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<chapter xmlns="http://docbook.org/ns/docbook"
  xmlns:xlink="http://www.w3.org/1999/xlink" version="5.0"
  xml:id="zfs">

  <info>
    <title>The Z File System (<acronym>ZFS</acronym>)</title>

    <authorgroup>
      <author>
	<personname>
	  <firstname>Tom</firstname>
	  <surname>Rhodes</surname>
	</personname>
	<contrib>Written by </contrib>
      </author>
      <author>
	<personname>
	  <firstname>Allan</firstname>
	  <surname>Jude</surname>
	</personname>
	<contrib>Written by </contrib>
      </author>
      <author>
	<personname>
	  <firstname>Benedict</firstname>
	  <surname>Reuschling</surname>
	</personname>
	<contrib>Written by </contrib>
      </author>
      <author>
	<personname>
	  <firstname>Warren</firstname>
	  <surname>Block</surname>
	</personname>
	<contrib>Written by </contrib>
      </author>
    </authorgroup>
  </info>

  <para>The <emphasis>Z File System</emphasis>, or
    <acronym>ZFS</acronym>, is an advanced file system designed to
    overcome many of the major problems found in previous
    designs.</para>

  <para>Originally developed at &sun;, ongoing open source
    <acronym>ZFS</acronym> development has moved to the <link
      xlink:href="http://open-zfs.org">OpenZFS Project</link>.</para>

  <para><acronym>ZFS</acronym> has three major design goals:</para>

  <itemizedlist>
    <listitem>
      <para>Data integrity: All data includes a
	<link linkend="zfs-term-checksum">checksum</link> of the data.
	When data is written, the checksum is calculated and written
	along with it.  When that data is later read back, the
	checksum is calculated again.  If the checksums do not match,
	a data error has been detected.  <acronym>ZFS</acronym> will
	attempt to automatically correct errors when data redundancy
	is available.</para>
    </listitem>

    <listitem>
      <para>Pooled storage: physical storage devices are added to a
	pool, and storage space is allocated from that shared pool.
	Space is available to all file systems, and can be increased
	by adding new storage devices to the pool.</para>
    </listitem>

    <listitem>
      <para>Performance: multiple caching mechanisms provide increased
	performance.  <link linkend="zfs-term-arc">ARC</link> is an
	advanced memory-based read cache.  A second level of
	disk-based read cache can be added with
	<link linkend="zfs-term-l2arc">L2ARC</link>, and disk-based
	synchronous write cache is available with
	<link linkend="zfs-term-zil">ZIL</link>.</para>
    </listitem>
  </itemizedlist>

  <para>A complete list of features and terminology is shown in
    <xref linkend="zfs-term"/>.</para>

  <sect1 xml:id="zfs-differences">
    <title>What Makes <acronym>ZFS</acronym> Different</title>

    <para><acronym>ZFS</acronym> is significantly different from any
      previous file system because it is more than just a file system.
      Combining the traditionally separate roles of volume manager and
      file system provides <acronym>ZFS</acronym> with unique
      advantages.  The file system is now aware of the underlying
      structure of the disks.  Traditional file systems could only be
      created on a single disk at a time.  If there were two disks
      then two separate file systems would have to be created.  In a
      traditional hardware <acronym>RAID</acronym> configuration, this
      problem was avoided by presenting the operating system with a
      single logical disk made up of the space provided by a number of
      physical disks, on top of which the operating system placed a
      file system.  Even in the case of software
      <acronym>RAID</acronym> solutions like those provided by
      <acronym>GEOM</acronym>, the <acronym>UFS</acronym> file system
      living on top of the <acronym>RAID</acronym> transform believed
      that it was dealing with a single device.
      <acronym>ZFS</acronym>'s combination of the volume manager and
      the file system solves this and allows the creation of many file
      systems all sharing a pool of available storage.  One of the
      biggest advantages to <acronym>ZFS</acronym>'s awareness of the
      physical layout of the disks is that existing file systems can
      be grown automatically when additional disks are added to the
      pool.  This new space is then made available to all of the file
      systems.  <acronym>ZFS</acronym> also has a number of different
      properties that can be applied to each file system, giving many
      advantages to creating a number of different file systems and
      datasets rather than a single monolithic file system.</para>
  </sect1>

  <sect1 xml:id="zfs-quickstart">
    <title>Quick Start Guide</title>

    <para>There is a startup mechanism that allows &os; to mount
      <acronym>ZFS</acronym> pools during system initialization.  To
      enable it, add this line to
      <filename>/etc/rc.conf</filename>:</para>

    <programlisting>zfs_enable="YES"</programlisting>

    <para>Then start the service:</para>

    <screen>&prompt.root; <userinput>service zfs start</userinput></screen>

    <para>The examples in this section assume three
      <acronym>SCSI</acronym> disks with the device names
      <filename><replaceable>da0</replaceable></filename>,
      <filename><replaceable>da1</replaceable></filename>, and
      <filename><replaceable>da2</replaceable></filename>.  Users
      of <acronym>SATA</acronym> hardware should instead use
      <filename><replaceable>ada</replaceable></filename> device
      names.</para>

    <sect2 xml:id="zfs-quickstart-single-disk-pool">
      <title>Single Disk Pool</title>

      <para>To create a simple, non-redundant pool using a single
	disk device:</para>

      <screen>&prompt.root; <userinput>zpool create <replaceable>example</replaceable> <replaceable>/dev/da0</replaceable></userinput></screen>

      <para>To view the new pool, review the output of
	<command>df</command>:</para>

      <screen>&prompt.root; <userinput>df</userinput>
Filesystem  1K-blocks    Used    Avail Capacity  Mounted on
/dev/ad0s1a   2026030  235230  1628718    13%    /
devfs               1       1        0   100%    /dev
/dev/ad0s1d  54098308 1032846 48737598     2%    /usr
example      17547136       0 17547136     0%    /example</screen>

      <para>This output shows that the <literal>example</literal> pool
	has been created and mounted.  It is now accessible as a file
	system.  Files can be created on it and users can browse
	it:</para>

      <screen>&prompt.root; <userinput>cd /example</userinput>
&prompt.root; <userinput>ls</userinput>
&prompt.root; <userinput>touch testfile</userinput>
&prompt.root; <userinput>ls -al</userinput>
total 4
drwxr-xr-x   2 root  wheel    3 Aug 29 23:15 .
drwxr-xr-x  21 root  wheel  512 Aug 29 23:12 ..
-rw-r--r--   1 root  wheel    0 Aug 29 23:15 testfile</screen>

      <para>However, this pool is not taking advantage of any
	<acronym>ZFS</acronym> features.  To create a dataset on this
	pool with compression enabled:</para>

      <screen>&prompt.root; <userinput>zfs create example/compressed</userinput>
&prompt.root; <userinput>zfs set compression=gzip example/compressed</userinput></screen>

      <para>The <literal>example/compressed</literal> dataset is now a
	<acronym>ZFS</acronym> compressed file system.  Try copying
	some large files to
	<filename>/example/compressed</filename>.</para>

      <para>Compression can be disabled with:</para>

      <screen>&prompt.root; <userinput>zfs set compression=off example/compressed</userinput></screen>

      <para>To unmount a file system, use
	<command>zfs umount</command> and then verify with
	<command>df</command>:</para>

      <screen>&prompt.root; <userinput>zfs umount example/compressed</userinput>
&prompt.root; <userinput>df</userinput>
Filesystem  1K-blocks    Used    Avail Capacity  Mounted on
/dev/ad0s1a   2026030  235232  1628716    13%    /
devfs               1       1        0   100%    /dev
/dev/ad0s1d  54098308 1032864 48737580     2%    /usr
example      17547008       0 17547008     0%    /example</screen>

      <para>To re-mount the file system to make it accessible again,
	use <command>zfs mount</command> and verify with
	<command>df</command>:</para>

      <screen>&prompt.root; <userinput>zfs mount example/compressed</userinput>
&prompt.root; <userinput>df</userinput>
Filesystem         1K-blocks    Used    Avail Capacity  Mounted on
/dev/ad0s1a          2026030  235234  1628714    13%    /
devfs                      1       1        0   100%    /dev
/dev/ad0s1d         54098308 1032864 48737580     2%    /usr
example             17547008       0 17547008     0%    /example
example/compressed  17547008       0 17547008     0%    /example/compressed</screen>

      <para>The pool and file system may also be observed by viewing
	the output from <command>mount</command>:</para>

      <screen>&prompt.root; <userinput>mount</userinput>
/dev/ad0s1a on / (ufs, local)
devfs on /dev (devfs, local)
/dev/ad0s1d on /usr (ufs, local, soft-updates)
example on /example (zfs, local)
example/compressed on /example/compressed (zfs, local)</screen>

      <para>After creation, <acronym>ZFS</acronym> datasets can be
	used like any file systems.  However, many other features are
	available which can be set on a per-dataset basis.  In the
	example below, a new file system called
	<literal>data</literal> is created.  Important files will be
	stored here, so it is configured to keep two copies of each
	data block:</para>

      <screen>&prompt.root; <userinput>zfs create example/data</userinput>
&prompt.root; <userinput>zfs set copies=2 example/data</userinput></screen>

      <para>It is now possible to see the data and space utilization
	by issuing <command>df</command>:</para>

      <screen>&prompt.root; <userinput>df</userinput>
Filesystem         1K-blocks    Used    Avail Capacity  Mounted on
/dev/ad0s1a          2026030  235234  1628714    13%    /
devfs                      1       1        0   100%    /dev
/dev/ad0s1d         54098308 1032864 48737580     2%    /usr
example             17547008       0 17547008     0%    /example
example/compressed  17547008       0 17547008     0%    /example/compressed
example/data        17547008       0 17547008     0%    /example/data</screen>

      <para>Notice that each file system on the pool has the same
	amount of available space.  This is the reason for using
	<command>df</command> in these examples, to show that the file
	systems use only the amount of space they need and all draw
	from the same pool.  <acronym>ZFS</acronym> eliminates
	concepts such as volumes and partitions, and allows multiple
	file systems to occupy the same pool.</para>

      <para>To destroy the file systems and then destroy the pool as
	it is no longer needed:</para>

      <screen>&prompt.root; <userinput>zfs destroy example/compressed</userinput>
&prompt.root; <userinput>zfs destroy example/data</userinput>
&prompt.root; <userinput>zpool destroy example</userinput></screen>
    </sect2>

    <sect2 xml:id="zfs-quickstart-raid-z">
      <title>RAID-Z</title>

      <para>Disks fail.  One method of avoiding data loss from disk
	failure is to implement <acronym>RAID</acronym>.
	<acronym>ZFS</acronym> supports this feature in its pool
	design.  <acronym>RAID-Z</acronym> pools require three or more
	disks but provide more usable space than mirrored
	pools.</para>

      <para>This example creates a <acronym>RAID-Z</acronym> pool,
	specifying the disks to add to the pool:</para>

      <screen>&prompt.root; <userinput>zpool create storage raidz da0 da1 da2</userinput></screen>

      <note>
	<para>&sun; recommends that the number of devices used in a
	  <acronym>RAID</acronym>-Z configuration be between three and
	  nine.  For environments requiring a single pool consisting
	  of 10 disks or more, consider breaking it up into smaller
	  <acronym>RAID-Z</acronym> groups.  If only two disks are
	  available and redundancy is a requirement, consider using a
	  <acronym>ZFS</acronym> mirror.  Refer to &man.zpool.8; for
	  more details.</para>
      </note>

      <para>The previous example created the
	<literal>storage</literal> zpool.  This example makes a new
	file system called <literal>home</literal> in that
	pool:</para>

      <screen>&prompt.root; <userinput>zfs create storage/home</userinput></screen>

      <para>Compression and keeping extra copies of directories
	and files can be enabled:</para>

      <screen>&prompt.root; <userinput>zfs set copies=2 storage/home</userinput>
&prompt.root; <userinput>zfs set compression=gzip storage/home</userinput></screen>

      <para>To make this the new home directory for users, copy the
	user data to this directory and create the appropriate
	symbolic links:</para>

      <screen>&prompt.root; <userinput>cp -rp /home/* /storage/home</userinput>
&prompt.root; <userinput>rm -rf /home /usr/home</userinput>
&prompt.root; <userinput>ln -s /storage/home /home</userinput>
&prompt.root; <userinput>ln -s /storage/home /usr/home</userinput></screen>

      <para>Users data is now stored on the freshly-created
	<filename>/storage/home</filename>.  Test by adding a new user
	and logging in as that user.</para>

      <para>Try creating a file system snapshot which can be rolled
	back later:</para>

      <screen>&prompt.root; <userinput>zfs snapshot storage/home@08-30-08</userinput></screen>

      <para>Snapshots can only be made of a full file system, not a
	single directory or file.</para>

      <para>The <literal>@</literal> character is a delimiter between
	the file system name or the volume name.  If an important
	directory has been accidentally deleted, the file system can
	be backed up, then rolled back to an earlier snapshot when the
	directory still existed:</para>

      <screen>&prompt.root; <userinput>zfs rollback storage/home@08-30-08</userinput></screen>

      <para>To list all available snapshots, run
	<command>ls</command> in the file system's
	<filename>.zfs/snapshot</filename> directory.  For example, to
	see the previously taken snapshot:</para>

      <screen>&prompt.root; <userinput>ls /storage/home/.zfs/snapshot</userinput></screen>

      <para>It is possible to write a script to perform regular
	snapshots on user data.  However, over time, snapshots can
	consume a great deal of disk space.  The previous snapshot can
	be removed using the command:</para>

      <screen>&prompt.root; <userinput>zfs destroy storage/home@08-30-08</userinput></screen>

      <para>After testing, <filename>/storage/home</filename> can be
	made the real <filename>/home</filename> using this
	command:</para>

      <screen>&prompt.root; <userinput>zfs set mountpoint=/home storage/home</userinput></screen>

      <para>Run <command>df</command> and <command>mount</command> to
	confirm that the system now treats the file system as the real
	<filename>/home</filename>:</para>

      <screen>&prompt.root; <userinput>mount</userinput>
/dev/ad0s1a on / (ufs, local)
devfs on /dev (devfs, local)
/dev/ad0s1d on /usr (ufs, local, soft-updates)
storage on /storage (zfs, local)
storage/home on /home (zfs, local)
&prompt.root; <userinput>df</userinput>
Filesystem   1K-blocks    Used    Avail Capacity  Mounted on
/dev/ad0s1a    2026030  235240  1628708    13%    /
devfs                1       1        0   100%    /dev
/dev/ad0s1d   54098308 1032826 48737618     2%    /usr
storage       26320512       0 26320512     0%    /storage
storage/home  26320512       0 26320512     0%    /home</screen>

      <para>This completes the <acronym>RAID-Z</acronym>
	configuration.  Daily status updates about the file systems
	created can be generated as part of the nightly
	&man.periodic.8; runs.  Add this line to
	<filename>/etc/periodic.conf</filename>:</para>

      <programlisting>daily_status_zfs_enable="YES"</programlisting>
    </sect2>

    <sect2 xml:id="zfs-quickstart-recovering-raid-z">
      <title>Recovering <acronym>RAID-Z</acronym></title>

      <para>Every software <acronym>RAID</acronym> has a method of
	monitoring its <literal>state</literal>.  The status of
	<acronym>RAID-Z</acronym> devices may be viewed with this
	command:</para>

      <screen>&prompt.root; <userinput>zpool status -x</userinput></screen>

      <para>If all pools are
	<link linkend="zfs-term-online">Online</link> and everything
	is normal, the message shows:</para>

      <screen>all pools are healthy</screen>

      <para>If there is an issue, perhaps a disk is in the
	<link linkend="zfs-term-offline">Offline</link> state, the
	pool state will look similar to:</para>

      <screen>  pool: storage
 state: DEGRADED
status: One or more devices has been taken offline by the administrator.
	Sufficient replicas exist for the pool to continue functioning in a
	degraded state.
action: Online the device using 'zpool online' or replace the device with
	'zpool replace'.
 scrub: none requested
config:

	NAME        STATE     READ WRITE CKSUM
	storage     DEGRADED     0     0     0
	  raidz1    DEGRADED     0     0     0
	    da0     ONLINE       0     0     0
	    da1     OFFLINE      0     0     0
	    da2     ONLINE       0     0     0

errors: No known data errors</screen>

      <para>This indicates that the device was previously taken
	offline by the administrator with this command:</para>

      <screen>&prompt.root; <userinput>zpool offline storage da1</userinput></screen>

      <para>Now the system can be powered down to replace
	<filename>da1</filename>.  When the system is back online,
	the failed disk can replaced in the pool:</para>

      <screen>&prompt.root; <userinput>zpool replace storage da1</userinput></screen>

      <para>From here, the status may be checked again, this time
	without <option>-x</option> so that all pools are
	shown:</para>

      <screen>&prompt.root; <userinput>zpool status storage</userinput>
 pool: storage
 state: ONLINE
 scrub: resilver completed with 0 errors on Sat Aug 30 19:44:11 2008
config:

	NAME        STATE     READ WRITE CKSUM
	storage     ONLINE       0     0     0
	  raidz1    ONLINE       0     0     0
	    da0     ONLINE       0     0     0
	    da1     ONLINE       0     0     0
	    da2     ONLINE       0     0     0

errors: No known data errors</screen>

      <para>In this example, everything is normal.</para>
    </sect2>

    <sect2 xml:id="zfs-quickstart-data-verification">
      <title>Data Verification</title>

      <para><acronym>ZFS</acronym> uses checksums to verify the
	integrity of stored data.  These are enabled automatically
	upon creation of file systems.</para>

      <warning>
	<para>Checksums can be disabled, but it is
	  <emphasis>not</emphasis> recommended!  Checksums take very
	  little storage space and provide data integrity.  Many
	  <acronym>ZFS</acronym> features will not work properly with
	  checksums disabled.  There is no noticeable performance gain
	  from disabling these checksums.</para>
      </warning>

      <para>Checksum verification is known as
	<emphasis>scrubbing</emphasis>.  Verify the data integrity of
	the <literal>storage</literal> pool with this command:</para>

      <screen>&prompt.root; <userinput>zpool scrub storage</userinput></screen>

      <para>The duration of a scrub depends on the amount of data
	stored.  Larger amounts of data will take proportionally
	longer to verify.  Scrubs are very <acronym>I/O</acronym>
	intensive, and only one scrub is allowed to run at a time.
	After the scrub completes, the status can be viewed with
	<command>status</command>:</para>

      <screen>&prompt.root; <userinput>zpool status storage</userinput>
 pool: storage
 state: ONLINE
 scrub: scrub completed with 0 errors on Sat Jan 26 19:57:37 2013
config:

	NAME        STATE     READ WRITE CKSUM
	storage     ONLINE       0     0     0
	  raidz1    ONLINE       0     0     0
	    da0     ONLINE       0     0     0
	    da1     ONLINE       0     0     0
	    da2     ONLINE       0     0     0

errors: No known data errors</screen>

      <para>The completion date of the last scrub operation is
	displayed to help track when another scrub is required.
	Routine scrubs help protect data from silent corruption and
	ensure the integrity of the pool.</para>

      <para>Refer to &man.zfs.8; and &man.zpool.8; for other
	<acronym>ZFS</acronym> options.</para>
    </sect2>
  </sect1>

  <sect1 xml:id="zfs-zpool">
    <title><command>zpool</command> Administration</title>

    <para><acronym>ZFS</acronym> administration is divided between two
      main utilities.  The <command>zpool</command> utility controls
      the operation of the pool and deals with adding, removing,
      replacing, and managing disks.  The
      <link linkend="zfs-zfs"><command>zfs</command></link> utility
      deals with creating, destroying, and managing datasets,
      both <link linkend="zfs-term-filesystem">file systems</link> and
      <link linkend="zfs-term-volume">volumes</link>.</para>

    <sect2 xml:id="zfs-zpool-create">
      <title>Creating and Destroying Storage Pools</title>

      <para>Creating a <acronym>ZFS</acronym> storage pool
	(<emphasis>zpool</emphasis>) involves making a number of
	decisions that are relatively permanent because the structure
	of the pool cannot be changed after the pool has been created.
	The most important decision is what types of vdevs into which
	to group the physical disks.  See the list of
	<link linkend="zfs-term-vdev">vdev types</link> for details
	about the possible options.  After the pool has been created,
	most vdev types do not allow additional disks to be added to
	the vdev.  The exceptions are mirrors, which allow additional
	disks to be added to the vdev, and stripes, which can be
	upgraded to mirrors by attaching an additional disk to the
	vdev.  Although additional vdevs can be added to expand a
	pool, the layout of the pool cannot be changed after pool
	creation.  Instead, the data must be backed up and the
	pool destroyed and recreated.</para>

      <para>Create a simple mirror pool:</para>

      <screen>&prompt.root; <userinput>zpool create <replaceable>mypool</replaceable> mirror <replaceable>/dev/ada1</replaceable> <replaceable>/dev/ada2</replaceable></userinput>
&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            ada1    ONLINE       0     0     0
            ada2    ONLINE       0     0     0

errors: No known data errors</screen>

      <para>Multiple vdevs can be created at once.  Specify multiple
	groups of disks separated by the vdev type keyword,
	<literal>mirror</literal> in this example:</para>

      <screen>&prompt.root; <userinput>zpool create <replaceable>mypool</replaceable> mirror <replaceable>/dev/ada1</replaceable> <replaceable>/dev/ada2</replaceable> mirror <replaceable>/dev/ada3</replaceable> <replaceable>/dev/ada4</replaceable></userinput>
  pool: mypool
 state: ONLINE
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            ada1    ONLINE       0     0     0
            ada2    ONLINE       0     0     0
          mirror-1  ONLINE       0     0     0
            ada3    ONLINE       0     0     0
            ada4    ONLINE       0     0     0

errors: No known data errors</screen>

      <para>Pools can also be constructed using partitions rather than
	whole disks.  Putting <acronym>ZFS</acronym> in a separate
	partition allows the same disk to have other partitions for
	other purposes.  In particular, partitions with bootcode and
	file systems needed for booting can be added.  This allows
	booting from disks that are also members of a pool.  There is
	no performance penalty on &os; when using a partition rather
	than a whole disk.  Using partitions also allows the
	administrator to <emphasis>under-provision</emphasis> the
	disks, using less than the full capacity.  If a future
	replacement disk of the same nominal size as the original
	actually has a slightly smaller capacity, the smaller
	partition will still fit, and the replacement disk can still
	be used.</para>

      <para>Create a
	<link linkend="zfs-term-vdev-raidz">RAID-Z2</link> pool using
	partitions:</para>

      <screen>&prompt.root; <userinput>zpool create <replaceable>mypool</replaceable> raidz2 <replaceable>/dev/ada0p3</replaceable> <replaceable>/dev/ada1p3</replaceable> <replaceable>/dev/ada2p3</replaceable> <replaceable>/dev/ada3p3</replaceable> <replaceable>/dev/ada4p3</replaceable> <replaceable>/dev/ada5p3</replaceable></userinput>
&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          raidz2-0  ONLINE       0     0     0
            ada0p3  ONLINE       0     0     0
            ada1p3  ONLINE       0     0     0
            ada2p3  ONLINE       0     0     0
            ada3p3  ONLINE       0     0     0
            ada4p3  ONLINE       0     0     0
            ada5p3  ONLINE       0     0     0

errors: No known data errors</screen>

      <para>A pool that is no longer needed can be destroyed so that
	the disks can be reused.  Destroying a pool involves first
	unmounting all of the datasets in that pool.  If the datasets
	are in use, the unmount operation will fail and the pool will
	not be destroyed.  The destruction of the pool can be forced
	with <option>-f</option>, but this can cause undefined
	behavior in applications which had open files on those
	datasets.</para>
    </sect2>

    <sect2 xml:id="zfs-zpool-attach">
      <title>Adding and Removing Devices</title>

      <para>There are two cases for adding disks to a zpool: attaching
	a disk to an existing vdev with
	<command>zpool attach</command>, or adding vdevs to the pool
	with <command>zpool add</command>.  Only some
	<link linkend="zfs-term-vdev">vdev types</link> allow disks to
	be added to the vdev after creation.</para>

      <para>A pool created with a single disk lacks redundancy.
	Corruption can be detected but
	not repaired, because there is no other copy of the data.

