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Diffstat (limited to 'sys/contrib/openzfs/module/zfs/spa_checkpoint.c')
| -rw-r--r-- | sys/contrib/openzfs/module/zfs/spa_checkpoint.c | 636 |
1 files changed, 636 insertions, 0 deletions
diff --git a/sys/contrib/openzfs/module/zfs/spa_checkpoint.c b/sys/contrib/openzfs/module/zfs/spa_checkpoint.c new file mode 100644 index 000000000000..5fb614467273 --- /dev/null +++ b/sys/contrib/openzfs/module/zfs/spa_checkpoint.c @@ -0,0 +1,636 @@ +/* + * 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) 2017 by Delphix. All rights reserved. + */ + +/* + * Storage Pool Checkpoint + * + * A storage pool checkpoint can be thought of as a pool-wide snapshot or + * a stable version of extreme rewind that guarantees no blocks from the + * checkpointed state will have been overwritten. It remembers the entire + * state of the storage pool (e.g. snapshots, dataset names, etc..) from the + * point that it was taken and the user can rewind back to that point even if + * they applied destructive operations on their datasets or even enabled new + * zpool on-disk features. If a pool has a checkpoint that is no longer + * needed, the user can discard it. + * + * == On disk data structures used == + * + * - The pool has a new feature flag and a new entry in the MOS. The feature + * flag is set to active when we create the checkpoint and remains active + * until the checkpoint is fully discarded. The entry in the MOS config + * (DMU_POOL_ZPOOL_CHECKPOINT) is populated with the uberblock that + * references the state of the pool when we take the checkpoint. The entry + * remains populated until we start discarding the checkpoint or we rewind + * back to it. + * + * - Each vdev contains a vdev-wide space map while the pool has a checkpoint, + * which persists until the checkpoint is fully discarded. The space map + * contains entries that have been freed in the current state of the pool + * but we want to keep around in case we decide to rewind to the checkpoint. + * [see vdev_checkpoint_sm] + * + * - Each metaslab's ms_sm space map behaves the same as without the + * checkpoint, with the only exception being the scenario when we free + * blocks that belong to the checkpoint. In this case, these blocks remain + * ALLOCATED in the metaslab's space map and they are added as FREE in the + * vdev's checkpoint space map. + * + * - Each uberblock has a field (ub_checkpoint_txg) which holds the txg that + * the uberblock was checkpointed. For normal uberblocks this field is 0. + * + * == Overview of operations == + * + * - To create a checkpoint, we first wait for the current TXG to be synced, + * so we can use the most recently synced uberblock (spa_ubsync) as the + * checkpointed uberblock. Then we use an early synctask to place that + * uberblock in MOS config, increment the feature flag for the checkpoint + * (marking it active), and setting spa_checkpoint_txg (see its use below) + * to the TXG of the checkpointed uberblock. We use an early synctask for + * the aforementioned operations to ensure that no blocks were dirtied + * between the current TXG and the TXG of the checkpointed uberblock + * (e.g the previous txg). + * + * - When a checkpoint exists, we need to ensure that the blocks that + * belong to the checkpoint are freed but never reused. This means that + * these blocks should never end up in the ms_allocatable or the ms_freeing + * trees of a metaslab. Therefore, whenever there is a checkpoint the new + * ms_checkpointing tree is used in addition to the aforementioned ones. + * + * Whenever a block is freed and we find out that it is referenced