +
+	    old_count - 1;
+	for (int i = 0; i < new_rm_hdr->bth_count + 1; i++)
+		new_start[i]->bth_parent = keep;
+	for (int i = 0; i <= keep_hdr->bth_count; i++) {
+		ASSERT3P(keep->btc_children[i]->bth_parent, ==, keep);
+		ASSERT3P(keep->btc_children[i], !=, rm_hdr);
+	}
+	zfs_btree_poison_node_at(tree, keep_hdr, keep_hdr->bth_count);
+
+	new_rm_hdr->bth_count = 0;
+	zfs_btree_node_destroy(tree, new_rm_hdr);
+	zfs_btree_remove_from_node(tree, parent, new_rm_hdr);
+}
+
+/* Remove the element at the specific location. */
+void
+zfs_btree_remove_idx(zfs_btree_t *tree, zfs_btree_index_t *where)
+{
+	size_t size = tree->bt_elem_size;
+	zfs_btree_hdr_t *hdr = where->bti_node;
+	uint64_t idx = where->bti_offset;
+	uint64_t capacity = P2ALIGN((BTREE_LEAF_SIZE -
+	    sizeof (zfs_btree_hdr_t)) / size, 2);
+
+	ASSERT(!where->bti_before);
+	if (tree->bt_bulk != NULL) {
+		/*
+		 * Leave bulk insert mode. Note that our index would be
+		 * invalid after we correct the tree, so we copy the value
+		 * we're planning to remove and find it again after
+		 * bulk_finish.
+		 */
+		uint8_t *value = zfs_btree_get(tree, where);
+		uint8_t *tmp = kmem_alloc(size, KM_SLEEP);
+		bmov(value, tmp, size);
+		zfs_btree_bulk_finish(tree);
+		VERIFY3P(zfs_btree_find(tree, tmp, where), !=, NULL);
+		kmem_free(tmp, size);
+		hdr = where->bti_node;
+		idx = where->bti_offset;
+	}
+
+	tree->bt_num_elems--;
+	/*
+	 * If the element happens to be in a core node, we move a leaf node's
+	 * element into its place and then remove the leaf node element. This
+	 * makes the rebalance logic not need to be recursive both upwards and
+	 * downwards.
+	 */
+	if (hdr->bth_core) {
+		zfs_btree_core_t *node = (zfs_btree_core_t *)hdr;
+		zfs_btree_hdr_t *left_subtree = node->btc_children[idx];
+		void *new_value = zfs_btree_last_helper(tree, left_subtree,
+		    where);
+		ASSERT3P(new_value, !=, NULL);
+
+		bmov(new_value, node->btc_elems + idx * size, size);
+
+		hdr = where->bti_node;
+		idx = where->bti_offset;
+		ASSERT(!where->bti_before);
+	}
+
+	/*
+	 * First, we'll update the leaf's metadata. Then, we shift any
+	 * elements after the idx to the left. After that, we rebalance if
+	 * needed.
+	 */
+	ASSERT(!hdr->bth_core);
+	zfs_btree_leaf_t *leaf = (zfs_btree_leaf_t *)hdr;
+	ASSERT3U(hdr->bth_count, >, 0);
+
+	uint64_t min_count = (capacity / 2) - 1;
+
+	/*
+	 * If we're over the minimum size or this is the root, just overwrite
+	 * the value and return.
+	 */
+	if (hdr->bth_count > min_count || hdr->bth_parent == NULL) {
+		hdr->bth_count--;
+		bt_shift_leaf_left(tree, leaf, idx + 1, hdr->bth_count - idx);
+		if (hdr->bth_parent == NULL) {
+			ASSERT0(tree->bt_height);
+			if (hdr->bth_count == 0) {
+				tree->bt_root = NULL;
+				tree->bt_height--;
+				zfs_btree_node_destroy(tree, &leaf->btl_hdr);
+			}
+		}
+		if (tree->bt_root != NULL)
+			zfs_btree_poison_node_at(tree, hdr, hdr->bth_count);
+		zfs_btree_verify(tree);
+		return;
+	}
+	ASSERT3U(hdr->bth_count, ==, min_count);
+
+	/*
+	 * Now we try to take a node from a sibling. We check left, then
+	 * right. If they exist and have more than the minimum number of
+	 * elements, we move the separator between us and them to our node
+	 * and move their closest element (last for left, first for right) to
+	 * the separator. Along the way we need to collapse the gap made by
+	 * idx, and (for our right neighbor) the gap made by removing their
+	 * first element.
+	 *
+	 * Note: this logic currently doesn't support taking from a neighbor
+	 * that isn't a sibling. This isn't critical functionality, but may be
+	 * worth implementing in the future for completeness' sake.
+	 */
+	zfs_btree_core_t *parent = hdr->bth_parent;
+	uint64_t parent_idx = zfs_btree_find_parent_idx(tree, hdr);
+
+	zfs_btree_hdr_t *l_hdr = (parent_idx == 0 ? NULL :
+	    parent->btc_children[parent_idx - 1]);
+	if (l_hdr != NULL && l_hdr->bth_count > min_count) {
+		/* We can take a node from the left neighbor. */
+		ASSERT(!l_hdr->bth_core);
+
+		/*
+		 * Move our elements back by one spot to make room for the
+		 * stolen element and overwrite the element being removed.
+		 */
+		bt_shift_leaf_right(tree, leaf, 0, idx);
+		uint8_t *separator = parent->btc_elems + (parent_idx - 1) *
+		    size;
+		uint8_t *take_elem = ((zfs_btree_leaf_t *)l_hdr)->btl_elems +
+		    (l_hdr->bth_count - 1) * size;
+		/* Move the separator to our first spot. */
+		bmov(separator, leaf->btl_elems, size);
+
+		/* Move our neighbor's last element to the separator. */
+		bmov(take_elem, separator, size);
+
+		/* Update the bookkeeping. */
+		l_hdr->bth_count--;
+		zfs_btree_poison_node_at(tree, l_hdr, l_hdr->bth_count);
+
+		zfs_btree_verify(tree);
+		return;
+	}
+
+	zfs_btree_hdr_t *r_hdr = (parent_idx == parent->btc_hdr.bth_count ?
+	    NULL : parent->btc_children[parent_idx + 1]);
+	if (r_hdr != NULL && r_hdr->bth_count > min_count) {
+		/* We can take a node from the right neighbor. */
+		ASSERT(!r_hdr->bth_core);
+		zfs_btree_leaf_t *neighbor = (zfs_btree_leaf_t *)r_hdr;
+
+		/*
+		 * Move our elements after the element being removed forwards
+		 * by one spot to make room for the stolen element and
+		 * overwrite the element being removed.
+		 */
+		bt_shift_leaf_left(tree, leaf, idx + 1, hdr->bth_count - idx -
+		    1);
+
+		uint8_t *separator = parent->btc_elems + parent_idx * size;
+		uint8_t *take_elem = ((zfs_btree_leaf_t *)r_hdr)->btl_elems;
+		/* Move the separator between us to our last spot. */
+		bmov(separator, leaf->btl_elems + (hdr->bth_count - 1) * size,
+		    size);
+
+		/* Move our neighbor's first element to the separator. */
+		bmov(take_elem, separator, size);
+
+		/* Update the bookkeeping. */
+		r_hdr->bth_count--;
+
+		/*
+		 * Move our neighbors elements forwards to overwrite the
+		 * stolen element.
+		 */
+		bt_shift_leaf_left(tree, neighbor, 1, r_hdr->bth_count);
+		zfs_btree_poison_node_at(tree, r_hdr, r_hdr->bth_count);
+		zfs_btree_verify(tree);
+		return;
+	}
+
+	/*
+	 * In this case, neither of our neighbors can spare an element, so we
+	 * need to merge with one of them. We prefer the left one,
+	 * arbitrarily. Move the separator into the leftmost merging node
+	 * (which may be us or the left neighbor), and then move the right
+	 * merging node's elements. Once that's done, we go back and delete
+	 * the element we're removing. Finally, go into the parent and delete
+	 * the right merging node and the separator. This may cause further
+	 * merging.
+	 */
+	zfs_btree_hdr_t *rm_hdr, *keep_hdr;
+	uint64_t new_idx = idx;
+	if (l_hdr != NULL) {
+		keep_hdr = l_hdr;
+		rm_hdr = hdr;
+		new_idx += keep_hdr->bth_count + 1; // 449
+	} else {
+		ASSERT3P(r_hdr, !=, NULL);
+		keep_hdr = hdr;
+		rm_hdr = r_hdr;
+		parent_idx++;
+	}
+
+	ASSERT(!keep_hdr->bth_core);
+	ASSERT(!rm_hdr->bth_core);
+	ASSERT3U(keep_hdr->bth_count, ==, min_count);
+	ASSERT3U(rm_hdr->bth_count, ==, min_count);
+
+	zfs_btree_leaf_t *keep = (zfs_btree_leaf_t *)keep_hdr;
+	zfs_btree_leaf_t *rm = (zfs_btree_leaf_t *)rm_hdr;
+
+	if (zfs_btree_verify_intensity >= 5) {
+		for (int i = 0; i < rm_hdr->bth_count + 1; i++) {
+			zfs_btree_verify_poison_at(tree, keep_hdr,
+			    keep_hdr->bth_count + i);
+		}
+	}
+	/*
+	 * Move the separator into the first open spot in the left
+	 * neighbor.
+	 */
+	uint8_t *out = keep->btl_elems + keep_hdr->bth_count * size;
+	uint8_t *separator = parent->btc_elems + (parent_idx - 1) *
+	    size;
+	bmov(separator, out, size);
+	keep_hdr->bth_count++;
+
+	/* Move our elements to the left neighbor. */
+	bt_transfer_leaf(tree, rm, 0, rm_hdr->bth_count, keep,
+	    keep_hdr->bth_count);
+
+	/* Update the bookkeeping. */
+	keep_hdr->bth_count += rm_hdr->bth_count;
+	ASSERT3U(keep_hdr->bth_count, ==, min_count * 2 + 1);
+
+	/* Remove the value from the node */
+	keep_hdr->bth_count--;
+	bt_shift_leaf_left(tree, keep, new_idx + 1, keep_hdr->bth_count -
+	    new_idx);
+	zfs_btree_poison_node_at(tree, keep_hdr, keep_hdr->bth_count);
+
+	rm_hdr->bth_count = 0;
+	zfs_btree_node_destroy(tree, rm_hdr);
+	/* Remove the emptied node from the parent. */
+	zfs_btree_remove_from_node(tree, parent, rm_hdr);
+	zfs_btree_verify(tree);
+}
+
+/* Remove the given value from the tree. */
+void
+zfs_btree_remove(zfs_btree_t *tree, const void *value)
+{
+	zfs_btree_index_t where = {0};
+	VERIFY3P(zfs_btree_find(tree, value, &where), !=, NULL);
+	zfs_btree_remove_idx(tree, &where);
+}
+
+/* Return the number of elements in the tree. */
+ulong_t
+zfs_btree_numnodes(zfs_btree_t *tree)
+{
+	return (tree->bt_num_elems);
+}
+
+/*
+ * This function is used to visit all the elements in the tree before
+ * destroying the tree. This allows the calling code to perform any cleanup it
+ * needs to do. This is more efficient than just removing the first element
+ * over and over, because it removes all rebalancing. Once the destroy_nodes()
+ * function has been called, no other btree operations are valid until it
+ * returns NULL, which point the only valid operation is zfs_btree_destroy().