	The <link linkend="zfs-term-copies">copies</link> property may
	be able to recover from a small failure such as a bad sector,
	but does not provide the same level of protection as mirroring
	or <acronym>RAID-Z</acronym>.  Starting with a pool consisting
	of a single disk vdev, <command>zpool attach</command> can be
	used to add an additional disk to the vdev, creating a mirror.
	<command>zpool attach</command> can also be used to add
	additional disks to a mirror group, increasing redundancy and
	read performance.  If the disks being used for the pool are
	partitioned, replicate the layout of the first disk on to the
	second, <command>gpart backup</command> and
	<command>gpart restore</command> can be used to make this
	process easier.</para>

      <para>Upgrade the single disk (stripe) vdev
	<replaceable>ada0p3</replaceable> to a mirror by attaching
	<replaceable>ada1p3</replaceable>:</para>

      <screen>&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          ada0p3    ONLINE       0     0     0

errors: No known data errors
&prompt.root; <userinput>zpool attach <replaceable>mypool</replaceable> <replaceable>ada0p3</replaceable> <replaceable>ada1p3</replaceable></userinput>
Make sure to wait until resilver is done before rebooting.

If you boot from pool 'mypool', you may need to update
boot code on newly attached disk 'ada1p3'.

Assuming you use GPT partitioning and 'da0' is your new boot disk
you may use the following command:

        gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 da0
&prompt.root; <userinput>gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 <replaceable>ada1</replaceable></userinput>
bootcode written to ada1
&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
status: One or more devices is currently being resilvered.  The pool will
        continue to function, possibly in a degraded state.
action: Wait for the resilver to complete.
  scan: resilver in progress since Fri May 30 08:19:19 2014
        527M scanned out of 781M at 47.9M/s, 0h0m to go
        527M resilvered, 67.53% done
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            ada0p3  ONLINE       0     0     0
            ada1p3  ONLINE       0     0     0  (resilvering)

errors: No known data errors
&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
  scan: resilvered 781M in 0h0m with 0 errors on Fri May 30 08:15:58 2014
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            ada0p3  ONLINE       0     0     0
            ada1p3  ONLINE       0     0     0

errors: No known data errors</screen>

      <para>When adding disks to the existing vdev is not an option,
	as for <acronym>RAID-Z</acronym>, an alternative method is to
	add another vdev to the pool.  Additional vdevs provide higher
	performance, distributing writes across the vdevs.  Each vdev
	is responsible for providing its own redundancy.  It is
	possible, but discouraged, to mix vdev types, like
	<literal>mirror</literal> and <literal>RAID-Z</literal>.
	Adding a non-redundant vdev to a pool containing mirror or
	<acronym>RAID-Z</acronym> vdevs risks the data on the entire
	pool.  Writes are distributed, so the failure of the
	non-redundant disk will result in the loss of a fraction of
	every block that has been written to the pool.</para>

      <para>Data is striped across each of the vdevs.  For example,
	with two mirror vdevs, this is effectively a
	<acronym>RAID</acronym> 10 that stripes writes across two sets
	of mirrors.  Space is allocated so that each vdev reaches 100%
	full at the same time.  There is a performance penalty if the
	vdevs have different amounts of free space, as a
	disproportionate amount of the data is written to the less
	full vdev.</para>

      <para>When attaching additional devices to a boot pool, remember
	to update the bootcode.</para>

      <para>Attach a second mirror group (<filename>ada2p3</filename>
	and <filename>ada3p3</filename>) to the existing
	mirror:</para>

      <screen>&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
  scan: resilvered 781M in 0h0m with 0 errors on Fri May 30 08:19:35 2014
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            ada0p3  ONLINE       0     0     0
            ada1p3  ONLINE       0     0     0

errors: No known data errors
&prompt.root; <userinput>zpool add <replaceable>mypool</replaceable> mirror <replaceable>ada2p3</replaceable> <replaceable>ada3p3</replaceable></userinput>
&prompt.root; <userinput>gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 <replaceable>ada2</replaceable></userinput>
bootcode written to ada2
&prompt.root; <userinput>gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 <replaceable>ada3</replaceable></userinput>
bootcode written to ada3
&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
  scan: scrub repaired 0 in 0h0m with 0 errors on Fri May 30 08:29:51 2014
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            ada0p3  ONLINE       0     0     0
            ada1p3  ONLINE       0     0     0
          mirror-1  ONLINE       0     0     0
            ada2p3  ONLINE       0     0     0
            ada3p3  ONLINE       0     0     0

errors: No known data errors</screen>

      <para>Currently, vdevs cannot be removed from a pool, and disks
	can only be removed from a mirror if there is enough remaining
	redundancy.  If only one disk in a mirror group remains, it
	ceases to be a mirror and reverts to being a stripe, risking
	the entire pool if that remaining disk fails.</para>

      <para>Remove a disk from a three-way mirror group:</para>

      <screen>&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
  scan: scrub repaired 0 in 0h0m with 0 errors on Fri May 30 08:29:51 2014
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            ada0p3  ONLINE       0     0     0
            ada1p3  ONLINE       0     0     0
            ada2p3  ONLINE       0     0     0

errors: No known data errors
&prompt.root; <userinput>zpool detach <replaceable>mypool</replaceable> <replaceable>ada2p3</replaceable></userinput>
&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
  scan: scrub repaired 0 in 0h0m with 0 errors on Fri May 30 08:29:51 2014
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            ada0p3  ONLINE       0     0     0
            ada1p3  ONLINE       0     0     0

errors: No known data errors</screen>
    </sect2>

    <sect2 xml:id="zfs-zpool-status">
      <title>Checking the Status of a Pool</title>

      <para>Pool status is important.  If a drive goes offline or a
	read, write, or checksum error is detected, the corresponding
	error count increases.  The <command>status</command> output
	shows the configuration and status of each device in the pool
	and the status of the entire pool.  Actions that need to be
	taken and details about the last <link
	  linkend="zfs-zpool-scrub"><command>scrub</command></link>
	are also shown.</para>

      <screen>&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
  scan: scrub repaired 0 in 2h25m with 0 errors on Sat Sep 14 04:25:50 2013
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          raidz2-0  ONLINE       0     0     0
            ada0p3  ONLINE       0     0     0
            ada1p3  ONLINE       0     0     0
            ada2p3  ONLINE       0     0     0
            ada3p3  ONLINE       0     0     0
            ada4p3  ONLINE       0     0     0
            ada5p3  ONLINE       0     0     0

errors: No known data errors</screen>
    </sect2>

    <sect2 xml:id="zfs-zpool-clear">
      <title>Clearing Errors</title>

      <para>When an error is detected, the read, write, or checksum
	counts are incremented.  The error message can be cleared and
	the counts reset with <command>zpool clear
	  <replaceable>mypool</replaceable></command>.  Clearing the
	error state can be important for automated scripts that alert
	the administrator when the pool encounters an error.  Further
	errors may not be reported if the old errors are not
	cleared.</para>
    </sect2>

    <sect2 xml:id="zfs-zpool-replace">
      <title>Replacing a Functioning Device</title>

      <para>There are a number of situations where it may be
	desirable to replace one disk with a different disk.  When
	replacing a working disk, the process keeps the old disk
	online during the replacement.  The pool never enters a
	<link linkend="zfs-term-degraded">degraded</link> state,
	reducing the risk of data loss.
	<command>zpool replace</command> copies all of the data from
	the old disk to the new one.  After the operation completes,
	the old disk is disconnected from the vdev.  If the new disk
	is larger than the old disk, it may be possible to grow the
	zpool, using the new space.  See <link
	  linkend="zfs-zpool-online">Growing a Pool</link>.</para>

      <para>Replace a functioning device in the pool:</para>

      <screen>&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            ada0p3  ONLINE       0     0     0
            ada1p3  ONLINE       0     0     0

errors: No known data errors
&prompt.root; <userinput>zpool replace <replaceable>mypool</replaceable> <replaceable>ada1p3</replaceable> <replaceable>ada2p3</replaceable></userinput>
Make sure to wait until resilver is done before rebooting.

If you boot from pool 'zroot', you may need to update
boot code on newly attached disk 'ada2p3'.

Assuming you use GPT partitioning and 'da0' is your new boot disk
you may use the following command:

        gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 da0
&prompt.root; <userinput>gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 <replaceable>ada2</replaceable></userinput>
&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
status: One or more devices is currently being resilvered.  The pool will
        continue to function, possibly in a degraded state.
action: Wait for the resilver to complete.
  scan: resilver in progress since Mon Jun  2 14:21:35 2014
        604M scanned out of 781M at 46.5M/s, 0h0m to go
        604M resilvered, 77.39% done
config:

        NAME             STATE     READ WRITE CKSUM
        mypool           ONLINE       0     0     0
          mirror-0       ONLINE       0     0     0
            ada0p3       ONLINE       0     0     0
            replacing-1  ONLINE       0     0     0
              ada1p3     ONLINE       0     0     0
              ada2p3     ONLINE       0     0     0  (resilvering)

errors: No known data errors
&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
  scan: resilvered 781M in 0h0m with 0 errors on Mon Jun  2 14:21:52 2014
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            ada0p3  ONLINE       0     0     0
            ada2p3  ONLINE       0     0     0

errors: No known data errors</screen>
    </sect2>

    <sect2 xml:id="zfs-zpool-resilver">
      <title>Dealing with Failed Devices</title>

      <para>When a disk in a pool fails, the vdev to which the disk
	belongs enters the
	<link linkend="zfs-term-degraded">degraded</link> state.  All
	of the data is still available, but performance may be reduced
	because missing data must be calculated from the available
	redundancy.  To restore the vdev to a fully functional state,
	the failed physical device must be replaced.
	<acronym>ZFS</acronym> is then instructed to begin the
	<link linkend="zfs-term-resilver">resilver</link> operation.
	Data that was on the failed device is recalculated from
	available redundancy and written to the replacement device.
	After completion, the vdev returns to
	<link linkend="zfs-term-online">online</link> status.</para>

      <para>If the vdev does not have any redundancy, or if multiple
	devices have failed and there is not enough redundancy to
	compensate, the pool enters the
	<link linkend="zfs-term-faulted">faulted</link> state.  If a
	sufficient number of devices cannot be reconnected to the
	pool, the pool becomes inoperative and data must be restored
	from backups.</para>

      <para>When replacing a failed disk, the name of the failed disk
	is replaced with the <acronym>GUID</acronym> of the device.
	A new device name parameter for
	<command>zpool replace</command> is not required if the
	replacement device has the same device name.</para>

      <para>Replace a failed disk using
	<command>zpool replace</command>:</para>

      <screen>&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: DEGRADED
status: One or more devices could not be opened.  Sufficient replicas exist for
        the pool to continue functioning in a degraded state.
action: Attach the missing device and online it using 'zpool online'.
   see: http://illumos.org/msg/ZFS-8000-2Q
  scan: none requested
config:

        NAME                    STATE     READ WRITE CKSUM
        mypool                  DEGRADED     0     0     0
          mirror-0              DEGRADED     0     0     0
            ada0p3              ONLINE       0     0     0
            316502962686821739  UNAVAIL      0     0     0  was /dev/ada1p3

errors: No known data errors
&prompt.root; <userinput>zpool replace <replaceable>mypool</replaceable> <replaceable>316502962686821739</replaceable> <replaceable>ada2p3</replaceable></userinput>
&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: DEGRADED
status: One or more devices is currently being resilvered.  The pool will
        continue to function, possibly in a degraded state.
action: Wait for the resilver to complete.
  scan: resilver in progress since Mon Jun  2 14:52:21 2014
        641M scanned out of 781M at 49.3M/s, 0h0m to go
        640M resilvered, 82.04% done
config:

        NAME                        STATE     READ WRITE CKSUM
        mypool                      DEGRADED     0     0     0
          mirror-0                  DEGRADED     0     0     0
            ada0p3                  ONLINE       0     0     0
            replacing-1             UNAVAIL      0     0     0
              15732067398082357289  UNAVAIL      0     0     0  was /dev/ada1p3/old
              ada2p3                ONLINE       0     0     0  (resilvering)

errors: No known data errors
&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
  scan: resilvered 781M in 0h0m with 0 errors on Mon Jun  2 14:52:38 2014
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            ada0p3  ONLINE       0     0     0
            ada2p3  ONLINE       0     0     0

errors: No known data errors</screen>
    </sect2>

    <sect2 xml:id="zfs-zpool-scrub">
      <title>Scrubbing a Pool</title>

      <para>It is recommended that pools be
	<link linkend="zfs-term-scrub">scrubbed</link> regularly,
	ideally at least once every month.  The
	<command>scrub</command> operation is very disk-intensive and
	will reduce performance while running.  Avoid high-demand
	periods when scheduling <command>scrub</command> or use <link
	  linkend="zfs-advanced-tuning-scrub_delay"><varname>vfs.zfs.scrub_delay</varname></link>
	to adjust the relative priority of the
	<command>scrub</command> to prevent it interfering with other
	workloads.</para>

      <screen>&prompt.root; <userinput>zpool scrub <replaceable>mypool</replaceable></userinput>
&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
  scan: scrub in progress since Wed Feb 19 20:52:54 2014
        116G scanned out of 8.60T at 649M/s, 3h48m to go
        0 repaired, 1.32% done
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          raidz2-0  ONLINE       0     0     0
            ada0p3  ONLINE       0     0     0
            ada1p3  ONLINE       0     0     0
            ada2p3  ONLINE       0     0     0
            ada3p3  ONLINE       0     0     0
            ada4p3  ONLINE       0     0     0
            ada5p3  ONLINE       0     0     0

errors: No known data errors</screen>

      <para>In the event that a scrub operation needs to be cancelled,
	issue <command>zpool scrub -s
	  <replaceable>mypool</replaceable></command>.</para>
    </sect2>

    <sect2 xml:id="zfs-zpool-selfheal">
      <title>Self-Healing</title>

      <para>The checksums stored with data blocks enable the file
	system to <emphasis>self-heal</emphasis>.  This feature will
	automatically repair data whose checksum does not match the
	one recorded on another device that is part of the storage
	pool.  For example, a mirror with two disks where one drive is
	starting to malfunction and cannot properly store the data any
	more.  This is even worse when the data has not been accessed
	for a long time, as with long term archive storage.
	Traditional file systems need to run algorithms that check and
	repair the data like &man.fsck.8;.  These commands take time,
	and in severe cases, an administrator has to manually decide
	which repair operation must be performed.  When
	<acronym>ZFS</acronym> detects a data block with a checksum
	that does not match, it tries to read the data from the mirror
	disk.  If that disk can provide the correct data, it will not
	only give that data to the application requesting it, but also
	correct the wrong data on the disk that had the bad checksum.
	This happens without any interaction from a system
	administrator during normal pool operation.</para>

      <para>The next example demonstrates this self-healing behavior.
	A mirrored pool of disks <filename>/dev/ada0</filename> and
	<filename>/dev/ada1</filename> is created.</para>

      <screen>&prompt.root; <userinput>zpool create <replaceable>healer</replaceable> mirror <replaceable>/dev/ada0</replaceable> <replaceable>/dev/ada1</replaceable></userinput>
&prompt.root; <userinput>zpool status <replaceable>healer</replaceable></userinput>
  pool: healer
 state: ONLINE
  scan: none requested
config:

    NAME        STATE     READ WRITE CKSUM
    healer      ONLINE       0     0     0
      mirror-0  ONLINE       0     0     0
       ada0     ONLINE       0     0     0
       ada1     ONLINE       0     0     0

errors: No known data errors
&prompt.root; <userinput>zpool list</userinput>
NAME     SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
healer   960M  92.5K   960M     0%  1.00x  ONLINE  -</screen>

      <para>Some important data that to be protected from data errors
	using the self-healing feature is copied to the pool.  A
	checksum of the pool is created for later comparison.</para>

      <screen>&prompt.root; <userinput>cp /some/important/data /healer</userinput>
&prompt.root; <userinput>zfs list</userinput>
NAME     SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
healer   960M  67.7M   892M     7%  1.00x  ONLINE  -
&prompt.root; <userinput>sha1 /healer > checksum.txt</userinput>
&prompt.root; <userinput>cat checksum.txt</userinput>
SHA1 (/healer) = 2753eff56d77d9a536ece6694bf0a82740344d1f</screen>

      <para>Data corruption is simulated by writing random data to the
	beginning of one of the disks in the mirror.  To prevent
	<acronym>ZFS</acronym> from healing the data as soon as it is
	detected, the pool is exported before the corruption and
	imported again afterwards.</para>

      <warning>
	<para>This is a dangerous operation that can destroy vital
	  data.  It is shown here for demonstrational purposes only
	  and should not be attempted during normal operation of a
	  storage pool.  Nor should this intentional corruption
	  example be run on any disk with a different file system on
	  it.  Do not use any other disk device names other than the
	  ones that are part of the pool.  Make certain that proper
	  backups of the pool are created before running the
	  command!</para>
      </warning>

      <screen>&prompt.root; <userinput>zpool export <replaceable>healer</replaceable></userinput>
&prompt.root; <userinput>dd if=/dev/random of=/dev/ada1 bs=1m count=200</userinput>
200+0 records in
200+0 records out
209715200 bytes transferred in 62.992162 secs (3329227 bytes/sec)
&prompt.root; <userinput>zpool import healer</userinput></screen>

      <para>The pool status shows that one device has experienced an
	error.  Note that applications reading data from the pool did
	not receive any incorrect data.  <acronym>ZFS</acronym>
	provided data from the <filename>ada0</filename> device with
	the correct checksums.  The device with the wrong checksum can
	be found easily as the <literal>CKSUM</literal> column
	contains a nonzero value.</para>

      <screen>&prompt.root; <userinput>zpool status <replaceable>healer</replaceable></userinput>
    pool: healer
   state: ONLINE
  status: One or more devices has experienced an unrecoverable error.  An
          attempt was made to correct the error.  Applications are unaffected.
  action: Determine if the device needs to be replaced, and clear the errors
          using 'zpool clear' or replace the device with 'zpool replace'.
     see: http://www.sun.com/msg/ZFS-8000-9P
    scan: none requested
  config:

      NAME        STATE     READ WRITE CKSUM
      healer      ONLINE       0     0     0
        mirror-0  ONLINE       0     0     0
         ada0     ONLINE       0     0     0
         ada1     ONLINE       0     0     1

errors: No known data errors</screen>

      <para>The error was detected and handled by using the redundancy
	present in the unaffected <filename>ada0</filename> mirror
	disk.  A checksum comparison with the original one will reveal
	whether the pool is consistent again.</para>

      <screen>&prompt.root; <userinput>sha1 /healer >> checksum.txt</userinput>
&prompt.root; <userinput>cat checksum.txt</userinput>
SHA1 (/healer) = 2753eff56d77d9a536ece6694bf0a82740344d1f
SHA1 (/healer) = 2753eff56d77d9a536ece6694bf0a82740344d1f</screen>

      <para>The two checksums that were generated before and after the
	intentional tampering with the pool data still match.  This
	shows how <acronym>ZFS</acronym> is capable of detecting and
	correcting any errors automatically when the checksums differ.
	Note that this is only possible when there is enough
	redundancy present in the pool.  A pool consisting of a single
	device has no self-healing capabilities.  That is also the
	reason why checksums are so important in
	<acronym>ZFS</acronym> and should not be disabled for any
	reason.  No &man.fsck.8; or similar file system consistency
	check program is required to detect and correct this and the
	pool was still available during the time there was a problem.
	A scrub operation is now required to overwrite the corrupted
	data on <filename>ada1</filename>.</para>

      <screen>&prompt.root; <userinput>zpool scrub <replaceable>healer</replaceable></userinput>
&prompt.root; <userinput>zpool status <replaceable>healer</replaceable></userinput>
  pool: healer
 state: ONLINE
status: One or more devices has experienced an unrecoverable error.  An
            attempt was made to correct the error.  Applications are unaffected.
action: Determine if the device needs to be replaced, and clear the errors
            using 'zpool clear' or replace the device with 'zpool replace'.
   see: http://www.sun.com/msg/ZFS-8000-9P
  scan: scrub in progress since Mon Dec 10 12:23:30 2012
        10.4M scanned out of 67.0M at 267K/s, 0h3m to go
        9.63M repaired, 15.56% done
config:

