Differential D27848 Diff 81411 en_US.ISO8859-1/books/handbook/zfs/chapter.xml

Changeset View

Standalone View

en_US.ISO8859-1/books/handbook/zfs/chapter.xml

Show First 20 Lines • Show All 2,922 Lines • ▼ Show 20 Lines	compression caused them to reach their quota limit.</para>
<para>Compression can have a similar unexpected interaction with		<para>Compression can have a similar unexpected interaction with
backups. Quotas are often used to limit how much data can be		backups. Quotas are often used to limit how much data can be
stored to ensure there is sufficient backup space available.		stored to ensure there is sufficient backup space available.
However since quotas do not consider compression, more data		However since quotas do not consider compression, more data
may be written than would fit with uncompressed		may be written than would fit with uncompressed
backups.</para>		backups.</para>
</sect2>		</sect2>

		<sect2 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; 12.1-RELEASE via
		<package>sysutils/openzfs</package> and has been the
		default in &os; 13-CURRENT since September 2020, and
		will by in &os; 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 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>
		</sect2>

<sect2 xml:id="zfs-zfs-deduplication">		<sect2 xml:id="zfs-zfs-deduplication">
<title>Deduplication</title>		<title>Deduplication</title>

<para>When enabled,		<para>When enabled,
<link linkend="zfs-term-deduplication">deduplication</link>		<link linkend="zfs-term-deduplication">deduplication</link>
uses the checksum of each block to detect duplicate blocks.		uses the checksum of each block to detect duplicate blocks.
When a new block is a duplicate of an existing block,		When a new block is a duplicate of an existing block,
<acronym>ZFS</acronym> writes an additional reference to the		<acronym>ZFS</acronym> writes an additional reference to the
▲ Show 20 Lines • Show All 1,420 Lines • Show Last 20 Lines