+ *
+ * example:
+ *
+ *      zfs_btree_index_t *cookie = NULL;
+ *      my_data_t *node;
+ *
+ *      while ((node = zfs_btree_destroy_nodes(tree, &cookie)) != NULL)
+ *              free(node->ptr);
+ *      zfs_btree_destroy(tree);
+ *
+ */
+void *
+zfs_btree_destroy_nodes(zfs_btree_t *tree, zfs_btree_index_t **cookie)
+{
+	if (*cookie == NULL) {
+		if (tree->bt_height == -1)
+			return (NULL);
+		*cookie = kmem_alloc(sizeof (**cookie), KM_SLEEP);
+		return (zfs_btree_first(tree, *cookie));
+	}
+
+	void *rval = zfs_btree_next_helper(tree, *cookie, *cookie,
+	    zfs_btree_node_destroy);
+	if (rval == NULL)   {
+		tree->bt_root = NULL;
+		tree->bt_height = -1;
+		tree->bt_num_elems = 0;
+		kmem_free(*cookie, sizeof (**cookie));
+		tree->bt_bulk = NULL;
+	}
+	return (rval);
+}
+
+static void
+zfs_btree_clear_helper(zfs_btree_t *tree, zfs_btree_hdr_t *hdr)
+{
+	if (hdr->bth_core) {
+		zfs_btree_core_t *btc = (zfs_btree_core_t *)hdr;
+		for (int i = 0; i <= hdr->bth_count; i++) {
+			zfs_btree_clear_helper(tree, btc->btc_children[i]);
+		}
+	}
+
+	zfs_btree_node_destroy(tree, hdr);
+}
+
+void
+zfs_btree_clear(zfs_btree_t *tree)
+{
+	if (tree->bt_root == NULL) {
+		ASSERT0(tree->bt_num_elems);
+		return;
+	}
+
+	zfs_btree_clear_helper(tree, tree->bt_root);
+	tree->bt_num_elems = 0;
+	tree->bt_root = NULL;
+	tree->bt_num_nodes = 0;
+	tree->bt_height = -1;
+	tree->bt_bulk = NULL;
+}
+
+void
+zfs_btree_destroy(zfs_btree_t *tree)
+{
+	ASSERT0(tree->bt_num_elems);
+	ASSERT3P(tree->bt_root, ==, NULL);
+}
+
+/* Verify that every child of this node has the correct parent pointer. */
+static void
+zfs_btree_verify_pointers_helper(zfs_btree_t *tree, zfs_btree_hdr_t *hdr)
+{
+	if (!hdr->bth_core)
+		return;
+
+	zfs_btree_core_t *node = (zfs_btree_core_t *)hdr;
+	for (int i = 0; i <= hdr->bth_count; i++) {
+		VERIFY3P(node->btc_children[i]->bth_parent, ==, hdr);
+		zfs_btree_verify_pointers_helper(tree, node->btc_children[i]);
+	}
+}
+
+/* Verify that every node has the correct parent pointer. */
+static void
+zfs_btree_verify_pointers(zfs_btree_t *tree)
+{
+	if (tree->bt_height == -1) {
+		VERIFY3P(tree->bt_root, ==, NULL);
+		return;
+	}
+	VERIFY3P(tree->bt_root->bth_parent, ==, NULL);
+	zfs_btree_verify_pointers_helper(tree, tree->bt_root);
+}
+
+/*
+ * Verify that all the current node and its children satisfy the count
+ * invariants, and return the total count in the subtree rooted in this node.
+ */
+static uint64_t
+zfs_btree_verify_counts_helper(zfs_btree_t *tree, zfs_btree_hdr_t *hdr)
+{
+	if (!hdr->bth_core) {
+		if (tree->bt_root != hdr && hdr != &tree->bt_bulk->btl_hdr) {
+			uint64_t capacity = P2ALIGN((BTREE_LEAF_SIZE -
+			    sizeof (zfs_btree_hdr_t)) / tree->bt_elem_size, 2);
+			VERIFY3U(hdr->bth_count, >=, (capacity / 2) - 1);
+		}
+
+		return (hdr->bth_count);
+	} else {
+
+		zfs_btree_core_t *node = (zfs_btree_core_t *)hdr;
+		uint64_t ret = hdr->bth_count;
+		if (tree->bt_root != hdr && tree->bt_bulk == NULL)
+			VERIFY3P(hdr->bth_count, >=, BTREE_CORE_ELEMS / 2 - 1);
+		for (int i = 0; i <= hdr->bth_count; i++) {
+			ret += zfs_btree_verify_counts_helper(tree,
+			    node->btc_children[i]);
+		}
+
+		return (ret);
+	}
+}
+
+/*
+ * Verify that all nodes satisfy the invariants and that the total number of
+ * elements is correct.
+ */
+static void
+zfs_btree_verify_counts(zfs_btree_t *tree)
+{
+	EQUIV(tree->bt_num_elems == 0, tree->bt_height == -1);
+	if (tree->bt_height == -1) {
+		return;
+	}
+	VERIFY3P(zfs_btree_verify_counts_helper(tree, tree->bt_root), ==,
+	    tree->bt_num_elems);
+}
+
+/*
+ * Check that the subtree rooted at this node has a uniform height. Returns
+ * the number of nodes under this node, to help verify bt_num_nodes.
+ */
+static uint64_t
+zfs_btree_verify_height_helper(zfs_btree_t *tree, zfs_btree_hdr_t *hdr,
+    int64_t height)
+{
+	if (!hdr->bth_core) {
+		VERIFY0(height);
+		return (1);
+	}
+
+	VERIFY(hdr->bth_core);
+	zfs_btree_core_t *node = (zfs_btree_core_t *)hdr;
+	uint64_t ret = 1;
+	for (int i = 0; i <= hdr->bth_count; i++) {
+		ret += zfs_btree_verify_height_helper(tree,
+		    node->btc_children[i], height - 1);
+	}
+	return (ret);
+}
+
+/*
+ * Check that the tree rooted at this node has a uniform height, and that the
+ * bt_height in the tree is correct.
+ */
+static void
+zfs_btree_verify_height(zfs_btree_t *tree)
+{
+	EQUIV(tree->bt_height == -1, tree->bt_root == NULL);
+	if (tree->bt_height == -1) {
+		return;
+	}
+
+	VERIFY3U(zfs_btree_verify_height_helper(tree, tree->bt_root,
+	    tree->bt_height), ==, tree->bt_num_nodes);
+}
+
+/*
+ * Check that the elements in this node are sorted, and that if this is a core
+ * node, the separators are properly between the subtrees they separaate and
+ * that the children also satisfy this requirement.
+ */
+static void
+zfs_btree_verify_order_helper(zfs_btree_t *tree, zfs_btree_hdr_t *hdr)
+{
+	size_t size = tree->bt_elem_size;
+	if (!hdr->bth_core) {
+		zfs_btree_leaf_t *leaf = (zfs_btree_leaf_t *)hdr;
+		for (int i = 1; i < hdr->bth_count; i++) {
+			VERIFY3S(tree->bt_compar(leaf->btl_elems + (i - 1) *
+			    size, leaf->btl_elems + i * size), ==, -1);
+		}
+		return;
+	}
+
+	zfs_btree_core_t *node = (zfs_btree_core_t *)hdr;
+	for (int i = 1; i < hdr->bth_count; i++) {
+		VERIFY3S(tree->bt_compar(node->btc_elems + (i - 1) * size,
+		    node->btc_elems + i * size), ==, -1);
+	}
+	for (int i = 0; i < hdr->bth_count; i++) {
+		uint8_t *left_child_last = NULL;
+		zfs_btree_hdr_t *left_child_hdr = node->btc_children[i];
+		if (left_child_hdr->bth_core) {
+			zfs_btree_core_t *left_child =
+			    (zfs_btree_core_t *)left_child_hdr;
+			left_child_last = left_child->btc_elems +
+			    (left_child_hdr->bth_count - 1) * size;
+		} else {
+			zfs_btree_leaf_t *left_child =
+			    (zfs_btree_leaf_t *)left_child_hdr;
+			left_child_last = left_child->btl_elems +
+			    (left_child_hdr->bth_count - 1) * size;
+		}
+		if (tree->bt_compar(node->btc_elems + i * size,
+		    left_child_last) != 1) {
+			panic("btree: compar returned %d (expected 1) at "
+			    "%px %d: compar(%px,  %px)", tree->bt_compar(
+			    node->btc_elems + i * size, left_child_last),
+			    (void *)node, i, (void *)(node->btc_elems + i *
+			    size), (void *)left_child_last);
+		}
+
+		uint8_t *right_child_first = NULL;
+		zfs_btree_hdr_t *right_child_hdr = node->btc_children[i + 1];
+		if (right_child_hdr->bth_core) {
+			zfs_btree_core_t *right_child =
+			    (zfs_btree_core_t *)right_child_hdr;
+			right_child_first = right_child->btc_elems;
+		} else {
+			zfs_btree_leaf_t *right_child =
+			    (zfs_btree_leaf_t *)right_child_hdr;
+			right_child_first = right_child->btl_elems;
+		}
+		if (tree->bt_compar(node->btc_elems + i * size,
+		    right_child_first) != -1) {
+			panic("btree: compar returned %d (expected -1) at "
+			    "%px %d: compar(%px,  %px)", tree->bt_compar(
+			    node->btc_elems + i * size, right_child_first),
+			    (void *)node, i, (void *)(node->btc_elems + i *
+			    size), (void *)right_child_first);
+		}
+	}
+	for (int i = 0; i <= hdr->bth_count; i++) {
+		zfs_btree_verify_order_helper(tree, node->btc_children[i]);
+	}
+}
+
+/* Check that all elements in the tree are in sorted order. */
+static void
+zfs_btree_verify_order(zfs_btree_t *tree)
+{
+	EQUIV(tree->bt_height == -1, tree->bt_root == NULL);
+	if (tree->bt_height == -1) {
+		return;
+	}
+
+	zfs_btree_verify_order_helper(tree, tree->bt_root);
+}
+
+#ifdef ZFS_DEBUG
+/* Check that all unused memory is poisoned correctly. */
+static void
+zfs_btree_verify_poison_helper(zfs_btree_t *tree, zfs_btree_hdr_t *hdr)
+{
+	size_t size = tree->bt_elem_size;
+	if (!hdr->bth_core) {
+		zfs_btree_leaf_t *leaf = (zfs_btree_leaf_t *)hdr;
+		uint8_t val = 0x0f;
+		for (int i = hdr->bth_count * size; i < BTREE_LEAF_SIZE -
+		    sizeof (zfs_btree_hdr_t); i++) {
+			VERIFY3U(leaf->btl_elems[i], ==, val);
+		}
+	} else {
+		zfs_btree_core_t *node = (zfs_btree_core_t *)hdr;
+		uint8_t val = 0x0f;
+		for (int i = hdr->bth_count * size; i < BTREE_CORE_ELEMS * size;
+		    i++) {
+			VERIFY3U(node->btc_elems[i], ==, val);
+		}
+
+		for (int i = hdr->bth_count + 1; i <= BTREE_CORE_ELEMS; i++) {
+			VERIFY3P(node->btc_children[i], ==,
+			    (zfs_btree_hdr_t *)BTREE_POISON);
+		}
+
+		for (int i = 0; i <= hdr->bth_count; i++) {
+			zfs_btree_verify_poison_helper(tree,
+			    node->btc_children[i]);
+		}
+	}
+}
+#endif
+
+/* Check that unused memory in the tree is still poisoned. */
+static void
+zfs_btree_verify_poison(zfs_btree_t *tree)
+{
+#ifdef ZFS_DEBUG
+	if (tree->bt_height == -1)
+		return;
+	zfs_btree_verify_poison_helper(tree, tree->bt_root);
+#endif
+}
+
+void
+zfs_btree_verify(zfs_btree_t *tree)
+{
+	if (zfs_btree_verify_intensity == 0)
+		return;
+	zfs_btree_verify_height(tree);
+	if (zfs_btree_verify_intensity == 1)
+		return;
+	zfs_btree_verify_pointers(tree);
+	if (zfs_btree_verify_intensity == 2)
+		return;
+	zfs_btree_verify_counts(tree);
+	if (zfs_btree_verify_intensity == 3)
+		return;
+	zfs_btree_verify_order(tree);
+
+	if (zfs_btree_verify_intensity == 4)
+		return;
+	zfs_btree_verify_poison(tree);
+}
diff --git a/sys/contrib/openzfs/module/zfs/dataset_kstats.c b/sys/contrib/openzfs/module/zfs/dataset_kstats.c
new file mode 100644
index 000000000000..e46a0926d557
--- /dev/null
+++ b/sys/contrib/openzfs/module/zfs/dataset_kstats.c
@@ -0,0 +1,215 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+
+/*
+ * Copyright (c) 2018 by Delphix. All rights reserved.