    NAME        STATE     READ WRITE CKSUM
    healer      ONLINE       0     0     0
      mirror-0  ONLINE       0     0     0
       ada0     ONLINE       0     0     0
       ada1     ONLINE       0     0   627  (repairing)

errors: No known data errors</screen>

      <para>The scrub operation reads data from
	<filename>ada0</filename> and rewrites any data with an
	incorrect checksum on <filename>ada1</filename>.  This is
	indicated by the <literal>(repairing)</literal> output from
	<command>zpool status</command>.  After the operation is
	complete, the pool status changes to:</para>

      <screen>&prompt.root; <userinput>zpool status <replaceable>healer</replaceable></userinput>
  pool: healer
 state: ONLINE
status: One or more devices has experienced an unrecoverable error.  An
        attempt was made to correct the error.  Applications are unaffected.
action: Determine if the device needs to be replaced, and clear the errors
             using 'zpool clear' or replace the device with 'zpool replace'.
   see: http://www.sun.com/msg/ZFS-8000-9P
  scan: scrub repaired 66.5M in 0h2m with 0 errors on Mon Dec 10 12:26:25 2012
config:

    NAME        STATE     READ WRITE CKSUM
    healer      ONLINE       0     0     0
      mirror-0  ONLINE       0     0     0
       ada0     ONLINE       0     0     0
       ada1     ONLINE       0     0 2.72K

errors: No known data errors</screen>

      <para>After the scrub operation completes and all the data
	has been synchronized from <filename>ada0</filename> to
	<filename>ada1</filename>, the error messages can be
	<link linkend="zfs-zpool-clear">cleared</link> from the pool
	status by running <command>zpool clear</command>.</para>

      <screen>&prompt.root; <userinput>zpool clear <replaceable>healer</replaceable></userinput>
&prompt.root; <userinput>zpool status <replaceable>healer</replaceable></userinput>
  pool: healer
 state: ONLINE
  scan: scrub repaired 66.5M in 0h2m with 0 errors on Mon Dec 10 12:26:25 2012
config:

    NAME        STATE     READ WRITE CKSUM
    healer      ONLINE       0     0     0
      mirror-0  ONLINE       0     0     0
       ada0     ONLINE       0     0     0
       ada1     ONLINE       0     0     0

errors: No known data errors</screen>

      <para>The pool is now back to a fully working state and all the
	errors have been cleared.</para>
    </sect2>

    <sect2 xml:id="zfs-zpool-online">
      <title>Growing a Pool</title>

      <para>The usable size of a redundant pool is limited by the
	capacity of the smallest device in each vdev.  The smallest
	device can be replaced with a larger device.  After completing
	a <link linkend="zfs-zpool-replace">replace</link> or
	<link linkend="zfs-term-resilver">resilver</link> operation,
	the pool can grow to use the capacity of the new device.  For
	example, consider a mirror of a 1&nbsp;TB drive and a
	2&nbsp;drive.  The usable space is 1&nbsp;TB.  Then the
	1&nbsp;TB is replaced with another 2&nbsp;TB drive, and the
	resilvering process duplicates existing data.  Because
	both of the devices now have 2&nbsp;TB capacity, the mirror's
	available space can be grown to 2&nbsp;TB.</para>

      <para>Expansion is triggered by using
	<command>zpool online -e</command> on each device.  After
	expansion of all devices, the additional space becomes
	available to the pool.</para>
    </sect2>

    <sect2 xml:id="zfs-zpool-import">
      <title>Importing and Exporting Pools</title>

      <para>Pools are <emphasis>exported</emphasis> before moving them
	to another system.  All datasets are unmounted, and each
	device is marked as exported but still locked so it cannot be
	used by other disk subsystems.  This allows pools to be
	<emphasis>imported</emphasis> on other machines, other
	operating systems that support <acronym>ZFS</acronym>, and
	even different hardware architectures (with some caveats, see
	&man.zpool.8;).  When a dataset has open files,
	<command> zpool export -f</command> can be used to force the
	export of a pool.  Use this with caution.  The datasets are
	forcibly unmounted, potentially resulting in unexpected
	behavior by the applications which had open files on those
	datasets.</para>

      <para>Export a pool that is not in use:</para>

      <screen>&prompt.root; <userinput>zpool export mypool</userinput></screen>

      <para>Importing a pool automatically mounts the datasets.  This
	may not be the desired behavior, and can be prevented with
	<command>zpool import -N</command>.
	<command>zpool import -o</command> sets temporary properties
	for this import only.
	<command>zpool import altroot=</command> allows importing a
	pool with a base mount point instead of the root of the file
	system.  If the pool was last used on a different system and
	was not properly exported, an import might have to be forced
	with <command>zpool import -f</command>.
	<command>zpool import -a</command> imports all pools that do
	not appear to be in use by another system.</para>

      <para>List all available pools for import:</para>

      <screen>&prompt.root; <userinput>zpool import</userinput>
   pool: mypool
     id: 9930174748043525076
  state: ONLINE
 action: The pool can be imported using its name or numeric identifier.
 config:

        mypool      ONLINE
          ada2p3    ONLINE</screen>

      <para>Import the pool with an alternative root directory:</para>

      <screen>&prompt.root; <userinput>zpool import -o altroot=<replaceable>/mnt</replaceable> <replaceable>mypool</replaceable></userinput>
&prompt.root; <userinput>zfs list</userinput>
zfs list
NAME                 USED  AVAIL  REFER  MOUNTPOINT
mypool               110K  47.0G    31K  /mnt/mypool</screen>
    </sect2>

    <sect2 xml:id="zfs-zpool-upgrade">
      <title>Upgrading a Storage Pool</title>

      <para>After upgrading &os;, or if a pool has been imported from
	a system using an older version of <acronym>ZFS</acronym>, the
	pool can be manually upgraded to the latest version of
	<acronym>ZFS</acronym> to support newer features.  Consider
	whether the pool may ever need to be imported on an older
	system before upgrading.  Upgrading is a one-way process.
	Older pools can be upgraded, but pools with newer features
	cannot be downgraded.</para>

      <para>Upgrade a v28 pool to support
	<literal>Feature Flags</literal>:</para>

      <screen>&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
status: The pool is formatted using a legacy on-disk format.  The pool can
        still be used, but some features are unavailable.
action: Upgrade the pool using 'zpool upgrade'.  Once this is done, the
        pool will no longer be accessible on software that does not support feat
        flags.
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
	    ada0    ONLINE       0     0     0
	    ada1    ONLINE       0     0     0

errors: No known data errors
&prompt.root; <userinput>zpool upgrade</userinput>
This system supports ZFS pool feature flags.

The following pools are formatted with legacy version numbers and can
be upgraded to use feature flags.  After being upgraded, these pools
will no longer be accessible by software that does not support feature
flags.

VER  POOL
---  ------------
28   mypool

Use 'zpool upgrade -v' for a list of available legacy versions.
Every feature flags pool has all supported features enabled.
&prompt.root; <userinput>zpool upgrade mypool</userinput>
This system supports ZFS pool feature flags.

Successfully upgraded 'mypool' from version 28 to feature flags.
Enabled the following features on 'mypool':
  async_destroy
  empty_bpobj
  lz4_compress
  multi_vdev_crash_dump</screen>

      <para>The newer features of <acronym>ZFS</acronym> will not be
	available until <command>zpool upgrade</command> has
	completed.  <command>zpool upgrade -v</command> can be used to
	see what new features will be provided by upgrading, as well
	as which features are already supported.</para>

      <para>Upgrade a pool to support additional feature flags:</para>

      <screen>&prompt.root; <userinput>zpool status</userinput>
  pool: mypool
 state: ONLINE
status: Some supported features are not enabled on the pool. The pool can
        still be used, but some features are unavailable.
action: Enable all features using 'zpool upgrade'. Once this is done,
        the pool may no longer be accessible by software that does not support
        the features. See zpool-features(7) for details.
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        mypool      ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
	    ada0    ONLINE       0     0     0
	    ada1    ONLINE       0     0     0

errors: No known data errors
&prompt.root; <userinput>zpool upgrade</userinput>
This system supports ZFS pool feature flags.

All pools are formatted using feature flags.


Some supported features are not enabled on the following pools. Once a
feature is enabled the pool may become incompatible with software
that does not support the feature. See zpool-features(7) for details.

POOL  FEATURE
---------------
zstore
      multi_vdev_crash_dump
      spacemap_histogram
      enabled_txg
      hole_birth
      extensible_dataset
      bookmarks
      filesystem_limits
&prompt.root; <userinput>zpool upgrade mypool</userinput>
This system supports ZFS pool feature flags.

Enabled the following features on 'mypool':
  spacemap_histogram
  enabled_txg
  hole_birth
  extensible_dataset
  bookmarks
  filesystem_limits</screen>

      <warning>
	<para>The boot code on systems that boot from a pool must be
	  updated to support the new pool version.  Use
	  <command>gpart bootcode</command> on the partition that
	  contains the boot code.  See &man.gpart.8; for more
	  information.</para>
      </warning>
    </sect2>

    <sect2 xml:id="zfs-zpool-history">
      <title>Displaying Recorded Pool History</title>

      <para>Commands that modify the pool are recorded.  Recorded
	actions include the creation of datasets, changing properties,
	or replacement of a disk.  This history is useful for
	reviewing how a pool was created and which user performed a
	specific action and when.  History is not kept in a log file,
	but is part of the pool itself.  The command to review this
	history is aptly named
	<command>zpool history</command>:</para>

      <screen>&prompt.root; <userinput>zpool history</userinput>
History for 'tank':
2013-02-26.23:02:35 zpool create tank mirror /dev/ada0 /dev/ada1
2013-02-27.18:50:58 zfs set atime=off tank
2013-02-27.18:51:09 zfs set checksum=fletcher4 tank
2013-02-27.18:51:18 zfs create tank/backup</screen>

      <para>The output shows <command>zpool</command> and
	<command>zfs</command> commands that were executed on the pool
	along with a timestamp.  Only commands that alter the pool in
	some way are recorded.  Commands like
	<command>zfs list</command> are not included.  When no pool
	name is specified, the history of all pools is
	displayed.</para>

      <para><command>zpool history</command> can show even more
	information when the options <option>-i</option> or
	<option>-l</option> are provided.  <option>-i</option>
	displays user-initiated events as well as internally logged
	<acronym>ZFS</acronym> events.</para>

      <screen>&prompt.root; <userinput>zpool history -i</userinput>
History for 'tank':
2013-02-26.23:02:35 [internal pool create txg:5] pool spa 28; zfs spa 28; zpl 5;uts  9.1-RELEASE 901000 amd64
2013-02-27.18:50:53 [internal property set txg:50] atime=0 dataset = 21
2013-02-27.18:50:58 zfs set atime=off tank
2013-02-27.18:51:04 [internal property set txg:53] checksum=7 dataset = 21
2013-02-27.18:51:09 zfs set checksum=fletcher4 tank
2013-02-27.18:51:13 [internal create txg:55] dataset = 39
2013-02-27.18:51:18 zfs create tank/backup</screen>

      <para>More details can be shown by adding <option>-l</option>.
	History records are shown in a long format, including
	information like the name of the user who issued the command
	and the hostname on which the change was made.</para>

      <screen>&prompt.root; <userinput>zpool history -l</userinput>
History for 'tank':
2013-02-26.23:02:35 zpool create tank mirror /dev/ada0 /dev/ada1 [user 0 (root) on :global]
2013-02-27.18:50:58 zfs set atime=off tank [user 0 (root) on myzfsbox:global]
2013-02-27.18:51:09 zfs set checksum=fletcher4 tank [user 0 (root) on myzfsbox:global]
2013-02-27.18:51:18 zfs create tank/backup [user 0 (root) on myzfsbox:global]</screen>

      <para>The output shows that the
	<systemitem class="username">root</systemitem> user created
	the mirrored pool with disks
	<filename>/dev/ada0</filename> and
	<filename>/dev/ada1</filename>.  The hostname
	<systemitem class="systemname">myzfsbox</systemitem> is also
	shown in the commands after the pool's creation.  The hostname
	display becomes important when the pool is exported from one
	system and imported on another.  The commands that are issued
	on the other system can clearly be distinguished by the
	hostname that is recorded for each command.</para>

      <para>Both options to <command>zpool history</command> can be
	combined to give the most detailed information possible for
	any given pool.  Pool history provides valuable information
	when tracking down the actions that were performed or when
	more detailed output is needed for debugging.</para>
    </sect2>

    <sect2 xml:id="zfs-zpool-iostat">
      <title>Performance Monitoring</title>

      <para>A built-in monitoring system can display pool
	<acronym>I/O</acronym> statistics in real time.  It shows the
	amount of free and used space on the pool, how many read and
	write operations are being performed per second, and how much
	<acronym>I/O</acronym> bandwidth is currently being utilized.
	By default, all pools in the system are monitored and
	displayed.  A pool name can be provided to limit monitoring to
	just that pool.  A basic example:</para>

      <screen>&prompt.root; <userinput>zpool iostat</userinput>
               capacity     operations    bandwidth
pool        alloc   free   read  write   read  write
----------  -----  -----  -----  -----  -----  -----
data         288G  1.53T      2     11  11.3K  57.1K</screen>

      <para>To continuously monitor <acronym>I/O</acronym> activity, a
	number can be specified as the last parameter, indicating a
	interval in seconds to wait between updates.  The next
	statistic line is printed after each interval.  Press
	<keycombo action="simul">
	  <keycap>Ctrl</keycap>
	  <keycap>C</keycap>
	</keycombo> to stop this continuous monitoring.
	Alternatively, give a second number on the command line after
	the interval to specify the total number of statistics to
	display.</para>

      <para>Even more detailed <acronym>I/O</acronym> statistics can
	be displayed with <option>-v</option>.  Each device in the
	pool is shown with a statistics line.  This is useful in
	seeing how many read and write operations are being performed
	on each device, and can help determine if any individual
	device is slowing down the pool.  This example shows a
	mirrored pool with two devices:</para>

      <screen>&prompt.root; <userinput>zpool iostat -v </userinput>
                            capacity     operations    bandwidth
pool                     alloc   free   read  write   read  write
-----------------------  -----  -----  -----  -----  -----  -----
data                      288G  1.53T      2     12  9.23K  61.5K
  mirror                  288G  1.53T      2     12  9.23K  61.5K
    ada1                     -      -      0      4  5.61K  61.7K
    ada2                     -      -      1      4  5.04K  61.7K
-----------------------  -----  -----  -----  -----  -----  -----</screen>
    </sect2>

    <sect2 xml:id="zfs-zpool-split">
      <title>Splitting a Storage Pool</title>

      <para>A pool consisting of one or more mirror vdevs can be split
	into two pools.  Unless otherwise specified, the last member
	of each mirror is detached and used to create a new pool
	containing the same data.  The operation should first be
	attempted with <option>-n</option>.  The details of the
	proposed operation are displayed without it actually being
	performed.  This helps confirm that the operation will do what
	the user intends.</para>
    </sect2>
  </sect1>

  <sect1 xml:id="zfs-zfs">
    <title><command>zfs</command> Administration</title>

    <para>The <command>zfs</command> utility is responsible for
      creating, destroying, and managing all <acronym>ZFS</acronym>
      datasets that exist within a pool.  The pool is managed using
      <link
	linkend="zfs-zpool"><command>zpool</command></link>.</para>

    <sect2 xml:id="zfs-zfs-create">
      <title>Creating and Destroying Datasets</title>

      <para>Unlike traditional disks and volume managers, space in
	<acronym>ZFS</acronym> is <emphasis>not</emphasis>
	preallocated.  With traditional file systems, after all of the
	space is partitioned and assigned, there is no way to add an
	additional file system without adding a new disk.  With
	<acronym>ZFS</acronym>, new file systems can be created at any
	time.  Each <link
	  linkend="zfs-term-dataset"><emphasis>dataset</emphasis></link>
	has properties including features like compression,
	deduplication, caching, and quotas, as well as other useful
	properties like readonly, case sensitivity, network file
	sharing, and a mount point.  Datasets can be nested inside
	each other, and child datasets will inherit properties from
	their parents.  Each dataset can be administered,
	<link linkend="zfs-zfs-allow">delegated</link>,
	<link linkend="zfs-zfs-send">replicated</link>,
	<link linkend="zfs-zfs-snapshot">snapshotted</link>,
	<link linkend="zfs-zfs-jail">jailed</link>, and destroyed as a
	unit.  There are many advantages to creating a separate
	dataset for each different type or set of files.  The only
	drawbacks to having an extremely large number of datasets is
	that some commands like <command>zfs list</command> will be
	slower, and the mounting of hundreds or even thousands of
	datasets can slow the &os; boot process.</para>

      <para>Create a new dataset and enable <link
	  linkend="zfs-term-compression-lz4">LZ4
	  compression</link> on it:</para>

      <screen>&prompt.root; <userinput>zfs list</userinput>
NAME                  USED  AVAIL  REFER  MOUNTPOINT
mypool                781M  93.2G   144K  none
mypool/ROOT           777M  93.2G   144K  none
mypool/ROOT/default   777M  93.2G   777M  /
mypool/tmp            176K  93.2G   176K  /tmp
mypool/usr            616K  93.2G   144K  /usr
mypool/usr/home       184K  93.2G   184K  /usr/home
mypool/usr/ports      144K  93.2G   144K  /usr/ports
mypool/usr/src        144K  93.2G   144K  /usr/src
mypool/var           1.20M  93.2G   608K  /var
mypool/var/crash      148K  93.2G   148K  /var/crash
mypool/var/log        178K  93.2G   178K  /var/log
mypool/var/mail       144K  93.2G   144K  /var/mail
mypool/var/tmp        152K  93.2G   152K  /var/tmp
&prompt.root; <userinput>zfs create -o compress=lz4 <replaceable>mypool/usr/mydataset</replaceable></userinput>
&prompt.root; <userinput>zfs list</userinput>
NAME                   USED  AVAIL  REFER  MOUNTPOINT
mypool                 781M  93.2G   144K  none
mypool/ROOT            777M  93.2G   144K  none
mypool/ROOT/default    777M  93.2G   777M  /
mypool/tmp             176K  93.2G   176K  /tmp
mypool/usr             704K  93.2G   144K  /usr
mypool/usr/home        184K  93.2G   184K  /usr/home
mypool/usr/mydataset  87.5K  93.2G  87.5K  /usr/mydataset
mypool/usr/ports       144K  93.2G   144K  /usr/ports
mypool/usr/src         144K  93.2G   144K  /usr/src
mypool/var            1.20M  93.2G   610K  /var
mypool/var/crash       148K  93.2G   148K  /var/crash
mypool/var/log         178K  93.2G   178K  /var/log
mypool/var/mail        144K  93.2G   144K  /var/mail
mypool/var/tmp         152K  93.2G   152K  /var/tmp</screen>

      <para>Destroying a dataset is much quicker than deleting all
	of the files that reside on the dataset, as it does not
	involve scanning all of the files and updating all of the
	corresponding metadata.</para>

      <para>Destroy the previously-created dataset:</para>

      <screen>&prompt.root; <userinput>zfs list</userinput>
NAME                   USED  AVAIL  REFER  MOUNTPOINT
mypool                 880M  93.1G   144K  none
mypool/ROOT            777M  93.1G   144K  none
mypool/ROOT/default    777M  93.1G   777M  /
mypool/tmp             176K  93.1G   176K  /tmp
mypool/usr             101M  93.1G   144K  /usr
mypool/usr/home        184K  93.1G   184K  /usr/home
mypool/usr/mydataset   100M  93.1G   100M  /usr/mydataset
mypool/usr/ports       144K  93.1G   144K  /usr/ports
mypool/usr/src         144K  93.1G   144K  /usr/src
mypool/var            1.20M  93.1G   610K  /var
mypool/var/crash       148K  93.1G   148K  /var/crash
mypool/var/log         178K  93.1G   178K  /var/log
mypool/var/mail        144K  93.1G   144K  /var/mail
mypool/var/tmp         152K  93.1G   152K  /var/tmp
&prompt.root; <userinput>zfs destroy <replaceable>mypool/usr/mydataset</replaceable></userinput>
&prompt.root; <userinput>zfs list</userinput>
NAME                  USED  AVAIL  REFER  MOUNTPOINT
mypool                781M  93.2G   144K  none
mypool/ROOT           777M  93.2G   144K  none
mypool/ROOT/default   777M  93.2G   777M  /
mypool/tmp            176K  93.2G   176K  /tmp
mypool/usr            616K  93.2G   144K  /usr
mypool/usr/home       184K  93.2G   184K  /usr/home
mypool/usr/ports      144K  93.2G   144K  /usr/ports
mypool/usr/src        144K  93.2G   144K  /usr/src
mypool/var           1.21M  93.2G   612K  /var
mypool/var/crash      148K  93.2G   148K  /var/crash
mypool/var/log        178K  93.2G   178K  /var/log
mypool/var/mail       144K  93.2G   144K  /var/mail
mypool/var/tmp        152K  93.2G   152K  /var/tmp</screen>

      <para>In modern versions of <acronym>ZFS</acronym>,
	<command>zfs destroy</command> is asynchronous, and the free
	space might take several minutes to appear in the pool.  Use
	<command>zpool get freeing
	  <replaceable>poolname</replaceable></command> to see the
	<literal>freeing</literal> property, indicating how many
	datasets are having their blocks freed in the background.
	If there are child datasets, like
	<link linkend="zfs-term-snapshot">snapshots</link> or other
	datasets, then the parent cannot be destroyed.  To destroy a
	dataset and all of its children, use <option>-r</option> to
	recursively destroy the dataset and all of its children.
	Use <option>-n</option> <option>-v</option>to list datasets
	and snapshots that would be destroyed by this operation, but
	do not actually destroy anything.  Space that would be
	reclaimed by destruction of snapshots is also shown.</para>
    </sect2>

    <sect2 xml:id="zfs-zfs-volume">
      <title>Creating and Destroying Volumes</title>

      <para>A volume is a special type of dataset.  Rather than being
	mounted as a file system, it is exposed as a block device
	under
	<filename>/dev/zvol/<replaceable>poolname</replaceable>/<replaceable>dataset</replaceable></filename>.
	This allows the volume to be used for other file systems, to
	back the disks of a virtual machine, or to be exported using
	protocols like <acronym>iSCSI</acronym> or
	<acronym>HAST</acronym>.</para>

