git: b5882a38ef - main - Add a section about Zstandard compression to the ZFS handbook

The branch main has been updated by allanjude:

URL: https://cgit.FreeBSD.org/doc/commit/?id=b5882a38ef4fefccf1f74b77947a35d276d25958

commit b5882a38ef4fefccf1f74b77947a35d276d25958
Author:     Allan Jude <[email protected]>
AuthorDate: 2020-12-22 03:23:36 +0000
Commit:     Allan Jude <[email protected]>
CommitDate: 2020-12-30 18:47:57 +0000

    Add a section about Zstandard compression to the ZFS handbook
    
    Reviewed by:    emaste, ygy, bcr, debdrup, [email protected]
    Sponsored by:   The FreeBSD Foundation
    Differential Revision:  https://reviews.freebsd.org/D27715
---
 en_US.ISO8859-1/books/handbook/zfs/chapter.xml | 66 ++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)

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
 	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">
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