+ * Copyright (c) 2018 Datto Inc.
+ */
+
+#include <sys/dataset_kstats.h>
+#include <sys/dmu_objset.h>
+#include <sys/dsl_dataset.h>
+#include <sys/spa.h>
+
+static dataset_kstat_values_t empty_dataset_kstats = {
+	{ "dataset_name",	KSTAT_DATA_STRING },
+	{ "writes",	KSTAT_DATA_UINT64 },
+	{ "nwritten",	KSTAT_DATA_UINT64 },
+	{ "reads",	KSTAT_DATA_UINT64 },
+	{ "nread",	KSTAT_DATA_UINT64 },
+	{ "nunlinks",	KSTAT_DATA_UINT64 },
+	{ "nunlinked",	KSTAT_DATA_UINT64 },
+};
+
+static int
+dataset_kstats_update(kstat_t *ksp, int rw)
+{
+	dataset_kstats_t *dk = ksp->ks_private;
+	ASSERT3P(dk->dk_kstats->ks_data, ==, ksp->ks_data);
+
+	if (rw == KSTAT_WRITE)
+		return (EACCES);
+
+	dataset_kstat_values_t *dkv = dk->dk_kstats->ks_data;
+	dkv->dkv_writes.value.ui64 =
+	    aggsum_value(&dk->dk_aggsums.das_writes);
+	dkv->dkv_nwritten.value.ui64 =
+	    aggsum_value(&dk->dk_aggsums.das_nwritten);
+	dkv->dkv_reads.value.ui64 =
+	    aggsum_value(&dk->dk_aggsums.das_reads);
+	dkv->dkv_nread.value.ui64 =
+	    aggsum_value(&dk->dk_aggsums.das_nread);
+	dkv->dkv_nunlinks.value.ui64 =
+	    aggsum_value(&dk->dk_aggsums.das_nunlinks);
+	dkv->dkv_nunlinked.value.ui64 =
+	    aggsum_value(&dk->dk_aggsums.das_nunlinked);
+
+	return (0);
+}
+
+void
+dataset_kstats_create(dataset_kstats_t *dk, objset_t *objset)
+{
+	/*
+	 * There should not be anything wrong with having kstats for
+	 * snapshots. Since we are not sure how useful they would be
+	 * though nor how much their memory overhead would matter in
+	 * a filesystem with many snapshots, we skip them for now.
+	 */
+	if (dmu_objset_is_snapshot(objset))
+		return;
+
+	/*
+	 * At the time of this writing, KSTAT_STRLEN is 255 in Linux,
+	 * and the spa_name can theoretically be up to 256 characters.
+	 * In reality though the spa_name can be 240 characters max
+	 * [see origin directory name check in pool_namecheck()]. Thus,
+	 * the naming scheme for the module name below should not cause
+	 * any truncations. In the event that a truncation does happen
+	 * though, due to some future change, we silently skip creating
+	 * the kstat and log the event.
+	 */
+	char kstat_module_name[KSTAT_STRLEN];
+	int n = snprintf(kstat_module_name, sizeof (kstat_module_name),
+	    "zfs/%s", spa_name(dmu_objset_spa(objset)));
+	if (n < 0) {
+		zfs_dbgmsg("failed to create dataset kstat for objset %lld: "
+		    " snprintf() for kstat module name returned %d",
+		    (unsigned long long)dmu_objset_id(objset), n);
+		return;
+	} else if (n >= KSTAT_STRLEN) {
+		zfs_dbgmsg("failed to create dataset kstat for objset %lld: "
+		    "kstat module name length (%d) exceeds limit (%d)",
+		    (unsigned long long)dmu_objset_id(objset),
+		    n, KSTAT_STRLEN);
+		return;
+	}
+
+	char kstat_name[KSTAT_STRLEN];
+	n = snprintf(kstat_name, sizeof (kstat_name), "objset-0x%llx",
+	    (unsigned long long)dmu_objset_id(objset));
+	if (n < 0) {
+		zfs_dbgmsg("failed to create dataset kstat for objset %lld: "
+		    " snprintf() for kstat name returned %d",
+		    (unsigned long long)dmu_objset_id(objset), n);
+		return;
+	}
+	ASSERT3U(n, <, KSTAT_STRLEN);
+
+	kstat_t *kstat = kstat_create(kstat_module_name, 0, kstat_name,
+	    "dataset", KSTAT_TYPE_NAMED,
+	    sizeof (empty_dataset_kstats) / sizeof (kstat_named_t),
+	    KSTAT_FLAG_VIRTUAL);
+	if (kstat == NULL)
+		return;
+
+	dataset_kstat_values_t *dk_kstats =
+	    kmem_alloc(sizeof (empty_dataset_kstats), KM_SLEEP);
+	bcopy(&empty_dataset_kstats, dk_kstats,
+	    sizeof (empty_dataset_kstats));
+
+	char *ds_name = kmem_zalloc(ZFS_MAX_DATASET_NAME_LEN, KM_SLEEP);
+	dsl_dataset_name(objset->os_dsl_dataset, ds_name);
+	KSTAT_NAMED_STR_PTR(&dk_kstats->dkv_ds_name) = ds_name;
+	KSTAT_NAMED_STR_BUFLEN(&dk_kstats->dkv_ds_name) =
+	    ZFS_MAX_DATASET_NAME_LEN;
+
+	kstat->ks_data = dk_kstats;
+	kstat->ks_update = dataset_kstats_update;
+	kstat->ks_private = dk;
+	kstat->ks_data_size += ZFS_MAX_DATASET_NAME_LEN;
+
+	kstat_install(kstat);
+	dk->dk_kstats = kstat;
+
+	aggsum_init(&dk->dk_aggsums.das_writes, 0);
+	aggsum_init(&dk->dk_aggsums.das_nwritten, 0);
+	aggsum_init(&dk->dk_aggsums.das_reads, 0);
+	aggsum_init(&dk->dk_aggsums.das_nread, 0);
+	aggsum_init(&dk->dk_aggsums.das_nunlinks, 0);
+	aggsum_init(&dk->dk_aggsums.das_nunlinked, 0);
+}
+
+void
+dataset_kstats_destroy(dataset_kstats_t *dk)
+{
+	if (dk->dk_kstats == NULL)
+		return;
+
+	dataset_kstat_values_t *dkv = dk->dk_kstats->ks_data;
+	kmem_free(KSTAT_NAMED_STR_PTR(&dkv->dkv_ds_name),
+	    KSTAT_NAMED_STR_BUFLEN(&dkv->dkv_ds_name));
+	kmem_free(dkv, sizeof (empty_dataset_kstats));
+
+	kstat_delete(dk->dk_kstats);
+	dk->dk_kstats = NULL;
+
+	aggsum_fini(&dk->dk_aggsums.das_writes);
+	aggsum_fini(&dk->dk_aggsums.das_nwritten);
+	aggsum_fini(&dk->dk_aggsums.das_reads);
+	aggsum_fini(&dk->dk_aggsums.das_nread);
+	aggsum_fini(&dk->dk_aggsums.das_nunlinks);
+	aggsum_fini(&dk->dk_aggsums.das_nunlinked);
+}
+
+void
+dataset_kstats_update_write_kstats(dataset_kstats_t *dk,
+    int64_t nwritten)
+{
+	ASSERT3S(nwritten, >=, 0);
+
+	if (dk->dk_kstats == NULL)
+		return;
+
+	aggsum_add(&dk->dk_aggsums.das_writes, 1);
+	aggsum_add(&dk->dk_aggsums.das_nwritten, nwritten);
+}
+
+void
+dataset_kstats_update_read_kstats(dataset_kstats_t *dk,
+    int64_t nread)
+{
+	ASSERT3S(nread, >=, 0);
+
+	if (dk->dk_kstats == NULL)
+		return;
+
+	aggsum_add(&dk->dk_aggsums.das_reads, 1);
+	aggsum_add(&dk->dk_aggsums.das_nread, nread);
+}
+
+void
+dataset_kstats_update_nunlinks_kstat(dataset_kstats_t *dk, int64_t delta)
+{
+	if (dk->dk_kstats == NULL)
+		return;
+
+	aggsum_add(&dk->dk_aggsums.das_nunlinks, delta);
+}
+
+void
+dataset_kstats_update_nunlinked_kstat(dataset_kstats_t *dk, int64_t delta)
+{
+	if (dk->dk_kstats == NULL)
+		return;
+
+	aggsum_add(&dk->dk_aggsums.das_nunlinked, delta);
+}
diff --git a/sys/contrib/openzfs/module/zfs/dbuf.c b/sys/contrib/openzfs/module/zfs/dbuf.c
new file mode 100644
index 000000000000..2de1f4e4c267
--- /dev/null
+++ b/sys/contrib/openzfs/module/zfs/dbuf.c
@@ -0,0 +1,4741 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
+ * Copyright (c) 2012, 2019 by Delphix. All rights reserved.
+ * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
+ * Copyright (c) 2014 Spectra Logic Corporation, All rights reserved.
+ * Copyright (c) 2019, Klara Inc.
+ * Copyright (c) 2019, Allan Jude
+ */
+
+#include <sys/zfs_context.h>
+#include <sys/arc.h>
+#include <sys/dmu.h>
+#include <sys/dmu_send.h>
+#include <sys/dmu_impl.h>
+#include <sys/dbuf.h>
+#include <sys/dmu_objset.h>
+#include <sys/dsl_dataset.h>
+#include <sys/dsl_dir.h>
+#include <sys/dmu_tx.h>
+#include <sys/spa.h>
+#include <sys/zio.h>
+#include <sys/dmu_zfetch.h>
+#include <sys/sa.h>
+#include <sys/sa_impl.h>
+#include <sys/zfeature.h>
+#include <sys/blkptr.h>
+#include <sys/range_tree.h>
+#include <sys/trace_zfs.h>
+#include <sys/callb.h>
+#include <sys/abd.h>
+#include <sys/vdev.h>
+#include <cityhash.h>
+#include <sys/spa_impl.h>
+
+kstat_t *dbuf_ksp;
+
+typedef struct dbuf_stats {
+	/*
+	 * Various statistics about the size of the dbuf cache.
+	 */
+	kstat_named_t cache_count;
+	kstat_named_t cache_size_bytes;
+	kstat_named_t cache_size_bytes_max;
+	/*
+	 * Statistics regarding the bounds on the dbuf cache size.
+	 */
+	kstat_named_t cache_target_bytes;
+	kstat_named_t cache_lowater_bytes;
+	kstat_named_t cache_hiwater_bytes;
+	/*
+	 * Total number of dbuf cache evictions that have occurred.
+	 */
+	kstat_named_t cache_total_evicts;
+	/*
+	 * The distribution of dbuf levels in the dbuf cache and
+	 * the total size of all dbufs at each level.
+	 */
+	kstat_named_t cache_levels[DN_MAX_LEVELS];
+	kstat_named_t cache_levels_bytes[DN_MAX_LEVELS];
+	/*
+	 * Statistics about the dbuf hash table.
+	 */
+	kstat_named_t hash_hits;
+	kstat_named_t hash_misses;
+	kstat_named_t hash_collisions;
+	kstat_named_t hash_elements;
+	kstat_named_t hash_elements_max;
+	/*
+	 * Number of sublists containing more than one dbuf in the dbuf
+	 * hash table. Keep track of the longest hash chain.
+	 */
+	kstat_named_t hash_chains;
+	kstat_named_t hash_chain_max;
+	/*
+	 * Number of times a dbuf_create() discovers that a dbuf was
+	 * already created and in the dbuf hash table.
+	 */
+	kstat_named_t hash_insert_race;
+	/*
+	 * Statistics about the size of the metadata dbuf cache.
+	 */
+	kstat_named_t metadata_cache_count;
+	kstat_named_t metadata_cache_size_bytes;
+	kstat_named_t metadata_cache_size_bytes_max;
+	/*
+	 * For diagnostic purposes, this is incremented whenever we can't add
+	 * something to the metadata cache because it's full, and instead put
+	 * the data in the regular dbuf cache.
+	 */
+	kstat_named_t metadata_cache_overflow;
+} dbuf_stats_t;
+
+dbuf_stats_t dbuf_stats = {
+	{ "cache_count",			KSTAT_DATA_UINT64 },
+	{ "cache_size_bytes",			KSTAT_DATA_UINT64 },
+	{ "cache_size_bytes_max",		KSTAT_DATA_UINT64 },
+	{ "cache_target_bytes",			KSTAT_DATA_UINT64 },
+	{ "cache_lowater_bytes",		KSTAT_DATA_UINT64 },
+	{ "cache_hiwater_bytes",		KSTAT_DATA_UINT64 },
+	{ "cache_total_evicts",			KSTAT_DATA_UINT64 },
+	{ { "cache_levels_N",			KSTAT_DATA_UINT64 } },
+	{ { "cache_levels_bytes_N",		KSTAT_DATA_UINT64 } },
+	{ "hash_hits",				KSTAT_DATA_UINT64 },
+	{ "hash_misses",			KSTAT_DATA_UINT64 },
+	{ "hash_collisions",			KSTAT_DATA_UINT64 },
+	{ "hash_elements",			KSTAT_DATA_UINT64 },
+	{ "hash_elements_max",			KSTAT_DATA_UINT64 },
+	{ "hash_chains",			KSTAT_DATA_UINT64 },
+	{ "hash_chain_max",			KSTAT_DATA_UINT64 },
+	{ "hash_insert_race",			KSTAT_DATA_UINT64 },
+	{ "metadata_cache_count",		KSTAT_DATA_UINT64 },
+	{ "metadata_cache_size_bytes",		KSTAT_DATA_UINT64 },
+	{ "metadata_cache_size_bytes_max",	KSTAT_DATA_UINT64 },
+	{ "metadata_cache_overflow",		KSTAT_DATA_UINT64 }
+};
+
+#define	DBUF_STAT_INCR(stat, val)	\
+	atomic_add_64(&dbuf_stats.stat.value.ui64, (val));
+#define	DBUF_STAT_DECR(stat, val)	\
+	DBUF_STAT_INCR(stat, -(val));
+#define	DBUF_STAT_BUMP(stat)		\
+	DBUF_STAT_INCR(stat, 1);
+#define	DBUF_STAT_BUMPDOWN(stat)	\
+	DBUF_STAT_INCR(stat, -1);
+#define	DBUF_STAT_MAX(stat, v) {					\
+	uint64_t _m;							\
+	while ((v) > (_m = dbuf_stats.stat.value.ui64) &&		\
+	    (_m != atomic_cas_64(&dbuf_stats.stat.value.ui64, _m, (v))))\
+		continue;						\
+}
+
+static boolean_t dbuf_undirty(dmu_buf_impl_t *db, dmu_tx_t *tx);
+static void dbuf_write(dbuf_dirty_record_t *dr, arc_buf_t *data, dmu_tx_t *tx);
+static void dbuf_sync_leaf_verify_bonus_dnode(dbuf_dirty_record_t *dr);
+static int dbuf_read_verify_dnode_crypt(dmu_buf_impl_t *db, uint32_t flags);
+
+extern inline void dmu_buf_init_user(dmu_buf_user_t *dbu,
+    dmu_buf_evict_func_t *evict_func_sync,
+    dmu_buf_evict_func_t *evict_func_async,
+    dmu_buf_t **clear_on_evict_dbufp);
+
+/*
+ * Global data structures and functions for the dbuf cache.
+ */
+static kmem_cache_t *dbuf_kmem_cache;
+static taskq_t *dbu_evict_taskq;
+
+static kthread_t *dbuf_cache_evict_thread;
+static kmutex_t dbuf_evict_lock;
+static kcondvar_t dbuf_evict_cv;
+static boolean_t dbuf_evict_thread_exit;
+
+/*
+ * There are two dbuf caches; each dbuf can only be in one of them at a time.
+ *
+ * 1. Cache of metadata dbufs, to help make read-heavy administrative commands
+ *    from /sbin/zfs run faster. The "metadata cache" specifically stores dbufs
+ *    that represent the metadata that describes filesystems/snapshots/
+ *    bookmarks/properties/etc. We only evict from this cache when we export a
+ *    pool, to short-circuit as much I/O as possible for all administrative
+ *    commands that need the metadata. There is no eviction policy for this
+ *    cache, because we try to only include types in it which would occupy a
+ *    very small amount of space per object but create a large impact on the
+ *    performance of these commands. Instead, after it reaches a maximum size
+ *    (which should only happen on very small memory systems with a very large
+ *    number of filesystem objects), we stop taking new dbufs into the
+ *    metadata cache, instead putting them in the normal dbuf cache.
+ *
+ * 2. LRU cache of dbufs. The dbuf cache maintains a list of dbufs that
+ *    are not currently held but have been recently released. These dbufs
+ *    are not eligible for arc eviction until they are aged out of the cache.
+ *    Dbufs that are aged out of the cache will be immediately destroyed and
+ *    become eligible for arc eviction.
+ *
+ * Dbufs are added to these caches once the last hold is released. If a dbuf is
+ * later accessed and still exists in the dbuf cache, then it will be removed
+ * from the cache and later re-added to the head of the cache.
+ *
+ * If a given dbuf meets the requirements for the metadata cache, it will go
+ * there, otherwise it will be considered for the generic LRU dbuf cache. The
+ * caches and the refcounts tracking their sizes are stored in an array indexed
+ * by those caches' matching enum values (from dbuf_cached_state_t).
+ */
+typedef struct dbuf_cache {
+	multilist_t *cache;
+	zfs_refcount_t size;
+} dbuf_cache_t;
+dbuf_cache_t dbuf_caches[DB_CACHE_MAX];
+
+/* Size limits for the caches */
+unsigned long dbuf_cache_max_bytes = ULONG_MAX;
+unsigned long dbuf_metadata_cache_max_bytes = ULONG_MAX;
+
+/* Set the default sizes of the caches to log2 fraction of arc size */
+int dbuf_cache_shift = 5;
+int dbuf_metadata_cache_shift = 6;
+
+static unsigned long dbuf_cache_target_bytes(void);
+static unsigned long dbuf_metadata_cache_target_bytes(void);
+
+/*
+ * The LRU dbuf cache uses a three-stage eviction policy:
+ *	- A low water marker designates when the dbuf eviction thread
+ *	should stop evicting from the dbuf cache.
+ *	- When we reach the maximum size (aka mid water mark), we
+ *	signal the eviction thread to run.
+ *	- The high water mark indicates when the eviction thread
+ *	is unable to keep up with the incoming load and eviction must
+ *	happen in the context of the calling thread.