      <para>A volume can be formatted with any file system, or used
	without a file system to store raw data.  To the user, a
	volume appears to be a regular disk.  Putting ordinary file
	systems on these <emphasis>zvols</emphasis> provides features
	that ordinary disks or file systems do not normally have.  For
	example, using the compression property on a 250&nbsp;MB
	volume allows creation of a compressed <acronym>FAT</acronym>
	file system.</para>

      <screen>&prompt.root; <userinput>zfs create -V 250m -o compression=on tank/fat32</userinput>
&prompt.root; <userinput>zfs list tank</userinput>
NAME USED AVAIL REFER MOUNTPOINT
tank 258M  670M   31K /tank
&prompt.root; <userinput>newfs_msdos -F32 /dev/zvol/tank/fat32</userinput>
&prompt.root; <userinput>mount -t msdosfs /dev/zvol/tank/fat32 /mnt</userinput>
&prompt.root; <userinput>df -h /mnt | grep fat32</userinput>
Filesystem           Size Used Avail Capacity Mounted on
/dev/zvol/tank/fat32 249M  24k  249M     0%   /mnt
&prompt.root; <userinput>mount | grep fat32</userinput>
/dev/zvol/tank/fat32 on /mnt (msdosfs, local)</screen>

      <para>Destroying a volume is much the same as destroying a
	regular file system dataset.  The operation is nearly
	instantaneous, but it may take several minutes for the free
	space to be reclaimed in the background.</para>
    </sect2>

    <sect2 xml:id="zfs-zfs-rename">
      <title>Renaming a Dataset</title>

      <para>The name of a dataset can be changed with
	<command>zfs rename</command>.  The parent of a dataset can
	also be changed with this command.  Renaming a dataset to be
	under a different parent dataset will change the value of
	those properties that are inherited from the parent dataset.
	When a dataset is renamed, it is unmounted and then remounted
	in the new location (which is inherited from the new parent
	dataset).  This behavior can be prevented with
	<option>-u</option>.</para>

      <para>Rename a dataset and move it to be under a different
	parent dataset:</para>

      <screen>&prompt.root; <userinput>zfs list</userinput>
NAME                   USED  AVAIL  REFER  MOUNTPOINT
mypool                 780M  93.2G   144K  none
mypool/ROOT            777M  93.2G   144K  none
mypool/ROOT/default    777M  93.2G   777M  /
mypool/tmp             176K  93.2G   176K  /tmp
mypool/usr             704K  93.2G   144K  /usr
mypool/usr/home        184K  93.2G   184K  /usr/home
mypool/usr/mydataset  87.5K  93.2G  87.5K  /usr/mydataset
mypool/usr/ports       144K  93.2G   144K  /usr/ports
mypool/usr/src         144K  93.2G   144K  /usr/src
mypool/var            1.21M  93.2G   614K  /var
mypool/var/crash       148K  93.2G   148K  /var/crash
mypool/var/log         178K  93.2G   178K  /var/log
mypool/var/mail        144K  93.2G   144K  /var/mail
mypool/var/tmp         152K  93.2G   152K  /var/tmp
&prompt.root; <userinput>zfs rename <replaceable>mypool/usr/mydataset</replaceable> <replaceable>mypool/var/newname</replaceable></userinput>
&prompt.root; <userinput>zfs list</userinput>
NAME                  USED  AVAIL  REFER  MOUNTPOINT
mypool                780M  93.2G   144K  none
mypool/ROOT           777M  93.2G   144K  none
mypool/ROOT/default   777M  93.2G   777M  /
mypool/tmp            176K  93.2G   176K  /tmp
mypool/usr            616K  93.2G   144K  /usr
mypool/usr/home       184K  93.2G   184K  /usr/home
mypool/usr/ports      144K  93.2G   144K  /usr/ports
mypool/usr/src        144K  93.2G   144K  /usr/src
mypool/var           1.29M  93.2G   614K  /var
mypool/var/crash      148K  93.2G   148K  /var/crash
mypool/var/log        178K  93.2G   178K  /var/log
mypool/var/mail       144K  93.2G   144K  /var/mail
mypool/var/newname   87.5K  93.2G  87.5K  /var/newname
mypool/var/tmp        152K  93.2G   152K  /var/tmp</screen>

      <para>Snapshots can also be renamed like this.  Due to the
	nature of snapshots, they cannot be renamed into a different
	parent dataset.  To rename a recursive snapshot, specify
	<option>-r</option>, and all snapshots with the same name in
	child datasets with also be renamed.</para>

      <screen>&prompt.root; <userinput>zfs list -t snapshot</userinput>
NAME                                USED  AVAIL  REFER  MOUNTPOINT
mypool/var/newname@first_snapshot      0      -  87.5K  -
&prompt.root; <userinput>zfs rename <replaceable>mypool/var/newname@first_snapshot</replaceable> <replaceable>new_snapshot_name</replaceable></userinput>
&prompt.root; <userinput>zfs list -t snapshot</userinput>
NAME                                   USED  AVAIL  REFER  MOUNTPOINT
mypool/var/newname@new_snapshot_name      0      -  87.5K  -</screen>
    </sect2>

    <sect2 xml:id="zfs-zfs-set">
      <title>Setting Dataset Properties</title>

      <para>Each <acronym>ZFS</acronym> dataset has a number of
	properties that control its behavior.  Most properties are
	automatically inherited from the parent dataset, but can be
	overridden locally.  Set a property on a dataset with
	<command>zfs set
	  <replaceable>property</replaceable>=<replaceable>value</replaceable>
	  <replaceable>dataset</replaceable></command>.  Most
	properties have a limited set of valid values,
	<command>zfs get</command> will display each possible property
	and valid values.  Most properties can be reverted to their
	inherited values using <command>zfs inherit</command>.</para>

      <para>User-defined properties can also be set.  They become part
	of the dataset configuration and can be used to provide
	additional information about the dataset or its contents.  To
	distinguish these custom properties from the ones supplied as
	part of <acronym>ZFS</acronym>, a colon (<literal>:</literal>)
	is used to create a custom namespace for the property.</para>

      <screen>&prompt.root; <userinput>zfs set <replaceable>custom</replaceable>:<replaceable>costcenter</replaceable>=<replaceable>1234</replaceable> <replaceable>tank</replaceable></userinput>
&prompt.root; <userinput>zfs get <replaceable>custom</replaceable>:<replaceable>costcenter</replaceable> <replaceable>tank</replaceable></userinput>
NAME PROPERTY           VALUE SOURCE
tank custom:costcenter  1234  local</screen>

      <para>To remove a custom property, use
	<command>zfs inherit</command> with <option>-r</option>.  If
	the custom property is not defined in any of the parent
	datasets, it will be removed completely (although the changes
	are still recorded in the pool's history).</para>

      <screen>&prompt.root; <userinput>zfs inherit -r <replaceable>custom</replaceable>:<replaceable>costcenter</replaceable> <replaceable>tank</replaceable></userinput>
&prompt.root; <userinput>zfs get <replaceable>custom</replaceable>:<replaceable>costcenter</replaceable> <replaceable>tank</replaceable></userinput>
NAME    PROPERTY           VALUE              SOURCE
tank    custom:costcenter  -                  -
&prompt.root; <userinput>zfs get all <replaceable>tank</replaceable> | grep <replaceable>custom</replaceable>:<replaceable>costcenter</replaceable></userinput>
&prompt.root;</screen>
    </sect2>

    <sect2 xml:id="zfs-zfs-snapshot">
      <title>Managing Snapshots</title>

      <para><link linkend="zfs-term-snapshot">Snapshots</link> are one
	of the most powerful features of <acronym>ZFS</acronym>.  A
	snapshot provides a read-only, point-in-time copy of the
	dataset.  With Copy-On-Write (<acronym>COW</acronym>),
	snapshots can be created quickly by preserving the older
	version of the data on disk.  If no snapshots exist, space is
	reclaimed for future use when data is rewritten or deleted.
	Snapshots preserve disk space by recording only the
	differences between the current dataset and a previous
	version.  Snapshots are allowed only on whole datasets, not on
	individual files or directories.  When a snapshot is created
	from a dataset, everything contained in it is duplicated.
	This includes the file system properties, files, directories,
	permissions, and so on.  Snapshots use no additional space
	when they are first created, only consuming space as the
	blocks they reference are changed.  Recursive snapshots taken
	with <option>-r</option> create a snapshot with the same name
	on the dataset and all of its children, providing a consistent
	moment-in-time snapshot of all of the file systems.  This can
	be important when an application has files on multiple
	datasets that are related or dependent upon each other.
	Without snapshots, a backup would have copies of the files
	from different points in time.</para>

      <para>Snapshots in <acronym>ZFS</acronym> provide a variety of
	features that even other file systems with snapshot
	functionality lack.  A typical example of snapshot use is to
	have a quick way of backing up the current state of the file
	system when a risky action like a software installation or a
	system upgrade is performed.  If the action fails, the
	snapshot can be rolled back and the system has the same state
	as when the snapshot was created.  If the upgrade was
	successful, the snapshot can be deleted to free up space.
	Without snapshots, a failed upgrade often requires a restore
	from backup, which is tedious, time consuming, and may require
	downtime during which the system cannot be used.  Snapshots
	can be rolled back quickly, even while the system is running
	in normal operation, with little or no downtime.  The time
	savings are enormous with multi-terabyte storage systems and
	the time required to copy the data from backup.  Snapshots are
	not a replacement for a complete backup of a pool, but can be
	used as a quick and easy way to store a copy of the dataset at
	a specific point in time.</para>

      <sect3 xml:id="zfs-zfs-snapshot-creation">
	<title>Creating Snapshots</title>

	<para>Snapshots are created  with <command>zfs snapshot
	    <replaceable>dataset</replaceable>@<replaceable>snapshotname</replaceable></command>.
	  Adding <option>-r</option> creates a snapshot recursively,
	  with the same name on all child datasets.</para>

	<para>Create a recursive snapshot of the entire pool:</para>

	<screen>&prompt.root; <userinput>zfs list -t all</userinput>
NAME                                   USED  AVAIL  REFER  MOUNTPOINT
mypool                                 780M  93.2G   144K  none
mypool/ROOT                            777M  93.2G   144K  none
mypool/ROOT/default                    777M  93.2G   777M  /
mypool/tmp                             176K  93.2G   176K  /tmp
mypool/usr                             616K  93.2G   144K  /usr
mypool/usr/home                        184K  93.2G   184K  /usr/home
mypool/usr/ports                       144K  93.2G   144K  /usr/ports
mypool/usr/src                         144K  93.2G   144K  /usr/src
mypool/var                            1.29M  93.2G   616K  /var
mypool/var/crash                       148K  93.2G   148K  /var/crash
mypool/var/log                         178K  93.2G   178K  /var/log
mypool/var/mail                        144K  93.2G   144K  /var/mail
mypool/var/newname                    87.5K  93.2G  87.5K  /var/newname
mypool/var/newname@new_snapshot_name      0      -  87.5K  -
mypool/var/tmp                         152K  93.2G   152K  /var/tmp
&prompt.root; <userinput>zfs snapshot -r <replaceable>mypool@my_recursive_snapshot</replaceable></userinput>
&prompt.root; <userinput>zfs list -t snapshot</userinput>
NAME                                        USED  AVAIL  REFER  MOUNTPOINT
mypool@my_recursive_snapshot                   0      -   144K  -
mypool/ROOT@my_recursive_snapshot              0      -   144K  -
mypool/ROOT/default@my_recursive_snapshot      0      -   777M  -
mypool/tmp@my_recursive_snapshot               0      -   176K  -
mypool/usr@my_recursive_snapshot               0      -   144K  -
mypool/usr/home@my_recursive_snapshot          0      -   184K  -
mypool/usr/ports@my_recursive_snapshot         0      -   144K  -
mypool/usr/src@my_recursive_snapshot           0      -   144K  -
mypool/var@my_recursive_snapshot               0      -   616K  -
mypool/var/crash@my_recursive_snapshot         0      -   148K  -
mypool/var/log@my_recursive_snapshot           0      -   178K  -
mypool/var/mail@my_recursive_snapshot          0      -   144K  -
mypool/var/newname@new_snapshot_name           0      -  87.5K  -
mypool/var/newname@my_recursive_snapshot       0      -  87.5K  -
mypool/var/tmp@my_recursive_snapshot           0      -   152K  -</screen>

	<para>Snapshots are not shown by a normal
	  <command>zfs list</command> operation.  To list snapshots,
	  <option>-t snapshot</option> is appended to
	  <command>zfs list</command>.  <option>-t all</option>
	  displays both file systems and snapshots.</para>

	<para>Snapshots are not mounted directly, so path is shown in
	  the <literal>MOUNTPOINT</literal> column.  There is no
	  mention of available disk space in the
	  <literal>AVAIL</literal> column, as snapshots cannot be
	  written to after they are created.  Compare the snapshot
	  to the original dataset from which it was created:</para>

	<screen>&prompt.root; <userinput>zfs list -rt all <replaceable>mypool/usr/home</replaceable></userinput>
NAME                                    USED  AVAIL  REFER  MOUNTPOINT
mypool/usr/home                         184K  93.2G   184K  /usr/home
mypool/usr/home@my_recursive_snapshot      0      -   184K  -</screen>

	<para>Displaying both the dataset and the snapshot together
	  reveals how snapshots work in
	  <link linkend="zfs-term-cow">COW</link> fashion.  They save
	  only the changes (<emphasis>delta</emphasis>) that were made
	  and not the complete file system contents all over again.
	  This means that snapshots take little space when few changes
	  are made.  Space usage can be made even more apparent by
	  copying a file to the dataset, then making a second
	  snapshot:</para>

	<screen>&prompt.root; <userinput>cp <replaceable>/etc/passwd</replaceable> <replaceable>/var/tmp</replaceable></userinput>
&prompt.root; zfs snapshot <replaceable>mypool/var/tmp</replaceable>@<replaceable>after_cp</replaceable>
&prompt.root; <userinput>zfs list -rt all <replaceable>mypool/var/tmp</replaceable></userinput>
NAME                                   USED  AVAIL  REFER  MOUNTPOINT
mypool/var/tmp                         206K  93.2G   118K  /var/tmp
mypool/var/tmp@my_recursive_snapshot    88K      -   152K  -
mypool/var/tmp@after_cp                   0      -   118K  -</screen>

	<para>The second snapshot contains only the changes to the
	  dataset after the copy operation.  This yields enormous
	  space savings.  Notice that the size of the snapshot
	  <replaceable>mypool/var/tmp@my_recursive_snapshot</replaceable>
	  also changed in the <literal>USED</literal>
	  column to indicate the changes between itself and the
	  snapshot taken afterwards.</para>
      </sect3>

      <sect3 xml:id="zfs-zfs-snapshot-diff">
	<title>Comparing Snapshots</title>

	<para>ZFS provides a built-in command to compare the
	  differences in content between two snapshots.  This is
	  helpful when many snapshots were taken over time and the
	  user wants to see how the file system has changed over time.
	  For example, <command>zfs diff</command> lets a user find
	  the latest snapshot that still contains a file that was
	  accidentally deleted.  Doing this for the two snapshots that
	  were created in the previous section yields this
	  output:</para>

	<screen>&prompt.root; <userinput>zfs list -rt all <replaceable>mypool/var/tmp</replaceable></userinput>
NAME                                   USED  AVAIL  REFER  MOUNTPOINT
mypool/var/tmp                         206K  93.2G   118K  /var/tmp
mypool/var/tmp@my_recursive_snapshot    88K      -   152K  -
mypool/var/tmp@after_cp                   0      -   118K  -
&prompt.root; <userinput>zfs diff <replaceable>mypool/var/tmp@my_recursive_snapshot</replaceable></userinput>
M       /var/tmp/
+       /var/tmp/passwd</screen>

	<para>The command lists the changes between the specified
	  snapshot (in this case
	  <literal><replaceable>mypool/var/tmp@my_recursive_snapshot</replaceable></literal>)
	  and the live file system.  The first column shows the
	  type of change:</para>

	<informaltable pgwide="1">
	  <tgroup cols="2">
	    <tbody valign="top">
	      <row>
		<entry>+</entry>
		<entry>The path or file was added.</entry>
	      </row>

	      <row>
		<entry>-</entry>
		<entry>The path or file was deleted.</entry>
	      </row>

	      <row>
		<entry>M</entry>
		<entry>The path or file was modified.</entry>
	      </row>

	      <row>
		<entry>R</entry>
		<entry>The path or file was renamed.</entry>
	      </row>
	    </tbody>
	  </tgroup>
	</informaltable>

	<para>Comparing the output with the table, it becomes clear
	  that <filename><replaceable>passwd</replaceable></filename>
	  was added after the snapshot
	  <literal><replaceable>mypool/var/tmp@my_recursive_snapshot</replaceable></literal>
	  was created.  This also resulted in a modification to the
	  parent directory mounted at
	  <literal><replaceable>/var/tmp</replaceable></literal>.</para>

	<para>Comparing two snapshots is helpful when using the
	  <acronym>ZFS</acronym> replication feature to transfer a
	  dataset to a different host for backup purposes.</para>

	<para>Compare two snapshots by providing the full dataset name
	  and snapshot name of both datasets:</para>

	<screen>&prompt.root; <userinput>cp /var/tmp/passwd /var/tmp/passwd.copy</userinput>
&prompt.root; <userinput>zfs snapshot <replaceable>mypool/var/tmp@diff_snapshot</replaceable></userinput>
&prompt.root; <userinput>zfs diff <replaceable>mypool/var/tmp@my_recursive_snapshot</replaceable> <replaceable>mypool/var/tmp@diff_snapshot</replaceable></userinput>
M       /var/tmp/
+       /var/tmp/passwd
+       /var/tmp/passwd.copy
&prompt.root; <userinput>zfs diff <replaceable>mypool/var/tmp@my_recursive_snapshot</replaceable> <replaceable>mypool/var/tmp@after_cp</replaceable></userinput>
M       /var/tmp/
+       /var/tmp/passwd</screen>

	<para>A backup administrator can compare two snapshots
	  received from the sending host and determine the actual
	  changes in the dataset.  See the
	  <link linkend="zfs-zfs-send">Replication</link> section for
	  more information.</para>
      </sect3>

      <sect3 xml:id="zfs-zfs-snapshot-rollback">
	<title>Snapshot Rollback</title>

	<para>When at least one snapshot is available, it can be
	  rolled back to at any time.  Most of the time this is the
	  case when the current state of the dataset is no longer
	  required and an older version is preferred.  Scenarios such
	  as local development tests have gone wrong, botched system
	  updates hampering the system's overall functionality, or the
	  requirement to restore accidentally deleted files or
	  directories are all too common occurrences.  Luckily,
	  rolling back a snapshot is just as easy as typing
	  <command>zfs rollback
	    <replaceable>snapshotname</replaceable></command>.
	  Depending on how many changes are involved, the operation
	  will finish in a certain amount of time.  During that time,
	  the dataset always remains in a consistent state, much like
	  a database that conforms to ACID principles is performing a
	  rollback.  This is happening while the dataset is live and
	  accessible without requiring a downtime.  Once the snapshot
	  has been rolled back, the dataset has the same state as it
	  had when the snapshot was originally taken.  All other data
	  in that dataset that was not part of the snapshot is
	  discarded.  Taking a snapshot of the current state of the
	  dataset before rolling back to a previous one is a good idea
	  when some data is required later.  This way, the user can
	  roll back and forth between snapshots without losing data
	  that is still valuable.</para>

	<para>In the first example, a snapshot is rolled back because
	  of a careless <command>rm</command> operation that removes
	  too much data than was intended.</para>

	<screen>&prompt.root; <userinput>zfs list -rt all <replaceable>mypool/var/tmp</replaceable></userinput>
NAME                                   USED  AVAIL  REFER  MOUNTPOINT
mypool/var/tmp                         262K  93.2G   120K  /var/tmp
mypool/var/tmp@my_recursive_snapshot    88K      -   152K  -
mypool/var/tmp@after_cp               53.5K      -   118K  -
mypool/var/tmp@diff_snapshot              0      -   120K  -
&prompt.user; <userinput>ls /var/tmp</userinput>
passwd          passwd.copy
&prompt.user; <userinput>rm /var/tmp/passwd*</userinput>
&prompt.user; <userinput>ls /var/tmp</userinput>
vi.recover
&prompt.user;</screen>

	<para>At this point, the user realized that too many files
	  were deleted and wants them back.  <acronym>ZFS</acronym>
	  provides an easy way to get them back using rollbacks, but
	  only when snapshots of important data are performed on a
	  regular basis.  To get the files back and start over from
	  the last snapshot, issue the command:</para>

	<screen>&prompt.root; <userinput>zfs rollback <replaceable>mypool/var/tmp@diff_snapshot</replaceable></userinput>
&prompt.user; <userinput>ls /var/tmp</userinput>
passwd          passwd.copy     vi.recover</screen>

	<para>The rollback operation restored the dataset to the state
	  of the last snapshot.  It is also possible to roll back to a
	  snapshot that was taken much earlier and has other snapshots
	  that were created after it.  When trying to do this,
	  <acronym>ZFS</acronym> will issue this warning:</para>

	<screen>&prompt.root; <userinput>zfs list -rt snapshot <replaceable>mypool/var/tmp</replaceable></userinput>
AME                                   USED  AVAIL  REFER  MOUNTPOINT
mypool/var/tmp@my_recursive_snapshot    88K      -   152K  -
mypool/var/tmp@after_cp               53.5K      -   118K  -
mypool/var/tmp@diff_snapshot              0      -   120K  -
&prompt.root; <userinput>zfs rollback <replaceable>mypool/var/tmp@my_recursive_snapshot</replaceable></userinput>
cannot rollback to 'mypool/var/tmp@my_recursive_snapshot': more recent snapshots exist
use '-r' to force deletion of the following snapshots:
mypool/var/tmp@after_cp
mypool/var/tmp@diff_snapshot</screen>

	<para>This warning means that snapshots exist between the
	  current state of the dataset and the snapshot to which the
	  user wants to roll back.  To complete the rollback, these
	  snapshots must be deleted.  <acronym>ZFS</acronym> cannot
	  track all the changes between different states of the
	  dataset, because snapshots are read-only.
	  <acronym>ZFS</acronym> will not delete the affected
	  snapshots unless the user specifies <option>-r</option> to
	  indicate that this is the desired action.  If that is the
	  intention, and the consequences of losing all intermediate
	  snapshots is understood, the command can be issued:</para>