by the + * checkpoint (we find out by comparing its birth to spa_checkpoint_txg), + * we place it in the ms_checkpointing tree instead of the ms_freeingtree. + * This way, we divide the blocks that are being freed into checkpointed + * and not-checkpointed blocks. + * + * In order to persist these frees, we write the extents from the + * ms_freeingtree to the ms_sm as usual, and the extents from the + * ms_checkpointing tree to the vdev_checkpoint_sm. This way, these + * checkpointed extents will remain allocated in the metaslab's ms_sm space + * map, and therefore won't be reused [see metaslab_sync()]. In addition, + * when we discard the checkpoint, we can find the entries that have + * actually been freed in vdev_checkpoint_sm. + * [see spa_checkpoint_discard_thread_sync()] + * + * - To discard the checkpoint we use an early synctask to delete the + * checkpointed uberblock from the MOS config, set spa_checkpoint_txg to 0, + * and wakeup the discarding zthr thread (an open-context async thread). + * We use an early synctask to ensure that the operation happens before any + * new data end up in the checkpoint's data structures. + * + * Once the synctask is done and the discarding zthr is awake, we discard + * the checkpointed data over multiple TXGs by having the zthr prefetching + * entries from vdev_checkpoint_sm and then starting a synctask that places + * them as free blocks into their respective ms_allocatable and ms_sm + * structures. + * [see spa_checkpoint_discard_thread()] + * + * When there are no entries left in the vdev_checkpoint_sm of all + * top-level vdevs, a final synctask runs that decrements the feature flag. + * + * - To rewind to the checkpoint, we first use the current uberblock and + * open the MOS so we can access the checkpointed uberblock from the MOS + * config. After we retrieve the checkpointed uberblock, we use it as the + * current uberblock for the pool by writing it to disk with an updated + * TXG, opening its version of the MOS, and moving on as usual from there. + * [see spa_ld_checkpoint_rewind()] + * + * An important note on rewinding to the checkpoint has to do with how we + * handle ZIL blocks. In the scenario of a rewind, we clear out any ZIL + * blocks that have not been claimed by the time we took the checkpoint + * as they should no longer be valid. + * [see comment in zil_claim()] + * + * == Miscellaneous information == + * + * - In the hypothetical event that we take a checkpoint, remove a vdev, + * and attempt to rewind, the rewind would fail as the checkpointed + * uberblock would reference data in the removed device. For this reason + * and others of similar nature, we disallow the following operations that + * can change the config: + * vdev removal and attach/detach, mirror splitting, and pool reguid. + * + * - As most of the checkpoint logic is implemented in the SPA and doesn't + * distinguish datasets when it comes to space accounting, having a + * checkpoint can potentially break the boundaries set by dataset + * reservations. + */ + +#include <sys/dmu_tx.h> +#include <sys/dsl_dir.h> +#include <sys/dsl_synctask.h> +#include <sys/metaslab_impl.h> +#include <sys/spa.h> +#include <sys/spa_impl.h> +#include <sys/spa_checkpoint.h> +#include <sys/vdev_impl.h> +#include <sys/zap.h> +#include <sys/zfeature.h> + +/* + * The following parameter limits the amount of memory to be used for the + * prefetching of the checkpoint space map done on each vdev while + * discarding the checkpoint. + * + * The reason it exists is because top-level vdevs with long checkpoint + * space maps can potentially take up a lot of memory depending on the + * amount of checkpointed data that has been freed within them while + * the pool had a checkpoint. + */ +unsigned long zfs_spa_discard_memory_limit = 16 * 1024 * 1024; + +int +spa_checkpoint_get_stats(spa_t *spa, pool_checkpoint_stat_t *pcs) +{ + if (!spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT)) + return (SET_ERROR(ZFS_ERR_NO_CHECKPOINT)); + + bzero(pcs, sizeof (pool_checkpoint_stat_t)); + + int error = zap_contains(spa_meta_objset(spa), + DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ZPOOL_CHECKPOINT); + ASSERT(error == 0 || error == ENOENT); + + if (error == ENOENT) + pcs->pcs_state = CS_CHECKPOINT_DISCARDING; + else + pcs->pcs_state = CS_CHECKPOINT_EXISTS; + + pcs->pcs_space = spa->spa_checkpoint_info.sci_dspace; + pcs->pcs_start_time = spa->spa_checkpoint_info.sci_timestamp; + + return (0); +} + +static void +spa_checkpoint_discard_complete_sync(void *arg, dmu_tx_t *tx) +{ + spa_t *spa = arg; + + spa->spa_checkpoint_info.sci_timestamp = 0; + + spa_feature_decr(spa, SPA_FEATURE_POOL_CHECKPOINT, tx); + spa_notify_waiters(spa); + + spa_history_log_internal(spa, "spa discard checkpoint", tx, + "finished discarding checkpointed state from the pool"); +} + +typedef struct spa_checkpoint_discard_sync_callback_arg { + vdev_t *sdc_vd; + uint64_t sdc_txg; + uint64_t sdc_entry_limit; +} spa_checkpoint_discard_sync_callback_arg_t; + +static int +spa_checkpoint_discard_sync_callback(space_map_entry_t *sme, void *arg) +{ + spa_checkpoint_discard_sync_callback_arg_t *sdc = arg; + vdev_t *vd = sdc->sdc_vd; + metaslab_t *ms = vd->vdev_ms[sme->sme_offset >> vd->vdev_ms_shift]; + uint64_t end = sme->sme_offset + sme->sme_run; + + if (sdc->sdc_entry_limit == 0) + return (SET_ERROR(EINTR)); + + /* + * Since the space map is not condensed, we know that + * none of its entries is crossing the boundaries of + * its respective metaslab. + * + * That said, there is no fundamental requirement that + * the checkpoint's space map entries should not cross + * metaslab boundaries. So if needed we could add code + * that handles metaslab-crossing segments in the future. + */ + VERIFY3U(sme->sme_type, ==, SM_FREE); + VERIFY3U(sme->sme_offset, >=, ms->ms_start); + VERIFY3U(end, <=, ms->ms_start + ms->ms_size); + + /* + * At this point we should not be processing any + * other frees concurrently, so the lock is technically + * unnecessary. We use the lock anyway though to + * potentially save ourselves from future headaches. + */ + mutex_enter(&ms->ms_lock); + if (range_tree_is_empty(ms->ms_freeing)) + vdev_dirty(vd, VDD_METASLAB, ms, sdc->sdc_txg); + range_tree_add(ms->ms_freeing, sme->sme_offset, sme->sme_run); + mutex_exit(&ms->ms_lock); + + ASSERT3U(vd->vdev_spa->spa_checkpoint_info.sci_dspace, >=, + sme->sme_run); + ASSERT3U(vd->vdev_stat.vs_checkpoint_space, >=, sme->sme_run); + + vd->vdev_spa->spa_checkpoint_info.sci_dspace -= sme->sme_run; + vd->vdev_stat.vs_checkpoint_space -= sme->sme_run; + sdc->sdc_entry_limit--; + + return (0); +} + +#ifdef ZFS_DEBUG +static void +spa_checkpoint_accounting_verify(spa_t *spa) +{ + vdev_t *rvd = spa->spa_root_vdev; + uint64_t ckpoint_sm_space_sum = 0; + uint64_t vs_ckpoint_space_sum = 0; + + for (uint64_t c = 0; c < rvd->vdev_children; c++) { + vdev_t *vd = rvd->vdev_child[c]; + + if (vd->vdev_checkpoint_sm != NULL) { + ckpoint_sm_space_sum += + -space_map_allocated(vd->vdev_checkpoint_sm); + vs_ckpoint_space_sum += + vd->vdev_stat.vs_checkpoint_space; + ASSERT3U(ckpoint_sm_space_sum, ==, + vs_ckpoint_space_sum); + } else { + ASSERT0(vd->vdev_stat.vs_checkpoint_space); + } + } + ASSERT3U(spa->spa_checkpoint_info.sci_dspace, ==, ckpoint_sm_space_sum); +} +#endif + +static void +spa_checkpoint_discard_thread_sync(void *arg, dmu_tx_t *tx) +{ + vdev_t *vd = arg; + int error; + + /* + * The space map callback is applied only to non-debug entries. + * Because the number of debug entries is less or equal to the + * number of non-debug entries, we want to ensure that we only + * read what we prefetched from open-context. + * + * Thus, we set the maximum entries that the space map callback + * will be applied to be half the entries that could fit in the + * imposed memory limit. + * + * Note that since this is a conservative estimate we also + * assume the worst case scenario in our computation where each + * entry is two-word. + */ + uint64_t max_entry_limit = + (zfs_spa_discard_memory_limit / (2 * sizeof (uint64_t))) >> 1; + + /* + * Iterate from the end of the space map towards the beginning, + * placing its entries on ms_freeing and removing them from the + * space map. The iteration stops if one of the following + * conditions is true: + * + * 1] We reached the beginning of the space map. At this point + * the space map should be completely empty and + * space_map_incremental_destroy should have returned 0. + * The next step would be to free and close the space map + * and remove its entry from its vdev's top zap. This allows + * spa_checkpoint_discard_thread() to move on to the next vdev. + * + * 2] We reached the memory limit (amount of memory used to hold + * space map entries in memory) and space_map_incremental_destroy + * returned EINTR. This means that there are entries remaining + * in the space map that will be cleared in a future invocation + * of this function by spa_checkpoint_discard_thread(). + */ + spa_checkpoint_discard_sync_callback_arg_t sdc; + sdc.sdc_vd = vd; + sdc.sdc_txg = tx->tx_txg; + sdc.sdc_entry_limit = max_entry_limit; + + uint64_t words_before = + space_map_length(vd->vdev_checkpoint_sm) / sizeof (uint64_t); + + error = space_map_incremental_destroy(vd->vdev_checkpoint_sm, + spa_checkpoint_discard_sync_callback, &sdc, tx); + + uint64_t words_after = + space_map_length(vd->vdev_checkpoint_sm) / sizeof (uint64_t); + +#ifdef ZFS_DEBUG + spa_checkpoint_accounting_verify(vd->vdev_spa); +#endif + + zfs_dbgmsg("discarding checkpoint: txg %llu, vdev id %d, " + "deleted %llu words - %llu words are left", + tx->tx_txg, vd->vdev_id, (words_before - words_after), + words_after); + + if (error != EINTR) { + if (error != 0) { + zfs_panic_recover("zfs: error %d was returned " + "while incrementally destroying the checkpoint " + "space map of vdev %llu\n", + error, vd->vdev_id); + } + ASSERT0(words_after); + ASSERT0(space_map_allocated(vd->vdev_checkpoint_sm)); + ASSERT0(space_map_length(vd->vdev_checkpoint_sm)); + + space_map_free(vd->vdev_checkpoint_sm, tx); + space_map_close(vd->vdev_checkpoint_sm); + vd->vdev_checkpoint_sm = NULL; + + VERIFY0(zap_remove(spa_meta_objset(vd->vdev_spa), + vd->vdev_top_zap, VDEV_TOP_ZAP_POOL_CHECKPOINT_SM, tx)); + } +} + +static boolean_t +spa_checkpoint_discard_is_done(spa_t *spa) +{ + vdev_t *rvd = spa->spa_root_vdev; + + ASSERT(!spa_has_checkpoint(spa)); + ASSERT(spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT)); + + for (uint64_t c = 0; c < rvd->vdev_children; c++) { + if (rvd->vdev_child[c]->vdev_checkpoint_sm != NULL) + return (B_FALSE); + ASSERT0(rvd->vdev_child[c]->vdev_stat.vs_checkpoint_space); + } + + return (B_TRUE); +} + +/* ARGSUSED */ +boolean_t +spa_checkpoint_discard_thread_check(void *arg, zthr_t *zthr) +{ + spa_t *spa = arg; + + if (!spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT)) + return (B_FALSE); + + if (spa_has_checkpoint(spa)) + return (B_FALSE); + + return (B_TRUE); +} + +void +spa_checkpoint_discard_thread(void *arg, zthr_t *zthr) +{ + spa_t *spa = arg; + vdev_t *rvd = spa->spa_root_vdev; + + for (uint64_t c = 0; c < rvd->vdev_children; c++) { + vdev_t *vd = rvd->vdev_child[c]; + + while (vd->vdev_checkpoint_sm != NULL) { + space_map_t *checkpoint_sm = vd->vdev_checkpoint_sm; + int numbufs; + dmu_buf_t **dbp; + + if (zthr_iscancelled(zthr)) + return; + + ASSERT3P(vd->vdev_ops, !