+ *
+ * The dbuf cache:
+ *                                                 (max size)
+ *                                      low water   mid water   hi water
+ * +----------------------------------------+----------+----------+
+ * |                                        |          |          |
+ * |                                        |          |          |
+ * |                                        |          |          |
+ * |                                        |          |          |
+ * +----------------------------------------+----------+----------+
+ *                                        stop        signal     evict
+ *                                      evicting     eviction   directly
+ *                                                    thread
+ *
+ * The high and low water marks indicate the operating range for the eviction
+ * thread. The low water mark is, by default, 90% of the total size of the
+ * cache and the high water mark is at 110% (both of these percentages can be
+ * changed by setting dbuf_cache_lowater_pct and dbuf_cache_hiwater_pct,
+ * respectively). The eviction thread will try to ensure that the cache remains
+ * within this range by waking up every second and checking if the cache is
+ * above the low water mark. The thread can also be woken up by callers adding
+ * elements into the cache if the cache is larger than the mid water (i.e max
+ * cache size). Once the eviction thread is woken up and eviction is required,
+ * it will continue evicting buffers until it's able to reduce the cache size
+ * to the low water mark. If the cache size continues to grow and hits the high
+ * water mark, then callers adding elements to the cache will begin to evict
+ * directly from the cache until the cache is no longer above the high water
+ * mark.
+ */
+
+/*
+ * The percentage above and below the maximum cache size.
+ */
+uint_t dbuf_cache_hiwater_pct = 10;
+uint_t dbuf_cache_lowater_pct = 10;
+
+/* ARGSUSED */
+static int
+dbuf_cons(void *vdb, void *unused, int kmflag)
+{
+	dmu_buf_impl_t *db = vdb;
+	bzero(db, sizeof (dmu_buf_impl_t));
+
+	mutex_init(&db->db_mtx, NULL, MUTEX_DEFAULT, NULL);
+	rw_init(&db->db_rwlock, NULL, RW_DEFAULT, NULL);
+	cv_init(&db->db_changed, NULL, CV_DEFAULT, NULL);
+	multilist_link_init(&db->db_cache_link);
+	zfs_refcount_create(&db->db_holds);
+
+	return (0);
+}
+
+/* ARGSUSED */
+static void
+dbuf_dest(void *vdb, void *unused)
+{
+	dmu_buf_impl_t *db = vdb;
+	mutex_destroy(&db->db_mtx);
+	rw_destroy(&db->db_rwlock);
+	cv_destroy(&db->db_changed);
+	ASSERT(!multilist_link_active(&db->db_cache_link));
+	zfs_refcount_destroy(&db->db_holds);
+}
+
+/*
+ * dbuf hash table routines
+ */
+static dbuf_hash_table_t dbuf_hash_table;
+
+static uint64_t dbuf_hash_count;
+
+/*
+ * We use Cityhash for this. It's fast, and has good hash properties without
+ * requiring any large static buffers.
+ */
+static uint64_t
+dbuf_hash(void *os, uint64_t obj, uint8_t lvl, uint64_t blkid)
+{
+	return (cityhash4((uintptr_t)os, obj, (uint64_t)lvl, blkid));
+}
+
+#define	DTRACE_SET_STATE(db, why) \
+	DTRACE_PROBE2(dbuf__state_change, dmu_buf_impl_t *, db,	\
+	    const char *, why)
+
+#define	DBUF_EQUAL(dbuf, os, obj, level, blkid)		\
+	((dbuf)->db.db_object == (obj) &&		\
+	(dbuf)->db_objset == (os) &&			\
+	(dbuf)->db_level == (level) &&			\
+	(dbuf)->db_blkid == (blkid))
+
+dmu_buf_impl_t *
+dbuf_find(objset_t *os, uint64_t obj, uint8_t level, uint64_t blkid)
+{
+	dbuf_hash_table_t *h = &dbuf_hash_table;
+	uint64_t hv;
+	uint64_t idx;
+	dmu_buf_impl_t *db;
+
+	hv = dbuf_hash(os, obj, level, blkid);
+	idx = hv & h->hash_table_mask;
+
+	mutex_enter(DBUF_HASH_MUTEX(h, idx));
+	for (db = h->hash_table[idx]; db != NULL; db = db->db_hash_next) {
+		if (DBUF_EQUAL(db, os, obj, level, blkid)) {
+			mutex_enter(&db->db_mtx);
+			if (db->db_state != DB_EVICTING) {
+				mutex_exit(DBUF_HASH_MUTEX(h, idx));
+				return (db);
+			}
+			mutex_exit(&db->db_mtx);
+		}
+	}
+	mutex_exit(DBUF_HASH_MUTEX(h, idx));
+	return (NULL);
+}
+
+static dmu_buf_impl_t *
+dbuf_find_bonus(objset_t *os, uint64_t object)
+{
+	dnode_t *dn;
+	dmu_buf_impl_t *db = NULL;
+
+	if (dnode_hold(os, object, FTAG, &dn) == 0) {
+		rw_enter(&dn->dn_struct_rwlock, RW_READER);
+		if (dn->dn_bonus != NULL) {
+			db = dn->dn_bonus;
+			mutex_enter(&db->db_mtx);
+		}
+		rw_exit(&dn->dn_struct_rwlock);
+		dnode_rele(dn, FTAG);
+	}
+	return (db);
+}
+
+/*
+ * Insert an entry into the hash table.  If there is already an element
+ * equal to elem in the hash table, then the already existing element
+ * will be returned and the new element will not be inserted.
+ * Otherwise returns NULL.
+ */
+static dmu_buf_impl_t *
+dbuf_hash_insert(dmu_buf_impl_t *db)
+{
+	dbuf_hash_table_t *h = &dbuf_hash_table;
+	objset_t *os = db->db_objset;
+	uint64_t obj = db->db.db_object;
+	int level = db->db_level;
+	uint64_t blkid, hv, idx;
+	dmu_buf_impl_t *dbf;
+	uint32_t i;
+
+	blkid = db->db_blkid;
+	hv = dbuf_hash(os, obj, level, blkid);
+	idx = hv & h->hash_table_mask;
+
+	mutex_enter(DBUF_HASH_MUTEX(h, idx));
+	for (dbf = h->hash_table[idx], i = 0; dbf != NULL;
+	    dbf = dbf->db_hash_next, i++) {
+		if (DBUF_EQUAL(dbf, os, obj, level, blkid)) {
+			mutex_enter(&dbf->db_mtx);
+			if (dbf->db_state != DB_EVICTING) {
+				mutex_exit(DBUF_HASH_MUTEX(h, idx));
+				return (dbf);
+			}
+			mutex_exit(&dbf->db_mtx);
+		}
+	}
+
+	if (i > 0) {
+		DBUF_STAT_BUMP(hash_collisions);
+		if (i == 1)
+			DBUF_STAT_BUMP(hash_chains);
+
+		DBUF_STAT_MAX(hash_chain_max, i);
+	}
+
+	mutex_enter(&db->db_mtx);
+	db->db_hash_next = h->hash_table[idx];
+	h->hash_table[idx] = db;
+	mutex_exit(DBUF_HASH_MUTEX(h, idx));
+	atomic_inc_64(&dbuf_hash_count);
+	DBUF_STAT_MAX(hash_elements_max, dbuf_hash_count);
+
+	return (NULL);
+}
+
+/*
+ * This returns whether this dbuf should be stored in the metadata cache, which
+ * is based on whether it's from one of the dnode types that store data related
+ * to traversing dataset hierarchies.
+ */
+static boolean_t
+dbuf_include_in_metadata_cache(dmu_buf_impl_t *db)
+{
+	DB_DNODE_ENTER(db);
+	dmu_object_type_t type = DB_DNODE(db)->dn_type;
+	DB_DNODE_EXIT(db);
+
+	/* Check if this dbuf is one of the types we care about */
+	if (DMU_OT_IS_METADATA_CACHED(type)) {
+		/* If we hit this, then we set something up wrong in dmu_ot */
+		ASSERT(DMU_OT_IS_METADATA(type));
+
+		/*
+		 * Sanity check for small-memory systems: don't allocate too
+		 * much memory for this purpose.
+		 */
+		if (zfs_refcount_count(
+		    &dbuf_caches[DB_DBUF_METADATA_CACHE].size) >
+		    dbuf_metadata_cache_target_bytes()) {
+			DBUF_STAT_BUMP(metadata_cache_overflow);
+			return (B_FALSE);
+		}
+
+		return (B_TRUE);
+	}
+
+	return (B_FALSE);
+}
+
+/*
+ * Remove an entry from the hash table.  It must be in the EVICTING state.
+ */
+static void
+dbuf_hash_remove(dmu_buf_impl_t *db)
+{
+	dbuf_hash_table_t *h = &dbuf_hash_table;
+	uint64_t hv, idx;
+	dmu_buf_impl_t *dbf, **dbp;
+
+	hv = dbuf_hash(db->db_objset, db->db.db_object,
+	    db->db_level, db->db_blkid);
+	idx = hv & h->hash_table_mask;
+
+	/*
+	 * We mustn't hold db_mtx to maintain lock ordering:
+	 * DBUF_HASH_MUTEX > db_mtx.
+	 */
+	ASSERT(zfs_refcount_is_zero(&db->db_holds));
+	ASSERT(db->db_state == DB_EVICTING);
+	ASSERT(!MUTEX_HELD(&db->db_mtx));
+
+	mutex_enter(DBUF_HASH_MUTEX(h, idx));
+	dbp = &h->hash_table[idx];
+	while ((dbf = *dbp) != db) {
+		dbp = &dbf->db_hash_next;
+		ASSERT(dbf != NULL);
+	}
+	*dbp = db->db_hash_next;
+	db->db_hash_next = NULL;
+	if (h->hash_table[idx] &&
+	    h->hash_table[idx]->db_hash_next == NULL)
+		DBUF_STAT_BUMPDOWN(hash_chains);
+	mutex_exit(DBUF_HASH_MUTEX(h, idx));
+	atomic_dec_64(&dbuf_hash_count);
+}
+
+typedef enum {
+	DBVU_EVICTING,
+	DBVU_NOT_EVICTING
+} dbvu_verify_type_t;
+
+static void
+dbuf_verify_user(dmu_buf_impl_t *db, dbvu_verify_type_t verify_type)
+{
+#ifdef ZFS_DEBUG
+	int64_t holds;
+
+	if (db->db_user == NULL)
+		return;
+
+	/* Only data blocks support the attachment of user data. */
+	ASSERT(db->db_level == 0);
+
+	/* Clients must resolve a dbuf before attaching user data. */
+	ASSERT(db->db.db_data != NULL);
+	ASSERT3U(db->db_state, ==, DB_CACHED);
+
+	holds = zfs_refcount_count(&db->db_holds);
+	if (verify_type == DBVU_EVICTING) {
+		/*
+		 * Immediate eviction occurs when holds == dirtycnt.
+		 * For normal eviction buffers, holds is zero on
+		 * eviction, except when dbuf_fix_old_data() calls
+		 * dbuf_clear_data().  However, the hold count can grow
+		 * during eviction even though db_mtx is held (see
+		 * dmu_bonus_hold() for an example), so we can only
+		 * test the generic invariant that holds >= dirtycnt.