	<screen>&prompt.root; <userinput>zfs rollback -r <replaceable>mypool/var/tmp@my_recursive_snapshot</replaceable></userinput>
&prompt.root; <userinput>zfs list -rt snapshot <replaceable>mypool/var/tmp</replaceable></userinput>
NAME                                   USED  AVAIL  REFER  MOUNTPOINT
mypool/var/tmp@my_recursive_snapshot     8K      -   152K  -
&prompt.user; <userinput>ls /var/tmp</userinput>
vi.recover</screen>

	<para>The output from <command>zfs list -t snapshot</command>
	  confirms that the intermediate snapshots
	  were removed as a result of
	  <command>zfs rollback -r</command>.</para>
      </sect3>

      <sect3 xml:id="zfs-zfs-snapshot-snapdir">
	<title>Restoring Individual Files from Snapshots</title>

	<para>Snapshots are mounted in a hidden directory under the
	  parent dataset:
	  <filename>.zfs/snapshots/<replaceable>snapshotname</replaceable></filename>.
	  By default, these directories will not be displayed even
	  when a standard <command>ls -a</command> is issued.
	  Although the directory is not displayed, it is there
	  nevertheless and can be accessed like any normal directory.
	  The property named <literal>snapdir</literal> controls
	  whether these hidden directories show up in a directory
	  listing.  Setting the property to <literal>visible</literal>
	  allows them to appear in the output of <command>ls</command>
	  and other commands that deal with directory contents.</para>

	<screen>&prompt.root; <userinput>zfs get snapdir <replaceable>mypool/var/tmp</replaceable></userinput>
NAME            PROPERTY  VALUE    SOURCE
mypool/var/tmp  snapdir   hidden   default
&prompt.user; <userinput>ls -a /var/tmp</userinput>
.               ..              passwd          vi.recover
&prompt.root; <userinput>zfs set snapdir=visible <replaceable>mypool/var/tmp</replaceable></userinput>
&prompt.user; <userinput>ls -a /var/tmp</userinput>
.               ..              .zfs            passwd          vi.recover</screen>

	<para>Individual files can easily be restored to a previous
	  state by copying them from the snapshot back to the parent
	  dataset.  The directory structure below
	  <filename>.zfs/snapshot</filename> has a directory named
	  exactly like the snapshots taken earlier to make it easier
	  to identify them.  In the next example, it is assumed that a
	  file is to be restored from the hidden
	  <filename>.zfs</filename> directory by copying it from the
	  snapshot that contained the latest version of the
	  file:</para>

	<screen>&prompt.root; <userinput>rm /var/tmp/passwd</userinput>
&prompt.user; <userinput>ls -a /var/tmp</userinput>
.               ..              .zfs            vi.recover
&prompt.root; <userinput>ls /var/tmp/.zfs/snapshot</userinput>
after_cp                my_recursive_snapshot
&prompt.root; <userinput>ls /var/tmp/.zfs/snapshot/<replaceable>after_cp</replaceable></userinput>
passwd          vi.recover
&prompt.root; <userinput>cp /var/tmp/.zfs/snapshot/<replaceable>after_cp/passwd</replaceable> <replaceable>/var/tmp</replaceable></userinput></screen>

	<para>When <command>ls .zfs/snapshot</command> was issued, the
	  <literal>snapdir</literal> property might have been set to
	  hidden, but it would still be possible to list the contents
	  of that directory.  It is up to the administrator to decide
	  whether these directories will be displayed.  It is possible
	  to display these for certain datasets and prevent it for
	  others.  Copying files or directories from this hidden
	  <filename>.zfs/snapshot</filename> is simple enough.  Trying
	  it the other way around results in this error:</para>

	<screen>&prompt.root; <userinput>cp <replaceable>/etc/rc.conf</replaceable> /var/tmp/.zfs/snapshot/<replaceable>after_cp/</replaceable></userinput>
cp: /var/tmp/.zfs/snapshot/after_cp/rc.conf: Read-only file system</screen>

	<para>The error reminds the user that snapshots are read-only
	  and can not be changed after creation.  No files can be
	  copied into or removed from snapshot directories because
	  that would change the state of the dataset they
	  represent.</para>

	<para>Snapshots consume space based on how much the parent
	  file system has changed since the time of the snapshot.  The
	  <literal>written</literal> property of a snapshot tracks how
	  much space is being used by the snapshot.</para>

	<para>Snapshots are destroyed and the space reclaimed with
	  <command>zfs destroy
	    <replaceable>dataset</replaceable>@<replaceable>snapshot</replaceable></command>.
	  Adding <option>-r</option> recursively removes all snapshots
	  with the same name under the parent dataset.  Adding
	  <option>-n -v</option> to the command displays a list of the
	  snapshots that would be deleted and an estimate of how much
	  space would be reclaimed without performing the actual
	  destroy operation.</para>
      </sect3>
    </sect2>

    <sect2 xml:id="zfs-zfs-clones">
      <title>Managing Clones</title>

      <para>A clone is a copy of a snapshot that is treated more like
	a regular dataset.  Unlike a snapshot, a clone is not read
	only, is mounted, and can have its own properties.  Once a
	clone has been created using <command>zfs clone</command>, the
	snapshot it was created from cannot be destroyed.  The
	child/parent relationship between the clone and the snapshot
	can be reversed using <command>zfs promote</command>.  After a
	clone has been promoted, the snapshot becomes a child of the
	clone, rather than of the original parent dataset.  This will
	change how the space is accounted, but not actually change the
	amount of space consumed.  The clone can be mounted at any
	point within the <acronym>ZFS</acronym> file system hierarchy,
	not just below the original location of the snapshot.</para>

      <para>To demonstrate the clone feature, this example dataset is
	used:</para>

      <screen>&prompt.root; <userinput>zfs list -rt all <replaceable>camino/home/joe</replaceable></userinput>
NAME                    USED  AVAIL  REFER  MOUNTPOINT
camino/home/joe         108K   1.3G    87K  /usr/home/joe
camino/home/joe@plans    21K      -  85.5K  -
camino/home/joe@backup    0K      -    87K  -</screen>

      <para>A typical use for clones is to experiment with a specific
	dataset while keeping the snapshot around to fall back to in
	case something goes wrong.  Since snapshots can not be
	changed, a read/write clone of a snapshot is created.  After
	the desired result is achieved in the clone, the clone can be
	promoted to a dataset and the old file system removed.  This
	is not strictly necessary, as the clone and dataset can
	coexist without problems.</para>

      <screen>&prompt.root; <userinput>zfs clone <replaceable>camino/home/joe</replaceable>@<replaceable>backup</replaceable> <replaceable>camino/home/joenew</replaceable></userinput>
&prompt.root; <userinput>ls /usr/home/joe*</userinput>
/usr/home/joe:
backup.txz     plans.txt

/usr/home/joenew:
backup.txz     plans.txt
&prompt.root; <userinput>df -h /usr/home</userinput>
Filesystem          Size    Used   Avail Capacity  Mounted on
usr/home/joe        1.3G     31k    1.3G     0%    /usr/home/joe
usr/home/joenew     1.3G     31k    1.3G     0%    /usr/home/joenew</screen>

      <para>After a clone is created it is an exact copy of the state
	the dataset was in when the snapshot was taken.  The clone can
	now be changed independently from its originating dataset.
	The only connection between the two is the snapshot.
	<acronym>ZFS</acronym> records this connection in the property
	<literal>origin</literal>.  Once the dependency between the
	snapshot and the clone has been removed by promoting the clone
	using <command>zfs promote</command>, the
	<literal>origin</literal> of the clone is removed as it is now
	an independent dataset.  This example demonstrates it:</para>

      <screen>&prompt.root; <userinput>zfs get origin <replaceable>camino/home/joenew</replaceable></userinput>
NAME                  PROPERTY  VALUE                     SOURCE
camino/home/joenew    origin    camino/home/joe@backup    -
&prompt.root; <userinput>zfs promote <replaceable>camino/home/joenew</replaceable></userinput>
&prompt.root; <userinput>zfs get origin <replaceable>camino/home/joenew</replaceable></userinput>
NAME                  PROPERTY  VALUE   SOURCE
camino/home/joenew    origin    -       -</screen>

      <para>After making some changes like copying
	<filename>loader.conf</filename> to the promoted clone, for
	example, the old directory becomes obsolete in this case.
	Instead, the promoted clone can replace it.  This can be
	achieved by two consecutive commands: <command>zfs
	  destroy</command> on the old dataset and <command>zfs
	  rename</command> on the clone to name it like the old
	dataset (it could also get an entirely different name).</para>

      <screen>&prompt.root; <userinput>cp <replaceable>/boot/defaults/loader.conf</replaceable> <replaceable>/usr/home/joenew</replaceable></userinput>
&prompt.root; <userinput>zfs destroy -f <replaceable>camino/home/joe</replaceable></userinput>
&prompt.root; <userinput>zfs rename <replaceable>camino/home/joenew</replaceable> <replaceable>camino/home/joe</replaceable></userinput>
&prompt.root; <userinput>ls /usr/home/joe</userinput>
backup.txz     loader.conf     plans.txt
&prompt.root; <userinput>df -h <replaceable>/usr/home</replaceable></userinput>
Filesystem          Size    Used   Avail Capacity  Mounted on
usr/home/joe        1.3G    128k    1.3G     0%    /usr/home/joe</screen>

      <para>The cloned snapshot is now handled like an ordinary
	dataset.  It contains all the data from the original snapshot
	plus the files that were added to it like
	<filename>loader.conf</filename>.  Clones can be used in
	different scenarios to provide useful features to ZFS users.
	For example, jails could be provided as snapshots containing
	different sets of installed applications.  Users can clone
	these snapshots and add their own applications as they see
	fit.  Once they are satisfied with the changes, the clones can
	be promoted to full datasets and provided to end users to work
	with like they would with a real dataset.  This saves time and
	administrative overhead when providing these jails.</para>
    </sect2>

    <sect2 xml:id="zfs-zfs-send">
      <title>Replication</title>

      <para>Keeping data on a single pool in one location exposes
	it to risks like theft and natural or human disasters.  Making
	regular backups of the entire pool is vital.
	<acronym>ZFS</acronym> provides a built-in serialization
	feature that can send a stream representation of the data to
	standard output.  Using this technique, it is possible to not
	only store the data on another pool connected to the local
	system, but also to send it over a network to another system.
	Snapshots are the basis for this replication (see the section
	on <link linkend="zfs-zfs-snapshot"><acronym>ZFS</acronym>
	  snapshots</link>).  The commands used for replicating data
	are <command>zfs send</command> and
	<command>zfs receive</command>.</para>

      <para>These examples demonstrate <acronym>ZFS</acronym>
	replication with these two pools:</para>

      <screen>&prompt.root; <userinput>zpool list</userinput>
NAME    SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
backup  960M    77K   896M     0%  1.00x  ONLINE  -
mypool  984M  43.7M   940M     4%  1.00x  ONLINE  -</screen>

      <para>The pool named <replaceable>mypool</replaceable> is the
	primary pool where data is written to and read from on a
	regular basis.  A second pool,
	<replaceable>backup</replaceable> is used as a standby in case
	the primary pool becomes unavailable.  Note that this
	fail-over is not done automatically by <acronym>ZFS</acronym>,
	but must be manually done by a system administrator when
	needed.  A snapshot is used to provide a consistent version of
	the file system to be replicated.  Once a snapshot of
	<replaceable>mypool</replaceable> has been created, it can be
	copied to the <replaceable>backup</replaceable> pool.  Only
	snapshots can be replicated.  Changes made since the most
	recent snapshot will not be included.</para>

      <screen>&prompt.root; <userinput>zfs snapshot <replaceable>mypool</replaceable>@<replaceable>backup1</replaceable></userinput>
&prompt.root; <userinput>zfs list -t snapshot</userinput>
NAME                    USED  AVAIL  REFER  MOUNTPOINT
mypool@backup1             0      -  43.6M  -</screen>

      <para>Now that a snapshot exists, <command>zfs send</command>
	can be used to create a stream representing the contents of
	the snapshot.  This stream can be stored as a file or received
	by another pool.  The stream is written to standard output,
	but must be redirected to a file or pipe or an error is
	produced:</para>

      <screen>&prompt.root; <userinput>zfs send <replaceable>mypool</replaceable>@<replaceable>backup1</replaceable></userinput>
Error: Stream can not be written to a terminal.
You must redirect standard output.</screen>

      <para>To back up a dataset with <command>zfs send</command>,
	redirect to a file located on the mounted backup pool.  Ensure
	that the pool has enough free space to accommodate the size of
	the snapshot being sent, which means all of the data contained
	in the snapshot, not just the changes from the previous
	snapshot.</para>

      <screen>&prompt.root; <userinput>zfs send <replaceable>mypool</replaceable>@<replaceable>backup1</replaceable> > <replaceable>/backup/backup1</replaceable></userinput>
&prompt.root; <userinput>zpool list</userinput>
NAME    SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
backup  960M  63.7M   896M     6%  1.00x  ONLINE  -
mypool  984M  43.7M   940M     4%  1.00x  ONLINE  -</screen>

      <para>The <command>zfs send</command> transferred all the data
	in the snapshot called <replaceable>backup1</replaceable> to
	the pool named <replaceable>backup</replaceable>.  Creating
	and sending these snapshots can be done automatically with a
	&man.cron.8; job.</para>

      <para>Instead of storing the backups as archive files,
	<acronym>ZFS</acronym> can receive them as a live file system,
	allowing the backed up data to be accessed directly.  To get
	to the actual data contained in those streams,
	<command>zfs receive</command> is used to transform the
	streams back into files and directories.  The example below
	combines <command>zfs send</command> and
	<command>zfs receive</command> using a pipe to copy the data
	from one pool to another.  The data can be used directly on
	the receiving pool after the transfer is complete.  A dataset
	can only be replicated to an empty dataset.</para>

      <screen>&prompt.root; <userinput>zfs snapshot <replaceable>mypool</replaceable>@<replaceable>replica1</replaceable></userinput>
&prompt.root; <userinput>zfs send -v <replaceable>mypool</replaceable>@<replaceable>replica1</replaceable> | zfs receive <replaceable>backup/mypool</replaceable></userinput>
send from @ to mypool@replica1 estimated size is 50.1M
total estimated size is 50.1M
TIME        SENT   SNAPSHOT

&prompt.root; <userinput>zpool list</userinput>
NAME    SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
backup  960M  63.7M   896M     6%  1.00x  ONLINE  -
mypool  984M  43.7M   940M     4%  1.00x  ONLINE  -</screen>

      <sect3 xml:id="zfs-send-incremental">
	<title>Incremental Backups</title>

	<para><command>zfs send</command> can also determine the
	  difference between two snapshots and send only the
	  differences between the two.  This saves disk space and
	  transfer time.  For example:</para>

	<screen>&prompt.root; <userinput>zfs snapshot <replaceable>mypool</replaceable>@<replaceable>replica2</replaceable></userinput>
&prompt.root; <userinput>zfs list -t snapshot</userinput>
NAME                    USED  AVAIL  REFER  MOUNTPOINT
mypool@replica1         5.72M      -  43.6M  -
mypool@replica2             0      -  44.1M  -
&prompt.root; <userinput>zpool list</userinput>
NAME    SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
backup  960M  61.7M   898M     6%  1.00x  ONLINE  -
mypool  960M  50.2M   910M     5%  1.00x  ONLINE  -</screen>

	<para>A second snapshot called
	  <replaceable>replica2</replaceable> was created.  This
	  second snapshot contains only the changes that were made to
	  the file system between now and the previous snapshot,
	  <replaceable>replica1</replaceable>.  Using
	  <command>zfs send -i</command> and indicating the pair of
	  snapshots generates an incremental replica stream containing
	  only the data that has changed.  This can only succeed if
	  the initial snapshot already exists on the receiving
	  side.</para>

	<screen>&prompt.root; <userinput>zfs send -v -i <replaceable>mypool</replaceable>@<replaceable>replica1</replaceable> <replaceable>mypool</replaceable>@<replaceable>replica2</replaceable> | zfs receive <replaceable>/backup/mypool</replaceable></userinput>
send from @replica1 to mypool@replica2 estimated size is 5.02M
total estimated size is 5.02M
TIME        SENT   SNAPSHOT

&prompt.root; <userinput>zpool list</userinput>
NAME    SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
backup  960M  80.8M   879M     8%  1.00x  ONLINE  -
mypool  960M  50.2M   910M     5%  1.00x  ONLINE  -

&prompt.root; <userinput>zfs list</userinput>
NAME                         USED  AVAIL  REFER  MOUNTPOINT
backup                      55.4M   240G   152K  /backup
backup/mypool               55.3M   240G  55.2M  /backup/mypool
mypool                      55.6M  11.6G  55.0M  /mypool

&prompt.root; <userinput>zfs list -t snapshot</userinput>
NAME                                         USED  AVAIL  REFER  MOUNTPOINT
backup/mypool@replica1                       104K      -  50.2M  -
backup/mypool@replica2                          0      -  55.2M  -
mypool@replica1                             29.9K      -  50.0M  -
mypool@replica2                                 0      -  55.0M  -</screen>

	<para>The incremental stream was successfully transferred.
	  Only the data that had changed was replicated, rather than
	  the entirety of <replaceable>replica1</replaceable>.  Only
	  the differences were sent, which took much less time to
	  transfer and saved disk space by not copying the complete
	  pool each time.  This is useful when having to rely on slow
	  networks or when costs per transferred byte must be
	  considered.</para>

	<para>A new file system,
	  <replaceable>backup/mypool</replaceable>, is available with
	  all of the files and data from the pool
	  <replaceable>mypool</replaceable>.  If <option>-P</option>
	  is specified, the properties of the dataset will be copied,
	  including compression settings, quotas, and mount points.
	  When <option>-R</option> is specified, all child datasets of
	  the indicated dataset will be copied, along with all of
	  their properties.  Sending and receiving can be automated so
	  that regular backups are created on the second pool.</para>
      </sect3>

      <sect3 xml:id="zfs-send-ssh">
	<title>Sending Encrypted Backups over
	  <application>SSH</application></title>

	<para>Sending streams over the network is a good way to keep a
	  remote backup, but it does come with a drawback.  Data sent
	  over the network link is not encrypted, allowing anyone to
	  intercept and transform the streams back into data without
	  the knowledge of the sending user.  This is undesirable,
	  especially when sending the streams over the internet to a
	  remote host.  <application>SSH</application> can be used to
	  securely encrypt data send over a network connection.  Since
	  <acronym>ZFS</acronym> only requires the stream to be
	  redirected from standard output, it is relatively easy to
	  pipe it through <application>SSH</application>.  To keep the
	  contents of the file system encrypted in transit and on the
	  remote system, consider using <link
	    xlink:href="http://wiki.freebsd.org/PEFS">PEFS</link>.</para>

	<para>A few settings and security precautions must be
	  completed first.  Only the necessary steps required for the
	  <command>zfs send</command> operation are shown here.  For
	  more information on <application>SSH</application>, see
	  <xref linkend="openssh"/>.</para>

	<para>This configuration is required:</para>

	<itemizedlist>
	  <listitem>
	    <para>Passwordless <application>SSH</application> access
	      between sending and receiving host using
	      <application>SSH</application> keys</para>
	  </listitem>

	  <listitem>
	    <para>Normally, the privileges of the
	      <systemitem class="username">root</systemitem> user are
	      needed to send and receive streams.  This requires
	      logging in to the receiving system as
	      <systemitem class="username">root</systemitem>.
	      However, logging in as
	      <systemitem class="username">root</systemitem> is
	      disabled by default for security reasons.  The
	      <link linkend="zfs-zfs-allow">ZFS Delegation</link>
	      system can be used to allow a
	      non-<systemitem class="username">root</systemitem> user
	      on each system to perform the respective send and
	      receive operations.</para>
	  </listitem>

	  <listitem>
	    <para>On the sending system:</para>

	    <screen>&prompt.root; <command>zfs allow -u someuser send,snapshot <replaceable>mypool</replaceable></command></screen>
	  </listitem>

	  <listitem>
	    <para>To mount the pool, the unprivileged user must own
	      the directory, and regular users must be allowed to
	      mount file systems.  On the receiving system:</para>

	    <screen>&prompt.root; sysctl vfs.usermount=1
vfs.usermount: 0 -> 1
&prompt.root; echo vfs.usermount=1 >> /etc/sysctl.conf
&prompt.root; <userinput>zfs create <replaceable>recvpool/backup</replaceable></userinput>
&prompt.root; <userinput>zfs allow -u <replaceable>someuser</replaceable> create,mount,receive <replaceable>recvpool/backup</replaceable></userinput>
&prompt.root; chown <replaceable>someuser</replaceable> <replaceable>/recvpool/backup</replaceable></screen>
	  </listitem>
	</itemizedlist>

	<para>The unprivileged user now has the ability to receive and
	  mount datasets, and the <replaceable>home</replaceable>
	  dataset can be replicated to the remote system:</para>

	<screen>&prompt.user; <userinput>zfs snapshot -r <replaceable>mypool/home</replaceable>@<replaceable>monday</replaceable></userinput>
&prompt.user; <userinput>zfs send -R <replaceable>mypool/home</replaceable>@<replaceable>monday</replaceable> | ssh <replaceable>someuser@backuphost</replaceable> zfs recv -dvu <replaceable>recvpool/backup</replaceable></userinput></screen>

	<para>A recursive snapshot called
	  <replaceable>monday</replaceable> is made of the file system
	  dataset <replaceable>home</replaceable> that resides on the
	  pool <replaceable>mypool</replaceable>.  Then it is sent
	  with <command>zfs send -R</command> to include the dataset,
	  all child datasets, snaphots, clones, and settings in the
	  stream.  The output is piped to the waiting
	  <command>zfs receive</command> on the remote host
	  <replaceable>backuphost</replaceable> through
	  <application>SSH</application>.  Using a fully qualified
	  domain name or IP address is recommended.  The receiving
	  machine writes the data to the
	  <replaceable>backup</replaceable> dataset on the
	  <replaceable>recvpool</replaceable> pool.  Adding
	  <option>-d</option> to <command>zfs recv</command>
	  overwrites the name of the pool on the receiving side with
	  the name of the snapshot.  <option>-u</option> causes the
	  file systems to not be mounted on the receiving side.  When
	  <option>-v</option> is included, more detail about the
	  transfer is shown, including elapsed time and the amount of
	  data transferred.</para>
      </sect3>
    </sect2>

