path: root/en_US.ISO8859-1
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authorAllan Jude <allanjude@FreeBSD.org>2020-12-22 03:23:36 +0000
committerAllan Jude <allanjude@FreeBSD.org>2020-12-30 18:47:57 +0000
commitb5882a38ef4fefccf1f74b77947a35d276d25958 (patch)
tree15f0008f8701cee46ced3f6461418a0e35a5d502 /en_US.ISO8859-1
parente9a6e6f104b7706ad46bc7b3d88536d3e57902f7 (diff)
Add a section about Zstandard compression to the ZFS handbook
Reviewed by: emaste, ygy, bcr, debdrup, pauamma@gundo.com Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D27715
Diffstat (limited to 'en_US.ISO8859-1')
1 files changed, 66 insertions, 0 deletions
diff --git a/en_US.ISO8859-1/books/handbook/zfs/chapter.xml b/en_US.ISO8859-1/books/handbook/zfs/chapter.xml
index 4bc0031fa0..547cff76e6 100644
--- a/en_US.ISO8859-1/books/handbook/zfs/chapter.xml
+++ b/en_US.ISO8859-1/books/handbook/zfs/chapter.xml
@@ -2926,6 +2926,72 @@ mypool/compressed_dataset logicalused 496G -</screen>
However since quotas do not consider compression, more data
may be written than would fit with uncompressed
+ <sect3 xml:id="zfs-zfs-compression-zstd">
+ <title>Zstandard Compression</title>
+ <para>In <acronym>OpenZFS</acronym> 2.0, a new compression
+ algorithm was added. Zstandard (<acronym>Zstd</acronym>)
+ offers higher compression ratios than the default
+ <acronym>LZ4</acronym> while offering much greater speeds
+ than the alternative, <acronym>gzip</acronym>.
+ <acronym>OpenZFS</acronym> 2.0 is available starting with
+ &os;&nbsp;12.1-RELEASE via
+ <package>sysutils/openzfs</package> and has been the
+ default in &os;&nbsp;13-CURRENT since September 2020, and
+ will by in &os;&nbsp;13.0-RELEASE.</para>
+ <para><acronym>Zstd</acronym> provides a large selection of
+ compression levels, providing fine-grained control over
+ performance versus compression ratio. One of the main
+ advantages of <acronym>Zstd</acronym> is that the
+ decompression speed is independent of the compression
+ level. For data that is written once but read many times,
+ <acronym>Zstd</acronym> allows the use of the highest
+ compression levels without a read performance
+ penalty.</para>
+ <para>Even when data is updated frequently, there are often
+ performance gains that come from enabling compression. One
+ of the biggest advantages comes from the compressed ARC
+ feature. <acronym>ZFS</acronym>'s Adaptive Replacement
+ Cache (<acronym>ARC</acronym>) caches the compressed version
+ of the data in <acronym>RAM</acronym>, decompressing it each
+ time it is needed. This allows the same amount of
+ <acronym>RAM</acronym> to store more data and metadata,
+ increasing the cache hit ratio.</para>
+ <para><acronym>ZFS</acronym> offers 19 levels of
+ <acronym>Zstd</acronym> compression, each offering
+ incrementally more space savings in exchange for slower
+ compression. The default level is
+ <literal>zstd-3</literal> and offers greater compression
+ than <acronym>LZ4</acronym> without being significantly
+ slower. Levels above 10 require significant amounts of
+ memory to compress each block, so they are discouraged on
+ systems with less than 16&nbsp;GB of <acronym>RAM</acronym>.
+ <acronym>ZFS</acronym> also implements a selection of the
+ <acronym>Zstd</acronym> <emphasis>fast</emphasis> levels,
+ which get correspondingly faster but offer lower
+ compression ratios. <acronym>ZFS</acronym> supports
+ <literal>zstd-fast-1</literal> through
+ <literal>zstd-fast-10</literal>,
+ <literal>zstd-fast-20</literal> through
+ <literal>zstd-fast-100</literal> in increments of 10, and
+ finally <literal>zstd-fast-500</literal> and
+ <literal>zstd-fast-1000</literal> which provide minimal
+ compression, but offer very high performance.</para>
+ <para>If ZFS is not able to allocate the required memory to
+ compress a block with <acronym>Zstd</acronym>, it will fall
+ back to storing the block uncompressed. This is unlikely
+ to happen outside of the highest levels of
+ <acronym>Zstd<acronym> on systems that are memory
+ constrained. The sysctl
+ <literal>kstat.zfs.misc.zstd.compress_alloc_fail</literal>
+ counts how many times this has occurred since the
+ <acronym>ZFS</acronym> module was loaded.</para>
+ </sect3>
<sect2 xml:id="zfs-zfs-deduplication">