+		 */
+		ASSERT3U(holds, >=, db->db_dirtycnt);
+	} else {
+		if (db->db_user_immediate_evict == TRUE)
+			ASSERT3U(holds, >=, db->db_dirtycnt);
+		else
+			ASSERT3U(holds, >, 0);
+	}
+#endif
+}
+
+static void
+dbuf_evict_user(dmu_buf_impl_t *db)
+{
+	dmu_buf_user_t *dbu = db->db_user;
+
+	ASSERT(MUTEX_HELD(&db->db_mtx));
+
+	if (dbu == NULL)
+		return;
+
+	dbuf_verify_user(db, DBVU_EVICTING);
+	db->db_user = NULL;
+
+#ifdef ZFS_DEBUG
+	if (dbu->dbu_clear_on_evict_dbufp != NULL)
+		*dbu->dbu_clear_on_evict_dbufp = NULL;
+#endif
+
+	/*
+	 * There are two eviction callbacks - one that we call synchronously
+	 * and one that we invoke via a taskq.  The async one is useful for
+	 * avoiding lock order reversals and limiting stack depth.
+	 *
+	 * Note that if we have a sync callback but no async callback,
+	 * it's likely that the sync callback will free the structure
+	 * containing the dbu.  In that case we need to take care to not
+	 * dereference dbu after calling the sync evict func.
+	 */
+	boolean_t has_async = (dbu->dbu_evict_func_async != NULL);
+
+	if (dbu->dbu_evict_func_sync != NULL)
+		dbu->dbu_evict_func_sync(dbu);
+
+	if (has_async) {
+		taskq_dispatch_ent(dbu_evict_taskq, dbu->dbu_evict_func_async,
+		    dbu, 0, &dbu->dbu_tqent);
+	}
+}
+
+boolean_t
+dbuf_is_metadata(dmu_buf_impl_t *db)
+{
+	/*
+	 * Consider indirect blocks and spill blocks to be meta data.
+	 */
+	if (db->db_level > 0 || db->db_blkid == DMU_SPILL_BLKID) {
+		return (B_TRUE);
+	} else {
+		boolean_t is_metadata;
+
+		DB_DNODE_ENTER(db);
+		is_metadata = DMU_OT_IS_METADATA(DB_DNODE(db)->dn_type);
+		DB_DNODE_EXIT(db);
+
+		return (is_metadata);
+	}
+}
+
+
+/*
+ * This function *must* return indices evenly distributed between all
+ * sublists of the multilist. This is needed due to how the dbuf eviction
+ * code is laid out; dbuf_evict_thread() assumes dbufs are evenly
+ * distributed between all sublists and uses this assumption when
+ * deciding which sublist to evict from and how much to evict from it.
+ */
+static unsigned int
+dbuf_cache_multilist_index_func(multilist_t *ml, void *obj)
+{
+	dmu_buf_impl_t *db = obj;
+
+	/*
+	 * The assumption here, is the hash value for a given
+	 * dmu_buf_impl_t will remain constant throughout it's lifetime
+	 * (i.e. it's objset, object, level and blkid fields don't change).
+	 * Thus, we don't need to store the dbuf's sublist index
+	 * on insertion, as this index can be recalculated on removal.
+	 *
+	 * Also, the low order bits of the hash value are thought to be
+	 * distributed evenly. Otherwise, in the case that the multilist
+	 * has a power of two number of sublists, each sublists' usage
+	 * would not be evenly distributed.
+	 */
+	return (dbuf_hash(db->db_objset, db->db.db_object,
+	    db->db_level, db->db_blkid) %
+	    multilist_get_num_sublists(ml));
+}
+
+/*
+ * The target size of the dbuf cache can grow with the ARC target,
+ * unless limited by the tunable dbuf_cache_max_bytes.
+ */
+static inline unsigned long
+dbuf_cache_target_bytes(void)
+{
+	return (MIN(dbuf_cache_max_bytes,
+	    arc_target_bytes() >> dbuf_cache_shift));
+}
+
+/*
+ * The target size of the dbuf metadata cache can grow with the ARC target,
+ * unless limited by the tunable dbuf_metadata_cache_max_bytes.
+ */
+static inline unsigned long
+dbuf_metadata_cache_target_bytes(void)
+{
+	return (MIN(dbuf_metadata_cache_max_bytes,
+	    arc_target_bytes() >> dbuf_metadata_cache_shift));
+}
+
+static inline uint64_t
+dbuf_cache_hiwater_bytes(void)
+{
+	uint64_t dbuf_cache_target = dbuf_cache_target_bytes();
+	return (dbuf_cache_target +
+	    (dbuf_cache_target * dbuf_cache_hiwater_pct) / 100);
+}
+
+static inline uint64_t
+dbuf_cache_lowater_bytes(void)
+{
+	uint64_t dbuf_cache_target = dbuf_cache_target_bytes();
+	return (dbuf_cache_target -
+	    (dbuf_cache_target * dbuf_cache_lowater_pct) / 100);
+}
+
+static inline boolean_t
+dbuf_cache_above_lowater(void)
+{
+	return (zfs_refcount_count(&dbuf_caches[DB_DBUF_CACHE].size) >
+	    dbuf_cache_lowater_bytes());
+}
+
+/*
+ * Evict the oldest eligible dbuf from the dbuf cache.
+ */
+static void
+dbuf_evict_one(void)
+{
+	int idx = multilist_get_random_index(dbuf_caches[DB_DBUF_CACHE].cache);
+	multilist_sublist_t *mls = multilist_sublist_lock(
+	    dbuf_caches[DB_DBUF_CACHE].cache, idx);
+
+	ASSERT(!MUTEX_HELD(&dbuf_evict_lock));
+
+	dmu_buf_impl_t *db = multilist_sublist_tail(mls);
+	while (db != NULL && mutex_tryenter(&db->db_mtx) == 0) {
+		db = multilist_sublist_prev(mls, db);
+	}
+
+	DTRACE_PROBE2(dbuf__evict__one, dmu_buf_impl_t *, db,
+	    multilist_sublist_t *, mls);
+
+	if (db != NULL) {
+		multilist_sublist_remove(mls, db);
+		multilist_sublist_unlock(mls);
+		(void) zfs_refcount_remove_many(
+		    &dbuf_caches[DB_DBUF_CACHE].size, db->db.db_size, db);
+		DBUF_STAT_BUMPDOWN(cache_levels[db->db_level]);
+		DBUF_STAT_BUMPDOWN(cache_count);
+		DBUF_STAT_DECR(cache_levels_bytes[db->db_level],
+		    db->db.db_size);
+		ASSERT3U(db->db_caching_status, ==, DB_DBUF_CACHE);
+		db->db_caching_status = DB_NO_CACHE;
+		dbuf_destroy(db);
+		DBUF_STAT_BUMP(cache_total_evicts);
+	} else {
+		multilist_sublist_unlock(mls);
+	}
+}
+
+/*
+ * The dbuf evict thread is responsible for aging out dbufs from the
+ * cache. Once the cache has reached it's maximum size, dbufs are removed
+ * and destroyed. The eviction thread will continue running until the size
+ * of the dbuf cache is at or below the maximum size. Once the dbuf is aged
+ * out of the cache it is destroyed and becomes eligible for arc eviction.
+ */
+/* ARGSUSED */
+static void
+dbuf_evict_thread(void *unused)
+{
+	callb_cpr_t cpr;
+
+	CALLB_CPR_INIT(&cpr, &dbuf_evict_lock, callb_generic_cpr, FTAG);
+
+	mutex_enter(&dbuf_evict_lock);
+	while (!dbuf_evict_thread_exit) {
+		while (!dbuf_cache_above_lowater() && !dbuf_evict_thread_exit) {
+			CALLB_CPR_SAFE_BEGIN(&cpr);
+			(void) cv_timedwait_sig_hires(&dbuf_evict_cv,
+			    &dbuf_evict_lock, SEC2NSEC(1), MSEC2NSEC(1), 0);
+			CALLB_CPR_SAFE_END(&cpr, &dbuf_evict_lock);
+		}
+		mutex_exit(&dbuf_evict_lock);
+
+		/*
+		 * Keep evicting as long as we're above the low water mark
+		 * for the cache. We do this without holding the locks to
+		 * minimize lock contention.
+		 */
+		while (dbuf_cache_above_lowater() && !dbuf_evict_thread_exit) {
+			dbuf_evict_one();
+		}
+
+		mutex_enter(&dbuf_evict_lock);
+	}
+
+	dbuf_evict_thread_exit = B_FALSE;
+	cv_broadcast(&dbuf_evict_cv);
+	CALLB_CPR_EXIT(&cpr);	/* drops dbuf_evict_lock */
+	thread_exit();
+}
+
+/*
+ * Wake up the dbuf eviction thread if the dbuf cache is at its max size.
+ * If the dbuf cache is at its high water mark, then evict a dbuf from the
+ * dbuf cache using the callers context.
+ */
+static void
+dbuf_evict_notify(uint64_t size)
+{
+	/*
+	 * We check if we should evict without holding the dbuf_evict_lock,
+	 * because it's OK to occasionally make the wrong decision here,
+	 * and grabbing the lock results in massive lock contention.
+	 */
+	if (size > dbuf_cache_target_bytes()) {
+		if (size > dbuf_cache_hiwater_bytes())
+			dbuf_evict_one();
+		cv_signal(&dbuf_evict_cv);
+	}
+}
+
+static int
+dbuf_kstat_update(kstat_t *ksp, int rw)
+{
+	dbuf_stats_t *ds = ksp->ks_data;
+
+	if (rw == KSTAT_WRITE) {
+		return (SET_ERROR(EACCES));
+	} else {
+		ds->metadata_cache_size_bytes.value.ui64 = zfs_refcount_count(
+		    &dbuf_caches[DB_DBUF_METADATA_CACHE].size);
+		ds->cache_size_bytes.value.ui64 =
+		    zfs_refcount_count(&dbuf_caches[DB_DBUF_CACHE].size);
+		ds->cache_target_bytes.value.ui64 = dbuf_cache_target_bytes();
+		ds->cache_hiwater_bytes.value.ui64 = dbuf_cache_hiwater_bytes();
+		ds->cache_lowater_bytes.value.ui64 = dbuf_cache_lowater_bytes();
+		ds->hash_elements.value.ui64 = dbuf_hash_count;
+	}
+
+	return (0);
+}
+
+void
+dbuf_init(void)
+{
+	uint64_t hsize = 1ULL << 16;
+	dbuf_hash_table_t *h = &dbuf_hash_table;
+	int i;
+
+	/*
+	 * The hash table is big enough to fill all of physical memory
+	 * with an average block size of zfs_arc_average_blocksize (default 8K).
+	 * By default, the table will take up
+	 * totalmem * sizeof(void*) / 8K (1MB per GB with 8-byte pointers).