    <sect2 xml:id="zfs-zfs-quota">
      <title>Dataset, User, and Group Quotas</title>

      <para><link linkend="zfs-term-quota">Dataset quotas</link> are
	used to restrict the amount of space that can be consumed
	by a particular dataset.
	<link linkend="zfs-term-refquota">Reference Quotas</link> work
	in very much the same way, but only count the space
	used by the dataset itself, excluding snapshots and child
	datasets.  Similarly,
	<link linkend="zfs-term-userquota">user</link> and
	<link linkend="zfs-term-groupquota">group</link> quotas can be
	used to prevent users or groups from using all of the
	space in the pool or dataset.</para>

      <para>To enforce a dataset quota of 10&nbsp;GB for
	<filename>storage/home/bob</filename>:</para>

      <screen>&prompt.root; <userinput>zfs set quota=10G storage/home/bob</userinput></screen>

      <para>To enforce a reference quota of 10&nbsp;GB for
	<filename>storage/home/bob</filename>:</para>

      <screen>&prompt.root; <userinput>zfs set refquota=10G storage/home/bob</userinput></screen>

      <para>To remove a quota of 10&nbsp;GB for
	<filename>storage/home/bob</filename>:</para>

      <screen>&prompt.root; <userinput>zfs set quota=none storage/home/bob</userinput></screen>

      <para>The general format is
	<literal>userquota@<replaceable>user</replaceable>=<replaceable>size</replaceable></literal>,
	and the user's name must be in one of these formats:</para>

      <itemizedlist>
	<listitem>
	  <para><acronym>POSIX</acronym> compatible name such as
	    <replaceable>joe</replaceable>.</para>
	</listitem>

	<listitem>
	  <para><acronym>POSIX</acronym> numeric ID such as
	    <replaceable>789</replaceable>.</para>
	</listitem>

	<listitem>
	  <para><acronym>SID</acronym> name
	    such as
	    <replaceable>joe.bloggs@example.com</replaceable>.</para>
	</listitem>

	<listitem>
	  <para><acronym>SID</acronym>
	    numeric ID such as
	    <replaceable>S-1-123-456-789</replaceable>.</para>
	</listitem>
      </itemizedlist>

      <para>For example, to enforce a user quota of 50&nbsp;GB for the
	user named <replaceable>joe</replaceable>:</para>

      <screen>&prompt.root; <userinput>zfs set userquota@joe=50G</userinput></screen>

      <para>To remove any quota:</para>

      <screen>&prompt.root; <userinput>zfs set userquota@joe=none</userinput></screen>

      <note>
	<para>User quota properties are not displayed by
	  <command>zfs get all</command>.
	  Non-<systemitem class="username">root</systemitem> users can
	  only see their own quotas unless they have been granted the
	  <literal>userquota</literal> privilege.  Users with this
	  privilege are able to view and set everyone's quota.</para>
      </note>

      <para>The general format for setting a group quota is:
	<literal>groupquota@<replaceable>group</replaceable>=<replaceable>size</replaceable></literal>.</para>

      <para>To set the quota for the group
	<replaceable>firstgroup</replaceable> to 50&nbsp;GB,
	use:</para>

      <screen>&prompt.root; <userinput>zfs set groupquota@firstgroup=50G</userinput></screen>

      <para>To remove the quota for the group
	<replaceable>firstgroup</replaceable>, or to make sure that
	one is not set, instead use:</para>

      <screen>&prompt.root; <userinput>zfs set groupquota@firstgroup=none</userinput></screen>

      <para>As with the user quota property,
	non-<systemitem class="username">root</systemitem> users can
	only see the quotas associated with the groups to which they
	belong.  However,
	<systemitem class="username">root</systemitem> or a user with
	the <literal>groupquota</literal> privilege can view and set
	all quotas for all groups.</para>

      <para>To display the amount of space used by each user on
	a file system or snapshot along with any quotas, use
	<command>zfs userspace</command>.  For group information, use
	<command>zfs groupspace</command>.  For more information about
	supported options or how to display only specific options,
	refer to &man.zfs.1;.</para>

      <para>Users with sufficient privileges, and
	<systemitem class="username">root</systemitem>, can list the
	quota for <filename>storage/home/bob</filename> using:</para>

      <screen>&prompt.root; <userinput>zfs get quota storage/home/bob</userinput></screen>
    </sect2>

    <sect2 xml:id="zfs-zfs-reservation">
      <title>Reservations</title>

      <para><link linkend="zfs-term-reservation">Reservations</link>
	guarantee a minimum amount of space will always be available
	on a dataset.  The reserved space will not be available to any
	other dataset.  This feature can be especially useful to
	ensure that free space is available for an important dataset
	or log files.</para>

      <para>The general format of the <literal>reservation</literal>
	property is
	<literal>reservation=<replaceable>size</replaceable></literal>,
	so to set a reservation of 10&nbsp;GB on
	<filename>storage/home/bob</filename>, use:</para>

      <screen>&prompt.root; <userinput>zfs set reservation=10G storage/home/bob</userinput></screen>

      <para>To clear any reservation:</para>

      <screen>&prompt.root; <userinput>zfs set reservation=none storage/home/bob</userinput></screen>

      <para>The same principle can be applied to the
	<literal>refreservation</literal> property for setting a
	<link linkend="zfs-term-refreservation">Reference
	  Reservation</link>, with the general format
	<literal>refreservation=<replaceable>size</replaceable></literal>.</para>

      <para>This command shows any reservations or refreservations
	that exist on <filename>storage/home/bob</filename>:</para>

      <screen>&prompt.root; <userinput>zfs get reservation storage/home/bob</userinput>
&prompt.root; <userinput>zfs get refreservation storage/home/bob</userinput></screen>
    </sect2>

    <sect2 xml:id="zfs-zfs-compression">
      <title>Compression</title>

      <para><acronym>ZFS</acronym> provides transparent compression.
	Compressing data at the block level as it is written not only
	saves space, but can also increase disk throughput.  If data
	is compressed by 25%, but the compressed data is written to
	the disk at the same rate as the uncompressed version,
	resulting in an effective write speed of 125%.  Compression
	can also be a great alternative to
	<link linkend="zfs-zfs-deduplication">Deduplication</link>
	because it does not require additional memory.</para>

      <para><acronym>ZFS</acronym> offers several different
	compression algorithms, each with different trade-offs.  With
	the introduction of <acronym>LZ4</acronym> compression in
	<acronym>ZFS</acronym> v5000, it is possible to enable
	compression for the entire pool without the large performance
	trade-off of other algorithms.  The biggest advantage to
	<acronym>LZ4</acronym> is the <emphasis>early abort</emphasis>
	feature.  If <acronym>LZ4</acronym> does not achieve at least
	12.5% compression in the first part of the data, the block is
	written uncompressed to avoid wasting CPU cycles trying to
	compress data that is either already compressed or
	uncompressible.  For details about the different compression
	algorithms available in <acronym>ZFS</acronym>, see the
	<link linkend="zfs-term-compression">Compression</link> entry
	in the terminology section.</para>

      <para>The administrator can monitor the effectiveness of
	compression using a number of dataset properties.</para>

      <screen>&prompt.root; <userinput>zfs get used,compressratio,compression,logicalused <replaceable>mypool/compressed_dataset</replaceable></userinput>
NAME        PROPERTY          VALUE     SOURCE
mypool/compressed_dataset  used              449G      -
mypool/compressed_dataset  compressratio     1.11x     -
mypool/compressed_dataset  compression       lz4       local
mypool/compressed_dataset  logicalused       496G      -</screen>

      <para>The dataset is currently using 449&nbsp;GB of space (the
	used property).  Without compression, it would have taken
	496&nbsp;GB of space (the <literal>logicallyused</literal>
	property).  This results in the 1.11:1 compression
	ratio.</para>

      <para>Compression can have an unexpected side effect when
	combined with
	<link linkend="zfs-term-userquota">User Quotas</link>.
	User quotas restrict how much space a user can consume on a
	dataset, but the measurements are based on how much space is
	used <emphasis>after compression</emphasis>.  So if a user has
	a quota of 10&nbsp;GB, and writes 10&nbsp;GB of compressible
	data, they will still be able to store additional data.  If
	they later update a file, say a database, with more or less
	compressible data, the amount of space available to them will
	change.  This can result in the odd situation where a user did
	not increase the actual amount of data (the
	<literal>logicalused</literal> property), but the change in
	compression caused them to reach their quota limit.</para>

      <para>Compression can have a similar unexpected interaction with
	backups.  Quotas are often used to limit how much data can be
	stored to ensure there is sufficient backup space available.
	However since quotas do not consider compression, more data
	may be written than would fit with uncompressed
	backups.</para>
    </sect2>

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

      <para>When enabled,
	<link linkend="zfs-term-deduplication">deduplication</link>
	uses the checksum of each block to detect duplicate blocks.
	When a new block is a duplicate of an existing block,
	<acronym>ZFS</acronym> writes an additional reference to the
	existing data instead of the whole duplicate block.
	Tremendous space savings are possible if the data contains
	many duplicated files or repeated information.  Be warned:
	deduplication requires an extremely large amount of memory,
	and most of the space savings can be had without the extra
	cost by enabling compression instead.</para>

      <para>To activate deduplication, set the
	<literal>dedup</literal> property on the target pool:</para>

      <screen>&prompt.root; <userinput>zfs set dedup=on <replaceable>pool</replaceable></userinput></screen>

      <para>Only new data being written to the pool will be
	deduplicated.  Data that has already been written to the pool
	will not be deduplicated merely by activating this option.  A
	pool with a freshly activated deduplication property will look
	like this example:</para>

      <screen>&prompt.root; <userinput>zpool list</userinput>
NAME  SIZE ALLOC  FREE CAP DEDUP HEALTH ALTROOT
pool 2.84G 2.19M 2.83G  0% 1.00x ONLINE -</screen>

      <para>The <literal>DEDUP</literal> column shows the actual rate
	of deduplication for the pool.  A value of
	<literal>1.00x</literal> shows that data has not been
	deduplicated yet.  In the next example, the ports tree is
	copied three times into different directories on the
	deduplicated pool created above.</para>

      <screen>&prompt.root; <userinput>zpool list</userinput>
for d in dir1 dir2 dir3; do
for> mkdir $d &amp;&amp; cp -R /usr/ports $d &amp;
for> done</screen>

      <para>Redundant data is detected and deduplicated:</para>

      <screen>&prompt.root; <userinput>zpool list</userinput>
NAME SIZE  ALLOC FREE CAP DEDUP HEALTH ALTROOT
pool 2.84G 20.9M 2.82G 0% 3.00x ONLINE -</screen>

      <para>The <literal>DEDUP</literal> column shows a factor of
	<literal>3.00x</literal>.  Multiple copies of the ports tree
	data was detected and deduplicated, using only a third of the
	space.  The potential for space savings can be enormous, but
	comes at the cost of having enough memory to keep track of the
	deduplicated blocks.</para>

      <para>Deduplication is not always beneficial, especially when
	the data on a pool is not redundant.
	<acronym>ZFS</acronym> can show potential space savings by
	simulating deduplication on an existing pool:</para>

      <screen>&prompt.root; <userinput>zdb -S <replaceable>pool</replaceable></userinput>
Simulated DDT histogram:

bucket              allocated                       referenced
______   ______________________________   ______________________________
refcnt   blocks   LSIZE   PSIZE   DSIZE   blocks   LSIZE   PSIZE   DSIZE
------   ------   -----   -----   -----   ------   -----   -----   -----
     1    2.58M    289G    264G    264G    2.58M    289G    264G    264G
     2     206K   12.6G   10.4G   10.4G     430K   26.4G   21.6G   21.6G
     4    37.6K    692M    276M    276M     170K   3.04G   1.26G   1.26G
     8    2.18K   45.2M   19.4M   19.4M    20.0K    425M    176M    176M
    16      174   2.83M   1.20M   1.20M    3.33K   48.4M   20.4M   20.4M
    32       40   2.17M    222K    222K    1.70K   97.2M   9.91M   9.91M
    64        9     56K   10.5K   10.5K      865   4.96M    948K    948K
   128        2   9.50K      2K      2K      419   2.11M    438K    438K
   256        5   61.5K     12K     12K    1.90K   23.0M   4.47M   4.47M
    1K        2      1K      1K      1K    2.98K   1.49M   1.49M   1.49M
 Total    2.82M    303G    275G    275G    3.20M    319G    287G    287G

dedup = 1.05, compress = 1.11, copies = 1.00, dedup * compress / copies = 1.16</screen>

      <para>After <command>zdb -S</command> finishes analyzing the
	pool, it shows the space reduction ratio that would be
	achieved by activating deduplication.  In this case,
	<literal>1.16</literal> is a very poor space saving ratio that
	is mostly provided by compression.  Activating deduplication
	on this pool would not save any significant amount of space,
	and is not worth the amount of memory required to enable
	deduplication.  Using the formula
	<emphasis>ratio = dedup * compress / copies</emphasis>,
	system administrators can plan the storage allocation,
	deciding whether the workload will contain enough duplicate
	blocks to justify the memory requirements.  If the data is
	reasonably compressible, the space savings may be very good.
	Enabling compression first is recommended, and compression can
	also provide greatly increased performance.  Only enable
	deduplication in cases where the additional savings will be
	considerable and there is sufficient memory for the <link
	  linkend="zfs-term-deduplication"><acronym>DDT</acronym></link>.</para>
    </sect2>

    <sect2 xml:id="zfs-zfs-jail">
      <title><acronym>ZFS</acronym> and Jails</title>

      <para><command>zfs jail</command> and the corresponding
	<literal>jailed</literal> property are used to delegate a
	<acronym>ZFS</acronym> dataset to a
	<link linkend="jails">Jail</link>.
	<command>zfs jail <replaceable>jailid</replaceable></command>
	attaches a dataset to the specified jail, and
	<command>zfs unjail</command> detaches it.  For the dataset to
	be controlled from within a jail, the
	<literal>jailed</literal> property must be set.  Once a
	dataset is jailed, it can no longer be mounted on the
	host because it may have mount points that would compromise
	the security of the host.</para>
    </sect2>
  </sect1>

  <sect1 xml:id="zfs-zfs-allow">
    <title>Delegated Administration</title>

    <para>A comprehensive permission delegation system allows
      unprivileged users to perform <acronym>ZFS</acronym>
      administration functions.  For example, if each user's home
      directory is a dataset, users can be given permission to create
      and destroy snapshots of their home directories.  A backup user
      can be given permission to use replication features.  A usage
      statistics script can be allowed to run with access only to the
      space utilization data for all users.  It is even possible to
      delegate the ability to delegate permissions.  Permission
      delegation is possible for each subcommand and most
      properties.</para>

    <sect2 xml:id="zfs-zfs-allow-create">
      <title>Delegating Dataset Creation</title>

      <para><command>zfs allow
	  <replaceable>someuser</replaceable> create
	  <replaceable>mydataset</replaceable></command> gives the
	specified user permission to create child datasets under the
	selected parent dataset.  There is a caveat: creating a new
	dataset involves mounting it.  That requires setting the
	&os; <literal>vfs.usermount</literal> &man.sysctl.8; to
	<literal>1</literal> to allow non-root users to mount a
	file system.  There is another restriction aimed at preventing
	abuse: non-<systemitem class="username">root</systemitem>
	users must own the mountpoint where the file system is to be
	mounted.</para>
    </sect2>

    <sect2 xml:id="zfs-zfs-allow-allow">
      <title>Delegating Permission Delegation</title>

      <para><command>zfs allow
	  <replaceable>someuser</replaceable> allow
	  <replaceable>mydataset</replaceable></command> gives the
	specified user the ability to assign any permission they have
	on the target dataset, or its children, to other users.  If a
	user has the <literal>snapshot</literal> permission and the
	<literal>allow</literal> permission, that user can then grant
	the <literal>snapshot</literal> permission to other
	users.</para>
    </sect2>
  </sect1>

  <sect1 xml:id="zfs-advanced">
    <title>Advanced Topics</title>

    <sect2 xml:id="zfs-advanced-tuning">
      <title>Tuning</title>

      <para>There are a number of tunables that can be adjusted to
	make <acronym>ZFS</acronym> perform best for different
	workloads.</para>

      <itemizedlist>
	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-arc_max"><emphasis><varname>vfs.zfs.arc_max</varname></emphasis>
	    - Maximum size of the <link
	      linkend="zfs-term-arc"><acronym>ARC</acronym></link>.
	    The default is all <acronym>RAM</acronym> less 1&nbsp;GB,
	    or one half of <acronym>RAM</acronym>, whichever is more.
	    However, a lower value should be used if the system will
	    be running any other daemons or processes that may require
	    memory.  This value can only be adjusted at boot time, and
	    is set in <filename>/boot/loader.conf</filename>.</para>
	</listitem>

	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-arc_meta_limit"><emphasis><varname>vfs.zfs.arc_meta_limit</varname></emphasis>
	    - Limit the portion of the
	    <link linkend="zfs-term-arc"><acronym>ARC</acronym></link>
	    that can be used to store metadata.  The default is one
	    fourth of <varname>vfs.zfs.arc_max</varname>.  Increasing
	    this value will improve performance if the workload
	    involves operations on a large number of files and
	    directories, or frequent metadata operations, at the cost
	    of less file data fitting in the <link
	      linkend="zfs-term-arc"><acronym>ARC</acronym></link>.
	    This value can only be adjusted at boot time, and is set
	    in <filename>/boot/loader.conf</filename>.</para>
	</listitem>

	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-arc_min"><emphasis><varname>vfs.zfs.arc_min</varname></emphasis>
	    - Minimum size of the <link
	      linkend="zfs-term-arc"><acronym>ARC</acronym></link>.
	    The default is one half of
	    <varname>vfs.zfs.arc_meta_limit</varname>.  Adjust this
	    value to prevent other applications from pressuring out
	    the entire <link
	      linkend="zfs-term-arc"><acronym>ARC</acronym></link>.
	    This value can only be adjusted at boot time, and is set
	    in <filename>/boot/loader.conf</filename>.</para>
	</listitem>

	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-vdev-cache-size"><emphasis><varname>vfs.zfs.vdev.cache.size</varname></emphasis>
	    - A preallocated amount of memory reserved as a cache for
	    each device in the pool.  The total amount of memory used
	    will be this value multiplied by the number of devices.
	    This value can only be adjusted at boot time, and is set
	    in <filename>/boot/loader.conf</filename>.</para>
	</listitem>

	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-min-auto-ashift"><emphasis><varname>vfs.zfs.min_auto_ashift</varname></emphasis>
	    - Minimum <varname>ashift</varname> (sector size) that
	    will be used automatically at pool creation time.  The
	    value is a power of two.  The default value of
	    <literal>9</literal> represents
	    <literal>2^9 = 512</literal>, a sector size of 512 bytes.
	    To avoid <emphasis>write amplification</emphasis> and get
	    the best performance, set this value to the largest sector
	    size used by a device in the pool.</para>

	  <para>Many drives have 4&nbsp;KB sectors.  Using the default
	    <varname>ashift</varname> of <literal>9</literal> with
	    these drives results in write amplification on these
	    devices.  Data that could be contained in a single
	    4&nbsp;KB write must instead be written in eight 512-byte
	    writes.  <acronym>ZFS</acronym> tries to read the native
	    sector size from all devices when creating a pool, but
	    many drives with 4&nbsp;KB sectors report that their
	    sectors are 512 bytes for compatibility.  Setting
	    <varname>vfs.zfs.min_auto_ashift</varname> to
	    <literal>12</literal> (<literal>2^12 = 4096</literal>)
	    before creating a pool forces <acronym>ZFS</acronym> to
	    use 4&nbsp;KB blocks for best performance on these
	    drives.</para>

	  <para>Forcing 4&nbsp;KB blocks is also useful on pools where
	    disk upgrades are planned.  Future disks are likely to use
	    4&nbsp;KB sectors, and <varname>ashift</varname> values
	    cannot be changed after a pool is created.</para>

	  <para>In some specific cases, the smaller 512-byte block
	    size might be preferable.  When used with 512-byte disks
	    for databases, or as storage for virtual machines, less
	    data is transferred during small random reads.  This can
	    provide better performance, especially when using a
	    smaller <acronym>ZFS</acronym> record size.</para>
	</listitem>

	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-prefetch_disable"><emphasis><varname>vfs.zfs.prefetch_disable</varname></emphasis>
	    - Disable prefetch.  A value of <literal>0</literal> is
	    enabled and <literal>1</literal> is disabled.  The default
	    is <literal>0</literal>, unless the system has less than
	    4&nbsp;GB of <acronym>RAM</acronym>.  Prefetch works by
	    reading larger blocks than were requested into the
	    <link linkend="zfs-term-arc"><acronym>ARC</acronym></link>
	    in hopes that the data will be needed soon.  If the
	    workload has a large number of random reads, disabling
	    prefetch may actually improve performance by reducing
	    unnecessary reads.  This value can be adjusted at any time
	    with &man.sysctl.8;.</para>
	</listitem>

	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-vdev-trim_on_init"><emphasis><varname>vfs.zfs.vdev.trim_on_init</varname></emphasis>
	    - Control whether new devices added to the pool have the
	    <literal>TRIM</literal> command run on them.  This ensures
	    the best performance and longevity for
	    <acronym>SSD</acronym>s, but takes extra time.  If the
	    device has already been secure erased, disabling this
	    setting will make the addition of the new device faster.
	    This value can be adjusted at any time with
	    &man.sysctl.8;.</para>
	</listitem>

	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-write_to_degraded"><emphasis><varname>vfs.zfs.write_to_degraded</varname></emphasis>
	    - Control whether new data is written to a vdev that is
	    in the <link linkend="zfs-term-degraded">DEGRADED</link>
	    state.  Defaults to <literal>0</literal>, preventing
	    writes to any top level vdev that is in a degraded state.
	    The administrator may wish to allow writing to degraded
	    vdevs to prevent the amount of free space across the vdevs
	    from becoming unbalanced, which will reduce read and write
	    performance.  This value can be adjusted at any time with
	    &man.sysctl.8;.</para>
	</listitem>

