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Add a section about Zstandard compression to the ZFS handbook

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Reviewed by:	emaste, ygy, bcr, debdrup, pauamma@gundo.com
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D27715
This commit is contained in:
Allan Jude 2020-12-22 03:23:36 +00:00
parent e9a6e6f104
commit b5882a38ef
1 changed files with 66 additions and 0 deletions

66
en_US.ISO8859-1/books/handbook/zfs/chapter.xml
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@ -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
backups.</para>
<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>
<sect2 xml:id="zfs-zfs-deduplication">