+	 */
+	while (hsize * zfs_arc_average_blocksize < physmem * PAGESIZE)
+		hsize <<= 1;
+
+retry:
+	h->hash_table_mask = hsize - 1;
+#if defined(_KERNEL)
+	/*
+	 * Large allocations which do not require contiguous pages
+	 * should be using vmem_alloc() in the linux kernel
+	 */
+	h->hash_table = vmem_zalloc(hsize * sizeof (void *), KM_SLEEP);
+#else
+	h->hash_table = kmem_zalloc(hsize * sizeof (void *), KM_NOSLEEP);
+#endif
+	if (h->hash_table == NULL) {
+		/* XXX - we should really return an error instead of assert */
+		ASSERT(hsize > (1ULL << 10));
+		hsize >>= 1;
+		goto retry;
+	}
+
+	dbuf_kmem_cache = kmem_cache_create("dmu_buf_impl_t",
+	    sizeof (dmu_buf_impl_t),
+	    0, dbuf_cons, dbuf_dest, NULL, NULL, NULL, 0);
+
+	for (i = 0; i < DBUF_MUTEXES; i++)
+		mutex_init(&h->hash_mutexes[i], NULL, MUTEX_DEFAULT, NULL);
+
+	dbuf_stats_init(h);
+
+	/*
+	 * All entries are queued via taskq_dispatch_ent(), so min/maxalloc
+	 * configuration is not required.
+	 */
+	dbu_evict_taskq = taskq_create("dbu_evict", 1, defclsyspri, 0, 0, 0);
+
+	for (dbuf_cached_state_t dcs = 0; dcs < DB_CACHE_MAX; dcs++) {
+		dbuf_caches[dcs].cache =
+		    multilist_create(sizeof (dmu_buf_impl_t),
+		    offsetof(dmu_buf_impl_t, db_cache_link),
+		    dbuf_cache_multilist_index_func);
+		zfs_refcount_create(&dbuf_caches[dcs].size);
+	}
+
+	dbuf_evict_thread_exit = B_FALSE;
+	mutex_init(&dbuf_evict_lock, NULL, MUTEX_DEFAULT, NULL);
+	cv_init(&dbuf_evict_cv, NULL, CV_DEFAULT, NULL);
+	dbuf_cache_evict_thread = thread_create(NULL, 0, dbuf_evict_thread,
+	    NULL, 0, &p0, TS_RUN, minclsyspri);
+
+	dbuf_ksp = kstat_create("zfs", 0, "dbufstats", "misc",
+	    KSTAT_TYPE_NAMED, sizeof (dbuf_stats) / sizeof (kstat_named_t),
+	    KSTAT_FLAG_VIRTUAL);
+	if (dbuf_ksp != NULL) {
+		for (i = 0; i < DN_MAX_LEVELS; i++) {
+			snprintf(dbuf_stats.cache_levels[i].name,
+			    KSTAT_STRLEN, "cache_level_%d", i);
+			dbuf_stats.cache_levels[i].data_type =
+			    KSTAT_DATA_UINT64;
+			snprintf(dbuf_stats.cache_levels_bytes[i].name,
+			    KSTAT_STRLEN, "cache_level_%d_bytes", i);
+			dbuf_stats.cache_levels_bytes[i].data_type =
+			    KSTAT_DATA_UINT64;
+		}
+		dbuf_ksp->ks_data = &dbuf_stats;
+		dbuf_ksp->ks_update = dbuf_kstat_update;
+		kstat_install(dbuf_ksp);
+	}
+}
+
+void
+dbuf_fini(void)
+{
+	dbuf_hash_table_t *h = &dbuf_hash_table;
+	int i;
+
+	dbuf_stats_destroy();
+
+	for (i = 0; i < DBUF_MUTEXES; i++)
+		mutex_destroy(&h->hash_mutexes[i]);
+#if defined(_KERNEL)
+	/*
+	 * Large allocations which do not require contiguous pages
+	 * should be using vmem_free() in the linux kernel
+	 */
+	vmem_free(h->hash_table, (h->hash_table_mask + 1) * sizeof (void *));
+#else
+	kmem_free(h->hash_table, (h->hash_table_mask + 1) * sizeof (void *));
+#endif
+	kmem_cache_destroy(dbuf_kmem_cache);
+	taskq_destroy(dbu_evict_taskq);
+
+	mutex_enter(&dbuf_evict_lock);
+	dbuf_evict_thread_exit = B_TRUE;
+	while (dbuf_evict_thread_exit) {
+		cv_signal(&dbuf_evict_cv);
+		cv_wait(&dbuf_evict_cv, &dbuf_evict_lock);
+	}
+	mutex_exit(&dbuf_evict_lock);
+
+	mutex_destroy(&dbuf_evict_lock);
+	cv_destroy(&dbuf_evict_cv);
+
+	for (dbuf_cached_state_t dcs = 0; dcs < DB_CACHE_MAX; dcs++) {
+		zfs_refcount_destroy(&dbuf_caches[dcs].size);
+		multilist_destroy(dbuf_caches[dcs].cache);
+	}
+
+	if (dbuf_ksp != NULL) {
+		kstat_delete(dbuf_ksp);
+		dbuf_ksp = NULL;
+	}
+}
+
+/*
+ * Other stuff.
+ */
+
+#ifdef ZFS_DEBUG
+static void
+dbuf_verify(dmu_buf_impl_t *db)
+{
+	dnode_t *dn;
+	dbuf_dirty_record_t *dr;
+	uint32_t txg_prev;
+
+	ASSERT(MUTEX_HELD(&db->db_mtx));
+
+	if (!(zfs_flags & ZFS_DEBUG_DBUF_VERIFY))
+		return;
+
+	ASSERT(db->db_objset != NULL);
+	DB_DNODE_ENTER(db);
+	dn = DB_DNODE(db);
+	if (dn == NULL) {
+		ASSERT(db->db_parent == NULL);
+		ASSERT(db->db_blkptr == NULL);
+	} else {
+		ASSERT3U(db->db.db_object, ==, dn->dn_object);
+		ASSERT3P(db->db_objset, ==, dn->dn_objset);
+		ASSERT3U(db->db_level, <, dn->dn_nlevels);
+		ASSERT(db->db_blkid == DMU_BONUS_BLKID ||
+		    db->db_blkid == DMU_SPILL_BLKID ||
+		    !avl_is_empty(&dn->dn_dbufs));
+	}
+	if (db->db_blkid == DMU_BONUS_BLKID) {
+		ASSERT(dn != NULL);
+		ASSERT3U(db->db.db_size, >=, dn->dn_bonuslen);
+		ASSERT3U(db->db.db_offset, ==, DMU_BONUS_BLKID);
+	} else if (db->db_blkid == DMU_SPILL_BLKID) {
+		ASSERT(dn != NULL);
+		ASSERT0(db->db.db_offset);
+	} else {
+		ASSERT3U(db->db.db_offset, ==, db->db_blkid * db->db.db_size);
+	}
+
+	if ((dr = list_head(&db->db_dirty_records)) != NULL) {
+		ASSERT(dr->dr_dbuf == db);
+		txg_prev = dr->dr_txg;
+		for (dr = list_next(&db->db_dirty_records, dr); dr != NULL;
+		    dr = list_next(&db->db_dirty_records, dr)) {
+			ASSERT(dr->dr_dbuf == db);
+			ASSERT(txg_prev > dr->dr_txg);
+			txg_prev = dr->dr_txg;
+		}
+	}
+
+	/*
+	 * We can't assert that db_size matches dn_datablksz because it
+	 * can be momentarily different when another thread is doing
+	 * dnode_set_blksz().
+	 */
+	if (db->db_level == 0 && db->db.db_object == DMU_META_DNODE_OBJECT) {
+		dr = db->db_data_pending;
+		/*
+		 * It should only be modified in syncing context, so
+		 * make sure we only have one copy of the data.
+		 */
+		ASSERT(dr == NULL || dr->dt.dl.dr_data == db->db_buf);
+	}
+
+	/* verify db->db_blkptr */
+	if (db->db_blkptr) {
+		if (db->db_parent == dn->dn_dbuf) {
+			/* db is pointed to by the dnode */
+			/* ASSERT3U(db->db_blkid, <, dn->dn_nblkptr); */
+			if (DMU_OBJECT_IS_SPECIAL(db->db.db_object))
+				ASSERT(db->db_parent == NULL);
+			else
+				ASSERT(db->db_parent != NULL);
+			if (db->db_blkid != DMU_SPILL_BLKID)
+				ASSERT3P(db->db_blkptr, ==,
+				    &dn->dn_phys->dn_blkptr[db->db_blkid]);
+		} else {
+			/* db is pointed to by an indirect block */
+			int epb __maybe_unused = db->db_parent->db.db_size >>
+			    SPA_BLKPTRSHIFT;
+			ASSERT3U(db->db_parent->db_level, ==, db->db_level+1);
+			ASSERT3U(db->db_parent->db.db_object, ==,
+			    db->db.db_object);
+			/*
+			 * dnode_grow_indblksz() can make this fail if we don't
+			 * have the parent's rwlock.  XXX indblksz no longer
+			 * grows.  safe to do this now?
+			 */
+			if (RW_LOCK_HELD(&db->db_parent->db_rwlock)) {
+				ASSERT3P(db->db_blkptr, ==,
+				    ((blkptr_t *)db->db_parent->db.db_data +
+				    db->db_blkid % epb));
+			}
+		}
+	}
+	if ((db->db_blkptr == NULL || BP_IS_HOLE(db->db_blkptr)) &&
+	    (db->db_buf == NULL || db->db_buf->b_data) &&
+	    db->db.db_data && db->db_blkid != DMU_BONUS_BLKID &&
+	    db->db_state != DB_FILL && !dn->dn_free_txg) {
+		/*
+		 * If the blkptr isn't set but they have nonzero data,
+		 * it had better be dirty, otherwise we'll lose that
+		 * data when we evict this buffer.
+		 *
+		 * There is an exception to this rule for indirect blocks; in
+		 * this case, if the indirect block is a hole, we fill in a few
+		 * fields on each of the child blocks (importantly, birth time)
+		 * to prevent hole birth times from being lost when you
+		 * partially fill in a hole.
+		 */
+		if (db->db_dirtycnt == 0) {
+			if (db->db_level == 0) {
+				uint64_t *buf = db->db.db_data;
+				int i;
+
+				for (i = 0; i < db->db.db_size >> 3; i++) {
+					ASSERT(buf[i] == 0);
+				}
+			} else {
+				blkptr_t *bps = db->db.db_data;
+				ASSERT3U(1 << DB_DNODE(db)->dn_indblkshift, ==,
+				    db->db.db_size);
+				/*
+				 * We want to verify that all the blkptrs in the
+				 * indirect block are holes, but we may have
+				 * automatically set up a few fields for them.
+				 * We iterate through each blkptr and verify
+				 * they only have those fields set.