	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-vdev-max_pending"><emphasis><varname>vfs.zfs.vdev.max_pending</varname></emphasis>
	    - Limit the number of pending I/O requests per device.
	    A higher value will keep the device command queue full
	    and may give higher throughput.  A lower value will reduce
	    latency.  This value can be adjusted at any time with
	    &man.sysctl.8;.</para>
	</listitem>

	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-top_maxinflight"><emphasis><varname>vfs.zfs.top_maxinflight</varname></emphasis>
	    - Maxmimum number of outstanding I/Os per top-level
	    <link linkend="zfs-term-vdev">vdev</link>.  Limits the
	    depth of the command queue to prevent high latency.  The
	    limit is per top-level vdev, meaning the limit applies to
	    each <link linkend="zfs-term-vdev-mirror">mirror</link>,
	    <link linkend="zfs-term-vdev-raidz">RAID-Z</link>, or
	    other vdev independently.  This value can be adjusted at
	    any time with &man.sysctl.8;.</para>
	</listitem>

	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-l2arc_write_max"><emphasis><varname>vfs.zfs.l2arc_write_max</varname></emphasis>
	    - Limit the amount of data written to the <link
	      linkend="zfs-term-l2arc"><acronym>L2ARC</acronym></link>
	    per second.  This tunable is designed to extend the
	    longevity of <acronym>SSD</acronym>s by limiting the
	    amount of data written to the device.  This value can be
	    adjusted at any time with &man.sysctl.8;.</para>
	</listitem>

	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-l2arc_write_boost"><emphasis><varname>vfs.zfs.l2arc_write_boost</varname></emphasis>
	    - The value of this tunable is added to <link
	      linkend="zfs-advanced-tuning-l2arc_write_max"><varname>vfs.zfs.l2arc_write_max</varname></link>
	    and increases the write speed to the
	    <acronym>SSD</acronym> until the first block is evicted
	    from the <link
	      linkend="zfs-term-l2arc"><acronym>L2ARC</acronym></link>.
	    This <quote>Turbo Warmup Phase</quote> is designed to
	    reduce the performance loss from an empty <link
	      linkend="zfs-term-l2arc"><acronym>L2ARC</acronym></link>
	    after a reboot.  This value can be adjusted at any time
	    with &man.sysctl.8;.</para>
	</listitem>

	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-scrub_delay"><emphasis><varname>vfs.zfs.scrub_delay</varname></emphasis>
	    - Number of ticks to delay between each I/O during a
	    <link
	      linkend="zfs-term-scrub"><command>scrub</command></link>.
	    To ensure that a <command>scrub</command> does not
	    interfere with the normal operation of the pool, if any
	    other <acronym>I/O</acronym> is happening the
	    <command>scrub</command> will delay between each command.
	    This value controls the limit on the total
	    <acronym>IOPS</acronym> (I/Os Per Second) generated by the
	    <command>scrub</command>.  The granularity of the setting
	    is determined by the value of <varname>kern.hz</varname>
	    which defaults to 1000 ticks per second.  This setting may
	    be changed, resulting in a different effective
	    <acronym>IOPS</acronym> limit.  The default value is
	    <literal>4</literal>, resulting in a limit of:
	    1000&nbsp;ticks/sec / 4 =
	    250&nbsp;<acronym>IOPS</acronym>.  Using a value of
	    <replaceable>20</replaceable> would give a limit of:
	    1000&nbsp;ticks/sec / 20 =
	    50&nbsp;<acronym>IOPS</acronym>.  The speed of
	    <command>scrub</command> is only limited when there has
	    been recent activity on the pool, as determined by <link
	      linkend="zfs-advanced-tuning-scan_idle"><varname>vfs.zfs.scan_idle</varname></link>.
	    This value can be adjusted at any time with
	    &man.sysctl.8;.</para>
	</listitem>

	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-resilver_delay"><emphasis><varname>vfs.zfs.resilver_delay</varname></emphasis>
	    - Number of milliseconds of delay inserted between
	    each I/O during a
	    <link linkend="zfs-term-resilver">resilver</link>.  To
	    ensure that a resilver does not interfere with the normal
	    operation of the pool, if any other I/O is happening the
	    resilver will delay between each command.  This value
	    controls the limit of total <acronym>IOPS</acronym> (I/Os
	    Per Second) generated by the resilver.  The granularity of
	    the setting is determined by the value of
	    <varname>kern.hz</varname> which defaults to 1000 ticks
	    per second.  This setting may be changed, resulting in a
	    different effective <acronym>IOPS</acronym> limit.  The
	    default value is 2, resulting in a limit of:
	    1000&nbsp;ticks/sec / 2 =
	    500&nbsp;<acronym>IOPS</acronym>.  Returning the pool to
	    an <link linkend="zfs-term-online">Online</link> state may
	    be more important if another device failing could
	    <link linkend="zfs-term-faulted">Fault</link> the pool,
	    causing data loss.  A value of 0 will give the resilver
	    operation the same priority as other operations, speeding
	    the healing process.  The speed of resilver is only
	    limited when there has been other recent activity on the
	    pool, as determined by <link
	      linkend="zfs-advanced-tuning-scan_idle"><varname>vfs.zfs.scan_idle</varname></link>.
	    This value can be adjusted at any time with
	    &man.sysctl.8;.</para>
	</listitem>

	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-scan_idle"><emphasis><varname>vfs.zfs.scan_idle</varname></emphasis>
	    - Number of milliseconds since the last operation before
	    the pool is considered idle.  When the pool is idle the
	    rate limiting for <link
	      linkend="zfs-term-scrub"><command>scrub</command></link>
	    and
	    <link linkend="zfs-term-resilver">resilver</link> are
	    disabled.  This value can be adjusted at any time with
	    &man.sysctl.8;.</para>
	</listitem>

	<listitem>
	  <para
	      xml:id="zfs-advanced-tuning-txg-timeout"><emphasis><varname>vfs.zfs.txg.timeout</varname></emphasis>
	    - Maximum number of seconds between
	    <link linkend="zfs-term-txg">transaction group</link>s.
	    The current transaction group will be written to the pool
	    and a fresh transaction group started if this amount of
	    time has elapsed since the previous transaction group.  A
	    transaction group my be triggered earlier if enough data
	    is written.  The default value is 5 seconds.  A larger
	    value may improve read performance by delaying
	    asynchronous writes, but this may cause uneven performance
	    when the transaction group is written.  This value can be
	    adjusted at any time with &man.sysctl.8;.</para>
	</listitem>
      </itemizedlist>
    </sect2>

<!-- These sections will be added in the future
    <sect2 xml:id="zfs-advanced-booting">
      <title>Booting Root on <acronym>ZFS</acronym> </title>

      <para></para>
    </sect2>

    <sect2 xml:id="zfs-advanced-beadm">
      <title><acronym>ZFS</acronym> Boot Environments</title>

      <para></para>
    </sect2>

    <sect2 xml:id="zfs-advanced-troubleshoot">
      <title>Troubleshooting</title>

      <para></para>
    </sect2>
-->

    <sect2 xml:id="zfs-advanced-i386">
      <title><acronym>ZFS</acronym> on i386</title>

      <para>Some of the features provided by <acronym>ZFS</acronym>
	are memory intensive, and may require tuning for maximum
	efficiency on systems with limited
	<acronym>RAM</acronym>.</para>

      <sect3>
	<title>Memory</title>

	<para>As a bare minimum, the total system memory should be at
	  least one gigabyte.  The amount of recommended
	  <acronym>RAM</acronym> depends upon the size of the pool and
	  which <acronym>ZFS</acronym> features are used.  A general
	  rule of thumb is 1&nbsp;GB of RAM for every 1&nbsp;TB of
	  storage.  If the deduplication feature is used, a general
	  rule of thumb is 5&nbsp;GB of RAM per TB of storage to be
	  deduplicated.  While some users successfully use
	  <acronym>ZFS</acronym> with less <acronym>RAM</acronym>,
	  systems under heavy load may panic due to memory exhaustion.
	  Further tuning may be required for systems with less than
	  the recommended RAM requirements.</para>
      </sect3>

      <sect3>
	<title>Kernel Configuration</title>

	<para>Due to the address space limitations of the
	  &i386; platform, <acronym>ZFS</acronym> users on the
	  &i386; architecture must add this option to a
	  custom kernel configuration file, rebuild the kernel, and
	  reboot:</para>

	<programlisting>options        KVA_PAGES=512</programlisting>

	<para>This expands the kernel address space, allowing
	  the <varname>vm.kvm_size</varname> tunable to be pushed
	  beyond the currently imposed limit of 1&nbsp;GB, or the
	  limit of 2&nbsp;GB for <acronym>PAE</acronym>.  To find the
	  most suitable value for this option, divide the desired
	  address space in megabytes by four.  In this example, it
	  is <literal>512</literal> for 2&nbsp;GB.</para>
      </sect3>

      <sect3>
	<title>Loader Tunables</title>

	<para>The <filename>kmem</filename> address space can be
	  increased on all &os; architectures.  On a test system with
	  1&nbsp;GB of physical memory, success was achieved with
	  these options added to
	  <filename>/boot/loader.conf</filename>, and the system
	  restarted:</para>

	<programlisting>vm.kmem_size="330M"
vm.kmem_size_max="330M"
vfs.zfs.arc_max="40M"
vfs.zfs.vdev.cache.size="5M"</programlisting>

	<para>For a more detailed list of recommendations for
	  <acronym>ZFS</acronym>-related tuning, see <link
	    xlink:href="http://wiki.freebsd.org/ZFSTuningGuide"></link>.</para>
      </sect3>
    </sect2>
  </sect1>

  <sect1 xml:id="zfs-links">
    <title>Additional Resources</title>

    <itemizedlist>
      <listitem>
	<para><link xlink:href="https://wiki.freebsd.org/ZFS">FreeBSD
	    Wiki - <acronym>ZFS</acronym></link></para>
      </listitem>

      <listitem>
	<para><link
	    xlink:href="https://wiki.freebsd.org/ZFSTuningGuide">FreeBSD
	    Wiki - <acronym>ZFS</acronym> Tuning</link></para>
      </listitem>

      <listitem>
	<para><link
	    xlink:href="http://wiki.illumos.org/display/illumos/ZFS">Illumos
	    Wiki - <acronym>ZFS</acronym></link></para>
      </listitem>

      <listitem>
	<para><link
	    xlink:href="http://docs.oracle.com/cd/E19253-01/819-5461/index.html">Oracle
	    Solaris <acronym>ZFS</acronym> Administration
	    Guide</link></para>
      </listitem>

      <listitem>
	<para><link
	    xlink:href="http://www.solarisinternals.com/wiki/index.php/ZFS_Evil_Tuning_Guide"><acronym>ZFS</acronym>
	    Evil Tuning Guide</link></para>
      </listitem>

      <listitem>
	<para><link
	    xlink:href="http://www.solarisinternals.com/wiki/index.php/ZFS_Best_Practices_Guide"><acronym>ZFS</acronym>
	    Best Practices Guide</link></para>
      </listitem>

      <listitem>
	<para><link
	    xlink:href="https://calomel.org/zfs_raid_speed_capacity.html">Calomel
	    Blog - <acronym>ZFS</acronym> Raidz Performance, Capacity
	    and Integrity</link></para>
      </listitem>
    </itemizedlist>
  </sect1>

  <sect1 xml:id="zfs-term">
    <title><acronym>ZFS</acronym> Features and Terminology</title>

    <para><acronym>ZFS</acronym> is a fundamentally different file
      system because it is more than just a file system.
      <acronym>ZFS</acronym> combines the roles of file system and
      volume manager, enabling additional storage devices to be added
      to a live system and having the new space available on all of
      the existing file systems in that pool immediately.  By
      combining the traditionally separate roles,
      <acronym>ZFS</acronym> is able to overcome previous limitations
      that prevented <acronym>RAID</acronym> groups being able to
      grow.  Each top level device in a zpool is called a
      <emphasis>vdev</emphasis>, which can be a simple disk or a
      <acronym>RAID</acronym> transformation such as a mirror or
      <acronym>RAID-Z</acronym> array.  <acronym>ZFS</acronym> file
      systems (called <emphasis>datasets</emphasis>) each have access
      to the combined free space of the entire pool.  As blocks are
      allocated from the pool, the space available to each file system
      decreases.  This approach avoids the common pitfall with
      extensive partitioning where free space becomes fragmented
      across the partitions.</para>

    <informaltable pgwide="1">
      <tgroup cols="2">
	<tbody valign="top">
	  <row>
	    <entry xml:id="zfs-term-zpool">zpool</entry>

	    <entry>A storage <emphasis>pool</emphasis> is the most
	      basic building block of <acronym>ZFS</acronym>.  A pool
	      is made up of one or more vdevs, the underlying devices
	      that store the data.  A pool is then used to create one
	      or more file systems (datasets) or block devices
	      (volumes).  These datasets and volumes share the pool of
	      remaining free space.  Each pool is uniquely identified
	      by a name and a <acronym>GUID</acronym>.  The features
	      available are determined by the <acronym>ZFS</acronym>
	      version number on the pool.

	      <note>
		<para>&os;&nbsp;9.0 and 9.1 include support for
		  <acronym>ZFS</acronym> version 28.  Later versions
		  use <acronym>ZFS</acronym> version 5000 with feature
		  flags.  The new feature flags system allows greater
		  cross-compatibility with other implementations of
		  <acronym>ZFS</acronym>.</para>
	      </note>
	    </entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-vdev">vdev&nbsp;Types</entry>

	    <entry>A pool is made up of one or more vdevs, which
	      themselves can be a single disk or a group of disks, in
	      the case of a <acronym>RAID</acronym> transform.  When
	      multiple vdevs are used, <acronym>ZFS</acronym> spreads
	      data across the vdevs to increase performance and
	      maximize usable space.

	      <itemizedlist>
		<listitem>
		  <para
		      xml:id="zfs-term-vdev-disk"><emphasis>Disk</emphasis>
		    - The most basic type of vdev is a standard block
		    device.  This can be an entire disk (such as
		    <filename><replaceable>/dev/ada0</replaceable></filename>
		    or
		    <filename><replaceable>/dev/da0</replaceable></filename>)
		    or a partition
		    (<filename><replaceable>/dev/ada0p3</replaceable></filename>).
		    On &os;, there is no performance penalty for using
		    a partition rather than the entire disk.  This
		    differs from recommendations made by the Solaris
		    documentation.</para>
		</listitem>

		<listitem>
		  <para
		      xml:id="zfs-term-vdev-file"><emphasis>File</emphasis>
		    - In addition to disks, <acronym>ZFS</acronym>
		    pools can be backed by regular files, this is
		    especially useful for testing and experimentation.
		    Use the full path to the file as the device path
		    in the zpool create command.  All vdevs must be
		    at least 128&nbsp;MB in size.</para>
		</listitem>

		<listitem>
		  <para
		      xml:id="zfs-term-vdev-mirror"><emphasis>Mirror</emphasis>
		    - When creating a mirror, specify the
		    <literal>mirror</literal> keyword followed by the
		    list of member devices for the mirror.  A mirror
		    consists of two or more devices, all data will be
		    written to all member devices.  A mirror vdev will
		    only hold as much data as its smallest member.  A
		    mirror vdev can withstand the failure of all but
		    one of its members without losing any data.</para>

		  <note>
		    <para>A regular single disk vdev can be upgraded
		      to a mirror vdev at any time with
		      <command>zpool
			<link
			  linkend="zfs-zpool-attach">attach</link></command>.</para>
		  </note>
		</listitem>

		<listitem>
		  <para
		      xml:id="zfs-term-vdev-raidz"><emphasis><acronym>RAID-Z</acronym></emphasis>
		    - <acronym>ZFS</acronym> implements
		    <acronym>RAID-Z</acronym>, a variation on standard
		    <acronym>RAID-5</acronym> that offers better
		    distribution of parity and eliminates the
		    <quote><acronym>RAID-5</acronym> write
		    hole</quote> in which the data and parity
		    information become inconsistent after an
		    unexpected restart.  <acronym>ZFS</acronym>
		    supports three levels of <acronym>RAID-Z</acronym>
		    which provide varying levels of redundancy in
		    exchange for decreasing levels of usable storage.
		    The types are named <acronym>RAID-Z1</acronym>
		    through <acronym>RAID-Z3</acronym> based on the
		    number of parity devices in the array and the
		    number of disks which can fail while the pool
		    remains operational.</para>

		  <para>In a <acronym>RAID-Z1</acronym> configuration
		    with four disks, each 1&nbsp;TB, usable storage is
		    3&nbsp;TB and the pool will still be able to
		    operate in degraded mode with one faulted disk.
		    If an additional disk goes offline before the
		    faulted disk is replaced and resilvered, all data
		    in the pool can be lost.</para>

		  <para>In a <acronym>RAID-Z3</acronym> configuration
		    with eight disks of 1&nbsp;TB, the volume will
		    provide 5&nbsp;TB of usable space and still be
		    able to operate with three faulted disks.  &sun;
		    recommends no more than nine disks in a single
		    vdev.  If the configuration has more disks, it is
		    recommended to divide them into separate vdevs and
		    the pool data will be striped across them.</para>

		  <para>A configuration of two
		    <acronym>RAID-Z2</acronym> vdevs consisting of 8
		    disks each would create something similar to a
		    <acronym>RAID-60</acronym> array.  A
		    <acronym>RAID-Z</acronym> group's storage capacity
		    is approximately the size of the smallest disk
		    multiplied by the number of non-parity disks.
		    Four 1&nbsp;TB disks in <acronym>RAID-Z1</acronym>
		    has an effective size of approximately 3&nbsp;TB,
		    and an array of eight 1&nbsp;TB disks in
		    <acronym>RAID-Z3</acronym> will yield 5&nbsp;TB of
		    usable space.</para>
		</listitem>

		<listitem>
		  <para
		      xml:id="zfs-term-vdev-spare"><emphasis>Spare</emphasis>
		    - <acronym>ZFS</acronym> has a special pseudo-vdev
		    type for keeping track of available hot spares.
		    Note that installed hot spares are not deployed
		    automatically; they must manually be configured to
		    replace the failed device using
		    <command>zfs replace</command>.</para>
		</listitem>

		<listitem>
		  <para
		      xml:id="zfs-term-vdev-log"><emphasis>Log</emphasis>
		    - <acronym>ZFS</acronym> Log Devices, also known
		    as <acronym>ZFS</acronym> Intent Log (<link
		      linkend="zfs-term-zil"><acronym>ZIL</acronym></link>)
		    move the intent log from the regular pool devices
		    to a dedicated device, typically an
		    <acronym>SSD</acronym>.  Having a dedicated log
		    device can significantly improve the performance
		    of applications with a high volume of synchronous
		    writes, especially databases.  Log devices can be
		    mirrored, but <acronym>RAID-Z</acronym> is not
		    supported.  If multiple log devices are used,
		    writes will be load balanced across them.</para>
		</listitem>

		<listitem>
		  <para
		      xml:id="zfs-term-vdev-cache"><emphasis>Cache</emphasis>
		    - Adding a cache vdev to a zpool will add the
		    storage of the cache to the <link
		      linkend="zfs-term-l2arc"><acronym>L2ARC</acronym></link>.
		    Cache devices cannot be mirrored.  Since a cache
		    device only stores additional copies of existing
		    data, there is no risk of data loss.</para>
		</listitem>
	      </itemizedlist></entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-txg">Transaction Group
	      (<acronym>TXG</acronym>)</entry>

	    <entry>Transaction Groups are the way changed blocks are
	      grouped together and eventually written to the pool.
	      Transaction groups are the atomic unit that
	      <acronym>ZFS</acronym> uses to assert consistency.  Each
	      transaction group is assigned a unique 64-bit
	      consecutive identifier.  There can be up to three active
	      transaction groups at a time, one in each of these three
	      states:

	      <itemizedlist>
		<listitem>
		  <para><emphasis>Open</emphasis> - When a new
		    transaction group is created, it is in the open
		    state, and accepts new writes.  There is always
		    a transaction group in the open state, however the
		    transaction group may refuse new writes if it has
		    reached a limit.  Once the open transaction group
		    has reached a limit, or the <link
		      linkend="zfs-advanced-tuning-txg-timeout"><varname>vfs.zfs.txg.timeout</varname></link>
		    has been reached, the transaction group advances
		    to the next state.</para>
		</listitem>

		<listitem>
		  <para><emphasis>Quiescing</emphasis> - A short state
		    that allows any pending operations to finish while
		    not blocking the creation of a new open
		    transaction group.  Once all of the transactions
		    in the group have completed, the transaction group
		    advances to the final state.</para>
		</listitem>

		<listitem>
		  <para><emphasis>Syncing</emphasis> - All of the data
		    in the transaction group is written to stable
		    storage.  This process will in turn modify other
		    data, such as metadata and space maps, that will
		    also need to be written to stable storage.  The
		    process of syncing involves multiple passes.  The
		    first, all of the changed data blocks, is the
		    biggest, followed by the metadata, which may take
		    multiple passes to complete.  Since allocating
		    space for the data blocks generates new metadata,
		    the syncing state cannot finish until a pass
		    completes that does not allocate any additional
		    space.  The syncing state is also where
		    <emphasis>synctasks</emphasis> are completed.
		    Synctasks are administrative operations, such as
		    creating or destroying snapshots and datasets,
		    that modify the uberblock are completed.  Once the
		    sync state is complete, the transaction group in
		    the quiescing state is advanced to the syncing
		    state.</para>
		</listitem>
	      </itemizedlist>

	      All administrative functions, such as <link
		linkend="zfs-term-snapshot"><command>snapshot</command></link>
	      are written as part of the transaction group.  When a
	      synctask is created, it is added to the currently open
	      transaction group, and that group is advanced as quickly
	      as possible to the syncing state to reduce the
	      latency of administrative commands.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-arc">Adaptive Replacement
	      Cache (<acronym>ARC</acronym>)</entry>