=, &vdev_indirect_ops); + + uint64_t size = MIN(space_map_length(checkpoint_sm), + zfs_spa_discard_memory_limit); + uint64_t offset = + space_map_length(checkpoint_sm) - size; + + /* + * Ensure that the part of the space map that will + * be destroyed by the synctask, is prefetched in + * memory before the synctask runs. + */ + int error = dmu_buf_hold_array_by_bonus( + checkpoint_sm->sm_dbuf, offset, size, + B_TRUE, FTAG, &numbufs, &dbp); + if (error != 0) { + zfs_panic_recover("zfs: error %d was returned " + "while prefetching checkpoint space map " + "entries of vdev %llu\n", + error, vd->vdev_id); + } + + VERIFY0(dsl_sync_task(spa->spa_name, NULL, + spa_checkpoint_discard_thread_sync, vd, + 0, ZFS_SPACE_CHECK_NONE)); + + dmu_buf_rele_array(dbp, numbufs, FTAG); + } + } + + VERIFY(spa_checkpoint_discard_is_done(spa)); + VERIFY0(spa->spa_checkpoint_info.sci_dspace); + VERIFY0(dsl_sync_task(spa->spa_name, NULL, + spa_checkpoint_discard_complete_sync, spa, + 0, ZFS_SPACE_CHECK_NONE)); +} + + +/* ARGSUSED */ +static int +spa_checkpoint_check(void *arg, dmu_tx_t *tx) +{ + spa_t *spa = dmu_tx_pool(tx)->dp_spa; + + if (!spa_feature_is_enabled(spa, SPA_FEATURE_POOL_CHECKPOINT)) + return (SET_ERROR(ENOTSUP)); + + if (!spa_top_vdevs_spacemap_addressable(spa)) + return (SET_ERROR(ZFS_ERR_VDEV_TOO_BIG)); + + if (spa->spa_removing_phys.sr_state == DSS_SCANNING) + return (SET_ERROR(ZFS_ERR_DEVRM_IN_PROGRESS)); + + if (spa->spa_checkpoint_txg != 0) + return (SET_ERROR(ZFS_ERR_CHECKPOINT_EXISTS)); + + if (spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT)) + return (SET_ERROR(ZFS_ERR_DISCARDING_CHECKPOINT)); + + return (0); +} + +/* ARGSUSED */ +static void +spa_checkpoint_sync(void *arg, dmu_tx_t *tx) +{ + dsl_pool_t *dp = dmu_tx_pool(tx); + spa_t *spa = dp->dp_spa; + uberblock_t checkpoint = spa->spa_ubsync; + + /* + * At this point, there should not be a checkpoint in the MOS. + */ + ASSERT3U(zap_contains(spa_meta_objset(spa), DMU_POOL_DIRECTORY_OBJECT, + DMU_POOL_ZPOOL_CHECKPOINT), ==, ENOENT); + + ASSERT0(spa->spa_checkpoint_info.sci_timestamp); + ASSERT0(spa->spa_checkpoint_info.sci_dspace); + + /* + * Since the checkpointed uberblock is the one that just got synced + * (we use spa_ubsync), its txg must be equal to the txg number of + * the txg we are syncing, minus 1. + */ + ASSERT3U(checkpoint.ub_txg, ==, spa->spa_syncing_txg - 1); + + /* + * Once the checkpoint is in place, we need to ensure that none of + * its blocks will be marked for reuse after it has been freed. + * When there is a checkpoint and a block is freed, we compare its + * birth txg to the txg of the checkpointed uberblock to see if the + * block is part of the checkpoint or not. Therefore, we have to set + * spa_checkpoint_txg before any frees happen in this txg (which is + * why this is done as an early_synctask as explained in the comment + * in spa_checkpoint()). + */ + spa->spa_checkpoint_txg = checkpoint.ub_txg; + spa->spa_checkpoint_info.sci_timestamp = checkpoint.ub_timestamp; + + checkpoint.ub_checkpoint_txg = checkpoint.ub_txg; + VERIFY0(zap_add(spa->spa_dsl_pool->dp_meta_objset, + DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ZPOOL_CHECKPOINT, + sizeof (uint64_t), sizeof (uberblock_t) / sizeof (uint64_t), + &checkpoint, tx)); + + /* + * Increment the feature refcount and thus activate the feature. + * Note that the feature will be deactivated when we've + * completely discarded all checkpointed state (both vdev + * space maps and uberblock). + */ + spa_feature_incr(spa, SPA_FEATURE_POOL_CHECKPOINT, tx); + + spa_history_log_internal(spa, "spa checkpoint", tx, + "checkpointed uberblock txg=%llu", (u_longlong_t)checkpoint.ub_txg); +} + +/* + * Create a checkpoint for the pool. + */ +int +spa_checkpoint(const char *pool) +{ + int error; + spa_t *spa; + + error = spa_open(pool, &spa, FTAG); + if (error != 0) + return (error); + + mutex_enter(&spa->spa_vdev_top_lock); + + /* + * Wait for current syncing txg to finish so the latest synced + * uberblock (spa_ubsync) has all the changes that we expect + * to see if we were to revert later to the checkpoint. In other + * words we want the checkpointed uberblock to include/reference + * all the changes that were pending at the time that we issued + * the checkpoint command. + */ + txg_wait_synced(spa_get_dsl(spa), 0); + + /* + * As the checkpointed uberblock references blocks from the previous + * txg (spa_ubsync) we want to ensure that are not freeing any of + * these blocks in the same txg that the following synctask will + * run. Thus, we run it as an early synctask, so the dirty changes + * that are synced to disk afterwards during zios and other synctasks + * do not reuse checkpointed blocks. + */ + error = dsl_early_sync_task(pool, spa_checkpoint_check, + spa_checkpoint_sync, NULL, 0, ZFS_SPACE_CHECK_NORMAL); + + mutex_exit(&spa->spa_vdev_top_lock); + + spa_close(spa, FTAG); + return (error); +} + +/* ARGSUSED */ +static int +spa_checkpoint_discard_check(void *arg, dmu_tx_t *tx) +{ + spa_t *spa = dmu_tx_pool(tx)->dp_spa; + + if (!spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT)) + return (SET_ERROR(ZFS_ERR_NO_CHECKPOINT)); + + if (spa->spa_checkpoint_txg == 0) + return (SET_ERROR(ZFS_ERR_DISCARDING_CHECKPOINT)); + + VERIFY0(zap_contains(spa_meta_objset(spa), + DMU_POOL_DIRECTORY_OBJECT, DMU_POOL_ZPOOL_CHECKPOINT)); + + return (0); +} + +/* ARGSUSED */ +static void +spa_checkpoint_discard_sync(void *arg, dmu_tx_t *tx) +{ + spa_t *spa = dmu_tx_pool(tx)->dp_spa; + + VERIFY0(zap_remove(spa_meta_objset(spa), DMU_POOL_DIRECTORY_OBJECT, + DMU_POOL_ZPOOL_CHECKPOINT, tx)); + + spa->spa_checkpoint_txg = 0; + + zthr_wakeup(spa->spa_checkpoint_discard_zthr); + + spa_history_log_internal(spa, "spa discard checkpoint", tx, + "started discarding checkpointed state from the pool"); +} + +/* + * Discard the checkpoint from a pool. + */ +int +spa_checkpoint_discard(const char *pool) +{ + /* + * Similarly to spa_checkpoint(), we want our synctask to run + * before any pending dirty data are written to disk so they + * won't end up in the checkpoint's data structures (e.g. + * ms_checkpointing and vdev_checkpoint_sm) and re-create any + * space maps that the discarding open-context thread has + * deleted. + * [see spa_discard_checkpoint_sync and spa_discard_checkpoint_thread] + */ + return (dsl_early_sync_task(pool, spa_checkpoint_discard_check, + spa_checkpoint_discard_sync, NULL, 0, + ZFS_SPACE_CHECK_DISCARD_CHECKPOINT)); +} + +EXPORT_SYMBOL(spa_checkpoint_get_stats); +EXPORT_SYMBOL(spa_checkpoint_discard_thread); +EXPORT_SYMBOL(spa_checkpoint_discard_thread_check); + +/* BEGIN CSTYLED */ +ZFS_MODULE_PARAM(zfs_spa, zfs_spa_, discard_memory_limit, ULONG, ZMOD_RW, + "Limit for memory used in prefetching the checkpoint space map done " + "on each vdev while discarding the checkpoint"); +/* END CSTYLED */ |