+				 */
+				for (int i = 0;
+				    i < db->db.db_size / sizeof (blkptr_t);
+				    i++) {
+					blkptr_t *bp = &bps[i];
+					ASSERT(ZIO_CHECKSUM_IS_ZERO(
+					    &bp->blk_cksum));
+					ASSERT(
+					    DVA_IS_EMPTY(&bp->blk_dva[0]) &&
+					    DVA_IS_EMPTY(&bp->blk_dva[1]) &&
+					    DVA_IS_EMPTY(&bp->blk_dva[2]));
+					ASSERT0(bp->blk_fill);
+					ASSERT0(bp->blk_pad[0]);
+					ASSERT0(bp->blk_pad[1]);
+					ASSERT(!BP_IS_EMBEDDED(bp));
+					ASSERT(BP_IS_HOLE(bp));
+					ASSERT0(bp->blk_phys_birth);
+				}
+			}
+		}
+	}
+	DB_DNODE_EXIT(db);
+}
+#endif
+
+static void
+dbuf_clear_data(dmu_buf_impl_t *db)
+{
+	ASSERT(MUTEX_HELD(&db->db_mtx));
+	dbuf_evict_user(db);
+	ASSERT3P(db->db_buf, ==, NULL);
+	db->db.db_data = NULL;
+	if (db->db_state != DB_NOFILL) {
+		db->db_state = DB_UNCACHED;
+		DTRACE_SET_STATE(db, "clear data");
+	}
+}
+
+static void
+dbuf_set_data(dmu_buf_impl_t *db, arc_buf_t *buf)
+{
+	ASSERT(MUTEX_HELD(&db->db_mtx));
+	ASSERT(buf != NULL);
+
+	db->db_buf = buf;
+	ASSERT(buf->b_data != NULL);
+	db->db.db_data = buf->b_data;
+}
+
+static arc_buf_t *
+dbuf_alloc_arcbuf_from_arcbuf(dmu_buf_impl_t *db, arc_buf_t *data)
+{
+	objset_t *os = db->db_objset;
+	spa_t *spa = os->os_spa;
+	arc_buf_contents_t type = DBUF_GET_BUFC_TYPE(db);
+	enum zio_compress compress_type;
+	uint8_t complevel;
+	int psize, lsize;
+
+	psize = arc_buf_size(data);
+	lsize = arc_buf_lsize(data);
+	compress_type = arc_get_compression(data);
+	complevel = arc_get_complevel(data);
+
+	if (arc_is_encrypted(data)) {
+		boolean_t byteorder;
+		uint8_t salt[ZIO_DATA_SALT_LEN];
+		uint8_t iv[ZIO_DATA_IV_LEN];
+		uint8_t mac[ZIO_DATA_MAC_LEN];
+		dnode_t *dn = DB_DNODE(db);
+
+		arc_get_raw_params(data, &byteorder, salt, iv, mac);
+		data = arc_alloc_raw_buf(spa, db, dmu_objset_id(os),
+		    byteorder, salt, iv, mac, dn->dn_type, psize, lsize,
+		    compress_type, complevel);
+	} else if (compress_type != ZIO_COMPRESS_OFF) {
+		ASSERT3U(type, ==, ARC_BUFC_DATA);
+		data = arc_alloc_compressed_buf(spa, db,
+		    psize, lsize, compress_type, complevel);
+	} else {
+		data = arc_alloc_buf(spa, db, type, psize);
+	}
+	return (data);
+}
+
+static arc_buf_t *
+dbuf_alloc_arcbuf(dmu_buf_impl_t *db)
+{
+	spa_t *spa = db->db_objset->os_spa;
+
+	return (arc_alloc_buf(spa, db, DBUF_GET_BUFC_TYPE(db), db->db.db_size));
+}
+
+/*
+ * Loan out an arc_buf for read.  Return the loaned arc_buf.
+ */
+arc_buf_t *
+dbuf_loan_arcbuf(dmu_buf_impl_t *db)
+{
+	arc_buf_t *abuf;
+
+	ASSERT(db->db_blkid != DMU_BONUS_BLKID);
+	mutex_enter(&db->db_mtx);
+	if (arc_released(db->db_buf) || zfs_refcount_count(&db->db_holds) > 1) {
+		int blksz = db->db.db_size;
+		spa_t *spa = db->db_objset->os_spa;
+
+		mutex_exit(&db->db_mtx);
+		abuf = arc_loan_buf(spa, B_FALSE, blksz);
+		bcopy(db->db.db_data, abuf->b_data, blksz);
+	} else {
+		abuf = db->db_buf;
+		arc_loan_inuse_buf(abuf, db);
+		db->db_buf = NULL;
+		dbuf_clear_data(db);
+		mutex_exit(&db->db_mtx);
+	}
+	return (abuf);
+}
+
+/*
+ * Calculate which level n block references the data at the level 0 offset
+ * provided.
+ */
+uint64_t
+dbuf_whichblock(const dnode_t *dn, const int64_t level, const uint64_t offset)
+{
+	if (dn->dn_datablkshift != 0 && dn->dn_indblkshift != 0) {
+		/*
+		 * The level n blkid is equal to the level 0 blkid divided by
+		 * the number of level 0s in a level n block.
+		 *
+		 * The level 0 blkid is offset >> datablkshift =
+		 * offset / 2^datablkshift.
+		 *
+		 * The number of level 0s in a level n is the number of block
+		 * pointers in an indirect block, raised to the power of level.
+		 * This is 2^(indblkshift - SPA_BLKPTRSHIFT)^level =
+		 * 2^(level*(indblkshift - SPA_BLKPTRSHIFT)).
+		 *
+		 * Thus, the level n blkid is: offset /
+		 * ((2^datablkshift)*(2^(level*(indblkshift-SPA_BLKPTRSHIFT))))
+		 * = offset / 2^(datablkshift + level *
+		 *   (indblkshift - SPA_BLKPTRSHIFT))
+		 * = offset >> (datablkshift + level *
+		 *   (indblkshift - SPA_BLKPTRSHIFT))
+		 */
+
+		const unsigned exp = dn->dn_datablkshift +
+		    level * (dn->dn_indblkshift - SPA_BLKPTRSHIFT);
+
+		if (exp >= 8 * sizeof (offset)) {
+			/* This only happens on the highest indirection level */
+			ASSERT3U(level, ==, dn->dn_nlevels - 1);
+			return (0);
+		}
+
+		ASSERT3U(exp, <, 8 * sizeof (offset));
+
+		return (offset >> exp);
+	} else {
+		ASSERT3U(offset, <, dn->dn_datablksz);
+		return (0);
+	}
+}
+
+/*
+ * This function is used to lock the parent of the provided dbuf. This should be
+ * used when modifying or reading db_blkptr.
+ */
+db_lock_type_t
+dmu_buf_lock_parent(dmu_buf_impl_t *db, krw_t rw, void *tag)
+{
+	enum db_lock_type ret = DLT_NONE;
+	if (db->db_parent != NULL) {
+		rw_enter(&db->db_parent->db_rwlock, rw);
+		ret = DLT_PARENT;
+	} else if (dmu_objset_ds(db->db_objset) != NULL) {
+		rrw_enter(&dmu_objset_ds(db->db_objset)->ds_bp_rwlock, rw,
+		    tag);
+		ret = DLT_OBJSET;
+	}
+	/*
+	 * We only return a DLT_NONE lock when it's the top-most indirect block
+	 * of the meta-dnode of the MOS.
+	 */
+	return (ret);
+}
+
+/*
+ * We need to pass the lock type in because it's possible that the block will
+ * move from being the topmost indirect block in a dnode (and thus, have no
+ * parent) to not the top-most via an indirection increase. This would cause a
+ * panic if we didn't pass the lock type in.
+ */
+void
+dmu_buf_unlock_parent(dmu_buf_impl_t *db, db_lock_type_t type, void *tag)
+{
+	if (type == DLT_PARENT)
+		rw_exit(&db->db_parent->db_rwlock);
+	else if (type == DLT_OBJSET)
+		rrw_exit(&dmu_objset_ds(db->db_objset)->ds_bp_rwlock, tag);
+}
+
+static void
+dbuf_read_done(zio_t *zio, const zbookmark_phys_t *zb, const blkptr_t *bp,
+    arc_buf_t *buf, void *vdb)
+{
+	dmu_buf_impl_t *db = vdb;
+
+	mutex_enter(&db->db_mtx);
+	ASSERT3U(db->db_state, ==, DB_READ);
+	/*
+	 * All reads are synchronous, so we must have a hold on the dbuf
+	 */
+	ASSERT(zfs_refcount_count(&db->db_holds) > 0);
+	ASSERT(db->db_buf == NULL);
+	ASSERT(db->db.db_data == NULL);
+	if (buf == NULL) {
+		/* i/o error */
+		ASSERT(zio == NULL || zio->io_error != 0);
+		ASSERT(db->db_blkid != DMU_BONUS_BLKID);
+		ASSERT3P(db->db_buf, ==, NULL);
+		db->db_state = DB_UNCACHED;
+		DTRACE_SET_STATE(db, "i/o error");
+	} else if (db->db_level == 0 && db->db_freed_in_flight) {
+		/* freed in flight */
+		ASSERT(zio == NULL || zio->io_error == 0);
+		arc_release(buf, db);
+		bzero(buf->b_data, db->db.db_size);
+		arc_buf_freeze(buf);
+		db->db_freed_in_flight = FALSE;
+		dbuf_set_data(db, buf);
+		db->db_state = DB_CACHED;
+		DTRACE_SET_STATE(db, "freed in flight");
+	} else {
+		/* success */
+		ASSERT(zio == NULL || zio->io_error == 0);
+		dbuf_set_data(db, buf);
+		db->db_state = DB_CACHED;
+		DTRACE_SET_STATE(db, "successful read");
+	}
+	cv_broadcast(&db->db_changed);
+	dbuf_rele_and_unlock(db, NULL, B_FALSE);
+}
+
+/*
+ * Shortcut for performing reads on bonus dbufs.  Returns
+ * an error if we fail to verify the dnode associated with
+ * a decrypted block. Otherwise success.
+ */
+static int
+dbuf_read_bonus(dmu_buf_impl_t *db, dnode_t *dn, uint32_t flags)
+{
+	int bonuslen, max_bonuslen, err;
+
+	err = dbuf_read_verify_dnode_crypt(db, flags);
+	if (err)
+		return (err);
+
+	bonuslen = MIN(dn->dn_bonuslen, dn->dn_phys->dn_bonuslen);
+	max_bonuslen = DN_SLOTS_TO_BONUSLEN(dn->dn_num_slots);
+	ASSERT(MUTEX_HELD(&db->db_mtx));
+	ASSERT(DB_DNODE_HELD(db));
+	ASSERT3U(bonuslen, <=, db->db.db_size);
+	db->db.db_data = kmem_alloc(max_bonuslen, KM_SLEEP);
+	arc_space_consume(max_bonuslen, ARC_SPACE_BONUS);
+	if (bonuslen < max_bonuslen)
+		bzero(db->db.db_data, max_bonuslen);
+	if (bonuslen)
+		bcopy(DN_BONUS(dn->dn_phys), db->db.db_data, bonuslen);
+	db->db_state = DB_CACHED;