	    <entry><acronym>ZFS</acronym> uses an Adaptive Replacement
	      Cache (<acronym>ARC</acronym>), rather than a more
	      traditional Least Recently Used (<acronym>LRU</acronym>)
	      cache.  An <acronym>LRU</acronym> cache is a simple list
	      of items in the cache, sorted by when each object was
	      most recently used.  New items are added to the top of
	      the list.  When the cache is full, items from the
	      bottom of the list are evicted to make room for more
	      active objects.  An <acronym>ARC</acronym> consists of
	      four lists; the Most Recently Used
	      (<acronym>MRU</acronym>) and Most Frequently Used
	      (<acronym>MFU</acronym>) objects, plus a ghost list for
	      each.  These ghost lists track recently evicted objects
	      to prevent them from being added back to the cache.
	      This increases the cache hit ratio by avoiding objects
	      that have a history of only being used occasionally.
	      Another advantage of using both an
	      <acronym>MRU</acronym> and <acronym>MFU</acronym> is
	      that scanning an entire file system would normally evict
	      all data from an <acronym>MRU</acronym> or
	      <acronym>LRU</acronym> cache in favor of this freshly
	      accessed content.  With <acronym>ZFS</acronym>, there is
	      also an <acronym>MFU</acronym> that only tracks the most
	      frequently used objects, and the cache of the most
	      commonly accessed blocks remains.</entry>
	  </row>

	  <row>
	    <entry
	      xml:id="zfs-term-l2arc"><acronym>L2ARC</acronym></entry>

	    <entry><acronym>L2ARC</acronym> is the second level
	      of the <acronym>ZFS</acronym> caching system.  The
	      primary <acronym>ARC</acronym> is stored in
	      <acronym>RAM</acronym>.  Since the amount of
	      available <acronym>RAM</acronym> is often limited,
	      <acronym>ZFS</acronym> can also use
	      <link linkend="zfs-term-vdev-cache">cache vdevs</link>.
	      Solid State Disks (<acronym>SSD</acronym>s) are often
	      used as these cache devices due to their higher speed
	      and lower latency compared to traditional spinning
	      disks.  <acronym>L2ARC</acronym> is entirely optional,
	      but having one will significantly increase read speeds
	      for files that are cached on the <acronym>SSD</acronym>
	      instead of having to be read from the regular disks.
	      <acronym>L2ARC</acronym> can also speed up <link
		linkend="zfs-term-deduplication">deduplication</link>
	      because a <acronym>DDT</acronym> that does not fit in
	      <acronym>RAM</acronym> but does fit in the
	      <acronym>L2ARC</acronym> will be much faster than a
	      <acronym>DDT</acronym> that must be read from disk.  The
	      rate at which data is added to the cache devices is
	      limited to prevent prematurely wearing out
	      <acronym>SSD</acronym>s with too many writes.  Until the
	      cache is full (the first block has been evicted to make
	      room), writing to the <acronym>L2ARC</acronym> is
	      limited to the sum of the write limit and the boost
	      limit, and afterwards limited to the write limit.  A
	      pair of &man.sysctl.8; values control these rate limits.
	      <link
		linkend="zfs-advanced-tuning-l2arc_write_max"><varname>vfs.zfs.l2arc_write_max</varname></link>
	      controls how many bytes are written to the cache per
	      second, while <link
		linkend="zfs-advanced-tuning-l2arc_write_boost"><varname>vfs.zfs.l2arc_write_boost</varname></link>
	      adds to this limit during the
	      <quote>Turbo Warmup Phase</quote> (Write Boost).</entry>
	  </row>

	  <row>
	    <entry
	      xml:id="zfs-term-zil"><acronym>ZIL</acronym></entry>

	    <entry><acronym>ZIL</acronym> accelerates synchronous
	      transactions by using storage devices like
	      <acronym>SSD</acronym>s that are faster than those used
	      in the main storage pool.  When an application requests
	      a synchronous write (a guarantee that the data has been
	      safely stored to disk rather than merely cached to be
	      written later), the data is written to the faster
	      <acronym>ZIL</acronym> storage, then later flushed out
	      to the regular disks.  This greatly reduces latency and
	      improves performance.  Only synchronous workloads like
	      databases will benefit from a <acronym>ZIL</acronym>.
	      Regular asynchronous writes such as copying files will
	      not use the <acronym>ZIL</acronym> at all.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-cow">Copy-On-Write</entry>

	    <entry>Unlike a traditional file system, when data is
	      overwritten on <acronym>ZFS</acronym>, the new data is
	      written to a different block rather than overwriting the
	      old data in place.  Only when this write is complete is
	      the metadata then updated to point to the new location.
	      In the event of a shorn write (a system crash or power
	      loss in the middle of writing a file), the entire
	      original contents of the file are still available and
	      the incomplete write is discarded.  This also means that
	      <acronym>ZFS</acronym> does not require a &man.fsck.8;
	      after an unexpected shutdown.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-dataset">Dataset</entry>

	    <entry><emphasis>Dataset</emphasis> is the generic term
	      for a <acronym>ZFS</acronym> file system, volume,
	      snapshot or clone.  Each dataset has a unique name in
	      the format
	      <replaceable>poolname/path@snapshot</replaceable>.
	      The root of the pool is technically a dataset as well.
	      Child datasets are named hierarchically like
	      directories.  For example,
	      <replaceable>mypool/home</replaceable>, the home
	      dataset, is a child of <replaceable>mypool</replaceable>
	      and inherits properties from it.  This can be expanded
	      further by creating
	      <replaceable>mypool/home/user</replaceable>.  This
	      grandchild dataset will inherit properties from the
	      parent and grandparent.  Properties on a child can be
	      set to override the defaults inherited from the parents
	      and grandparents.  Administration of datasets and their
	      children can be
	      <link linkend="zfs-zfs-allow">delegated</link>.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-filesystem">File system</entry>

	    <entry>A <acronym>ZFS</acronym> dataset is most often used
	      as a file system.  Like most other file systems, a
	      <acronym>ZFS</acronym> file system is mounted somewhere
	      in the systems directory hierarchy and contains files
	      and directories of its own with permissions, flags, and
	      other metadata.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-volume">Volume</entry>

	    <entry>In additional to regular file system datasets,
	      <acronym>ZFS</acronym> can also create volumes, which
	      are block devices.  Volumes have many of the same
	      features, including copy-on-write, snapshots, clones,
	      and checksumming.  Volumes can be useful for running
	      other file system formats on top of
	      <acronym>ZFS</acronym>, such as <acronym>UFS</acronym>
	      virtualization, or exporting <acronym>iSCSI</acronym>
	      extents.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-snapshot">Snapshot</entry>

	    <entry>The
	      <link linkend="zfs-term-cow">copy-on-write</link>
	      (<acronym>COW</acronym>) design of
	      <acronym>ZFS</acronym> allows for nearly instantaneous,
	      consistent snapshots with arbitrary names.  After taking
	      a snapshot of a dataset, or a recursive snapshot of a
	      parent dataset that will include all child datasets, new
	      data is written to new blocks, but the old blocks are
	      not reclaimed as free space.  The snapshot contains
	      the original version of the file system, and the live
	      file system contains any changes made since the snapshot
	      was taken.  No additional space is used.  As new data is
	      written to the live file system, new blocks are
	      allocated to store this data.  The apparent size of the
	      snapshot will grow as the blocks are no longer used in
	      the live file system, but only in the snapshot.  These
	      snapshots can be mounted read only to allow for the
	      recovery of previous versions of files.  It is also
	      possible to
	      <link linkend="zfs-zfs-snapshot">rollback</link> a live
	      file system to a specific snapshot, undoing any changes
	      that took place after the snapshot was taken.  Each
	      block in the pool has a reference counter which keeps
	      track of how many snapshots, clones, datasets, or
	      volumes make use of that block.  As files and snapshots
	      are deleted, the reference count is decremented.  When a
	      block is no longer referenced, it is reclaimed as free
	      space.  Snapshots can also be marked with a
	      <link linkend="zfs-zfs-snapshot">hold</link>.  When a
	      snapshot is held, any attempt to destroy it will return
	      an <literal>EBUSY</literal> error.  Each snapshot can
	      have multiple holds, each with a unique name.  The
	      <link linkend="zfs-zfs-snapshot">release</link> command
	      removes the hold so the snapshot can deleted.  Snapshots
	      can be taken on volumes, but they can only be cloned or
	      rolled back, not mounted independently.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-clone">Clone</entry>

	    <entry>Snapshots can also be cloned.  A clone is a
	      writable version of a snapshot, allowing the file system
	      to be forked as a new dataset.  As with a snapshot, a
	      clone initially consumes no additional space.  As
	      new data is written to a clone and new blocks are
	      allocated, the apparent size of the clone grows.  When
	      blocks are overwritten in the cloned file system or
	      volume, the reference count on the previous block is
	      decremented.  The snapshot upon which a clone is based
	      cannot be deleted because the clone depends on it.  The
	      snapshot is the parent, and the clone is the child.
	      Clones can be <emphasis>promoted</emphasis>, reversing
	      this dependency and making the clone the parent and the
	      previous parent the child.  This operation requires no
	      additional space.  Because the amount of space used by
	      the parent and child is reversed, existing quotas and
	      reservations might be affected.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-checksum">Checksum</entry>

	    <entry>Every block that is allocated is also checksummed.
	      The checksum algorithm used is a per-dataset property,
	      see <link
		linkend="zfs-zfs-set"><command>set</command></link>.
	      The checksum of each block is transparently validated as
	      it is read, allowing <acronym>ZFS</acronym> to detect
	      silent corruption.  If the data that is read does not
	      match the expected checksum, <acronym>ZFS</acronym> will
	      attempt to recover the data from any available
	      redundancy, like mirrors or <acronym>RAID-Z</acronym>).
	      Validation of all checksums can be triggered with <link
		linkend="zfs-term-scrub"><command>scrub</command></link>.
	      Checksum algorithms include:

	      <itemizedlist>
		<listitem>
		  <para><literal>fletcher2</literal></para>
		</listitem>

		<listitem>
		  <para><literal>fletcher4</literal></para>
		</listitem>

		<listitem>
		  <para><literal>sha256</literal></para>
		</listitem>
	      </itemizedlist>

	      The <literal>fletcher</literal> algorithms are faster,
	      but <literal>sha256</literal> is a strong cryptographic
	      hash and has a much lower chance of collisions at the
	      cost of some performance.  Checksums can be disabled,
	      but it is not recommended.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-compression">Compression</entry>

	    <entry>Each dataset has a compression property, which
	      defaults to off.  This property can be set to one of a
	      number of compression algorithms.  This will cause all
	      new data that is written to the dataset to be
	      compressed.  Beyond a reduction in space used, read and
	      write throughput often increases because fewer blocks
	      are read or written.

	      <itemizedlist>
		<listitem xml:id="zfs-term-compression-lz4">
		  <para><emphasis><acronym>LZ4</acronym></emphasis> -
		    Added in <acronym>ZFS</acronym> pool version
		    5000 (feature flags), <acronym>LZ4</acronym> is
		    now the recommended compression algorithm.
		    <acronym>LZ4</acronym> compresses approximately
		    50% faster than <acronym>LZJB</acronym> when
		    operating on compressible data, and is over three
		    times faster when operating on uncompressible
		    data.  <acronym>LZ4</acronym> also decompresses
		    approximately 80% faster than
		    <acronym>LZJB</acronym>.  On modern
		    <acronym>CPU</acronym>s, <acronym>LZ4</acronym>
		    can often compress at over 500&nbsp;MB/s, and
		    decompress at over 1.5&nbsp;GB/s (per single CPU
		    core).</para>

		  <note>
		    <para><acronym>LZ4</acronym> compression is
		      only available after &os;&nbsp;9.2.</para>
		  </note>
		</listitem>

		<listitem xml:id="zfs-term-compression-lzjb">
		  <para><emphasis><acronym>LZJB</acronym></emphasis> -
		    The default compression algorithm.  Created by
		    Jeff Bonwick (one of the original creators of
		    <acronym>ZFS</acronym>).  <acronym>LZJB</acronym>
		    offers good compression with less
		    <acronym>CPU</acronym> overhead compared to
		    <acronym>GZIP</acronym>.  In the future, the
		    default compression algorithm will likely change
		    to <acronym>LZ4</acronym>.</para>
		</listitem>

		<listitem xml:id="zfs-term-compression-gzip">
		  <para><emphasis><acronym>GZIP</acronym></emphasis> -
		    A popular stream compression algorithm available
		    in <acronym>ZFS</acronym>.  One of the main
		    advantages of using <acronym>GZIP</acronym> is its
		    configurable level of compression.  When setting
		    the <literal>compress</literal> property, the
		    administrator can choose the level of compression,
		    ranging from <literal>gzip1</literal>, the lowest
		    level of compression, to <literal>gzip9</literal>,
		    the highest level of compression.  This gives the
		    administrator control over how much
		    <acronym>CPU</acronym> time to trade for saved
		    disk space.</para>
		</listitem>

		<listitem xml:id="zfs-term-compression-zle">
		  <para><emphasis><acronym>ZLE</acronym></emphasis> -
		    Zero Length Encoding is a special compression
		    algorithm that only compresses continuous runs of
		    zeros.  This compression algorithm is only useful
		    when the dataset contains large blocks of
		    zeros.</para>
		</listitem>
	      </itemizedlist></entry>
	  </row>

	  <row>
	    <entry
	      xml:id="zfs-term-copies">Copies</entry>

	    <entry>When set to a value greater than 1, the
	      <literal>copies</literal> property instructs
	      <acronym>ZFS</acronym> to maintain multiple copies of
	      each block in the
	      <link linkend="zfs-term-filesystem">File System</link>
	      or
	      <link linkend="zfs-term-volume">Volume</link>.  Setting
	      this property on important datasets provides additional
	      redundancy from which to recover a block that does not
	      match its checksum.  In pools without redundancy, the
	      copies feature is the only form of redundancy.  The
	      copies feature can recover from a single bad sector or
	      other forms of minor corruption, but it does not protect
	      the pool from the loss of an entire disk.</entry>
	  </row>

	  <row>
	    <entry
	      xml:id="zfs-term-deduplication">Deduplication</entry>

	    <entry>Checksums make it possible to detect duplicate
	      blocks of data as they are written.  With deduplication,
	      the reference count of an existing, identical block is
	      increased, saving storage space.  To detect duplicate
	      blocks, a deduplication table (<acronym>DDT</acronym>)
	      is kept in memory.  The table contains a list of unique
	      checksums, the location of those blocks, and a reference
	      count.  When new data is written, the checksum is
	      calculated and compared to the list.  If a match is
	      found, the existing block is used.  The
	      <acronym>SHA256</acronym> checksum algorithm is used
	      with deduplication to provide a secure cryptographic
	      hash.  Deduplication is tunable.  If
	      <literal>dedup</literal> is <literal>on</literal>, then
	      a matching checksum is assumed to mean that the data is
	      identical.  If <literal>dedup</literal> is set to
	      <literal>verify</literal>, then the data in the two
	      blocks will be checked byte-for-byte to ensure it is
	      actually identical.  If the data is not identical, the
	      hash collision will be noted and the two blocks will be
	      stored separately.  Because <acronym>DDT</acronym> must
	      store the hash of each unique block, it consumes a very
	      large amount of memory.  A general rule of thumb is
	      5-6&nbsp;GB of ram per 1&nbsp;TB of deduplicated data).
	      In situations where it is not practical to have enough
	      <acronym>RAM</acronym> to keep the entire
	      <acronym>DDT</acronym> in memory, performance will
	      suffer greatly as the <acronym>DDT</acronym> must be
	      read from disk before each new block is written.
	      Deduplication can use <acronym>L2ARC</acronym> to store
	      the <acronym>DDT</acronym>, providing a middle ground
	      between fast system memory and slower disks.  Consider
	      using compression instead, which often provides nearly
	      as much space savings without the additional memory
	      requirement.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-scrub">Scrub</entry>

	    <entry>Instead of a consistency check like &man.fsck.8;,
	      <acronym>ZFS</acronym> has <command>scrub</command>.
	      <command>scrub</command> reads all data blocks stored on
	      the pool and verifies their checksums against the known
	      good checksums stored in the metadata.  A periodic check
	      of all the data stored on the pool ensures the recovery
	      of any corrupted blocks before they are needed.  A scrub
	      is not required after an unclean shutdown, but is
	      recommended at least once every three months.  The
	      checksum of each block is verified as blocks are read
	      during normal use, but a scrub makes certain that even
	      infrequently used blocks are checked for silent
	      corruption.  Data security is improved, especially in
	      archival storage situations.  The relative priority of
	      <command>scrub</command> can be adjusted with <link
		linkend="zfs-advanced-tuning-scrub_delay"><varname>vfs.zfs.scrub_delay</varname></link>
	      to prevent the scrub from degrading the performance of
	      other workloads on the pool.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-quota">Dataset Quota</entry>

	    <entry><acronym>ZFS</acronym> provides very fast and
	      accurate dataset, user, and group space accounting in
	      addition to quotas and space reservations.  This gives
	      the administrator fine grained control over how space is
	      allocated and allows space to be reserved for critical
	      file systems.

	      <para><acronym>ZFS</acronym> supports different types of
		quotas: the dataset quota, the <link
		  linkend="zfs-term-refquota">reference
		  quota (<acronym>refquota</acronym>)</link>, the
		<link linkend="zfs-term-userquota">user
		  quota</link>, and the
		<link linkend="zfs-term-groupquota">group
		  quota</link>.</para>

	      <para>Quotas limit the amount of space that a dataset
		and all of its descendants, including snapshots of the
		dataset, child datasets, and the snapshots of those
		datasets, can consume.</para>

	      <note>
		<para>Quotas cannot be set on volumes, as the
		  <literal>volsize</literal> property acts as an
		  implicit quota.</para>
	      </note></entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-refquota">Reference
	      Quota</entry>

	    <entry>A reference quota limits the amount of space a
	      dataset can consume by enforcing a hard limit.  However,
	      this hard limit includes only space that the dataset
	      references and does not include space used by
	      descendants, such as file systems or snapshots.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-userquota">User
	      Quota</entry>

	    <entry>User quotas are useful to limit the amount of space
	      that can be used by the specified user.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-groupquota">Group
	      Quota</entry>

	    <entry>The group quota limits the amount of space that a
	      specified group can consume.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-reservation">Dataset
	      Reservation</entry>

	    <entry>The <literal>reservation</literal> property makes
	      it possible to guarantee a minimum amount of space for a
	      specific dataset and its descendants.  If a 10&nbsp;GB
	      reservation is set on
	      <filename>storage/home/bob</filename>, and another
	      dataset tries to use all of the free space, at least
	      10&nbsp;GB of space is reserved for this dataset.  If a
	      snapshot is taken of
	      <filename>storage/home/bob</filename>, the space used by
	      that snapshot is counted against the reservation.  The
	      <link
		linkend="zfs-term-refreservation"><literal>refreservation</literal></link>
	      property works in a similar way, but it
	      <emphasis>excludes</emphasis> descendants like
	      snapshots.

	      <para>Reservations of any sort are useful in many
		situations, such as planning and testing the
		suitability of disk space allocation in a new system,
		or ensuring that enough space is available on file
		systems for audio logs or system recovery procedures
		and files.</para>
	    </entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-refreservation">Reference
	      Reservation</entry>

	    <entry>The <literal>refreservation</literal> property
	      makes it possible to guarantee a minimum amount of
	      space for the use of a specific dataset
	      <emphasis>excluding</emphasis> its descendants.  This
	      means that if a 10&nbsp;GB reservation is set on
	      <filename>storage/home/bob</filename>, and another
	      dataset tries to use all of the free space, at least
	      10&nbsp;GB of space is reserved for this dataset.  In
	      contrast to a regular
	      <link linkend="zfs-term-reservation">reservation</link>,
	      space used by snapshots and descendant datasets is not
	      counted against the reservation.  For example, if a
	      snapshot is taken of
	      <filename>storage/home/bob</filename>, enough disk space
	      must exist outside of the
	      <literal>refreservation</literal> amount for the
	      operation to succeed.  Descendants of the main data set
	      are not counted in the <literal>refreservation</literal>
	      amount and so do not encroach on the space set.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-resilver">Resilver</entry>

	    <entry>When a disk fails and is replaced, the new disk
	      must be filled with the data that was lost.  The process
	      of using the parity information distributed across the
	      remaining drives to calculate and write the missing data
	      to the new drive is called
	      <emphasis>resilvering</emphasis>.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-online">Online</entry>

	    <entry>A pool or vdev in the <literal>Online</literal>
	      state has all of its member devices connected and fully
	      operational.  Individual devices in the
	      <literal>Online</literal> state are functioning
	      normally.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-offline">Offline</entry>

	    <entry>Individual devices can be put in an
	      <literal>Offline</literal> state by the administrator if
	      there is sufficient redundancy to avoid putting the pool
	      or vdev into a
	      <link linkend="zfs-term-faulted">Faulted</link> state.
	      An administrator may choose to offline a disk in
	      preparation for replacing it, or to make it easier to
	      identify.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-degraded">Degraded</entry>

	    <entry>A pool or vdev in the <literal>Degraded</literal>
	      state has one or more disks that have been disconnected
	      or have failed.  The pool is still usable, but if
	      additional devices fail, the pool could become
	      unrecoverable.  Reconnecting the missing devices or
	      replacing the failed disks will return the pool to an
	      <link linkend="zfs-term-online">Online</link> state
	      after the reconnected or new device has completed the
	      <link linkend="zfs-term-resilver">Resilver</link>
	      process.</entry>
	  </row>

	  <row>
	    <entry xml:id="zfs-term-faulted">Faulted</entry>

	    <entry>A pool or vdev in the <literal>Faulted</literal>
	      state is no longer operational.  The data on it can no
	      longer be accessed.  A pool or vdev enters the
	      <literal>Faulted</literal> state when the number of
	      missing or failed devices exceeds the level of
	      redundancy in the vdev.  If missing devices can be
	      reconnected, the pool will return to a
	      <link linkend="zfs-term-online">Online</link> state.  If
	      there is insufficient redundancy to compensate for the
	      number of failed disks, then the contents of the pool
	      are lost and must be restored from backups.</entry>
	  </row>
	</tbody>
      </tgroup>
    </informaltable>
  </sect1>
</chapter>