path: root/en_US.ISO8859-1/books/handbook/filesystems/chapter.sgml

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     The FreeBSD Documentation Project
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<chapter id="filesystems">
  <chapterinfo>
    <authorgroup>
      <author>
	<firstname>Tom</firstname>
	<surname>Rhodes</surname>
	<contrib>Written by </contrib>
      </author>
    </authorgroup>
  </chapterinfo>

  <title>File Systems Support</title>

  <sect1 id="filesystems-synopsis">
    <title>Synopsis</title>

    <indexterm><primary>File Systems</primary></indexterm>
    <indexterm>
      <primary>File Systems Support</primary>
      <see>File Systems</see>
    </indexterm>

    <para>File systems are an integral part of any operating system.
      They allow for users to upload and store files, provide access
      to data, and of course, make hard drives useful.  Different
      operating systems usually have one major aspect in common, that
      is their native file system.  On &os; this file system is known
      as the Fast File System or <acronym>FFS</acronym> which is built
      on the original Unix&trade; File System, also known as
      <acronym>UFS</acronym>.  This is the native file system on &os;
      which is placed on hard disks for access to data.</para>

    <para>&os; also supports a multitude of different file systems to
      provide support for accessing data from other operating systems
      locally, i.e.&nbsp;data stored on locally attached
      <acronym>USB</acronym> storage devices, flash drives, and hard
      disks.  There is also support for some non-native file systems.
      These are file systems developed on other
      operating systems, like the &linux; Extended File System
      (<acronym>EXT</acronym>), and the &sun; Z File System
      (<acronym>ZFS</acronym>).</para>

    <para>There are different levels of support for the various file
      systems in &os;.  Some will require a kernel module to be loaded,
      others may require a toolset to be installed.  This chapter is
      designed to help users of &os; access other file systems on their
      systems, starting with the &sun; Z file
      system.</para>

    <para>After reading this chapter, you will know:</para>

    <itemizedlist>
      <listitem>
	<para>The difference between native and supported file systems.</para>
      </listitem>

      <listitem>
	<para>What file systems are supported by &os;.</para>
      </listitem>

      <listitem>
	<para>How to enable, configure, access and make use of
	  non-native file systems.</para>
      </listitem>
    </itemizedlist>

    <para>Before reading this chapter, you should:</para>

    <itemizedlist>
      <listitem>
	<para>Understand &unix; and &os; basics
	  (<xref linkend="basics">).</para>
      </listitem>

      <listitem>
	<para>Be familiar with
	  the basics of kernel configuration/compilation
	  (<xref linkend="kernelconfig">).</para>
      </listitem>

      <listitem>
	<para>Feel comfortable installing third party software
	  in &os; (<xref linkend="ports">).</para>
      </listitem>

      <listitem>
	<para>Have some familiarity with disks, storage and
	  device names in &os; (<xref linkend="disks">).</para>
      </listitem>
    </itemizedlist>
  </sect1>

  <sect1 id="filesystems-zfs">
    <title>The Z File System (ZFS)</title>

    <para>The Z&nbsp;file system, developed by &sun;, is a new
      technology designed to use a pooled storage method.  This means
      that space is only used as it is needed for data storage.  It
      has also been designed for maximum data integrity, supporting
      data snapshots, multiple copies, and data checksums.  A new
      data replication model, known as <acronym>RAID</acronym>-Z has
      been added.  The <acronym>RAID</acronym>-Z model is similar
      to <acronym>RAID</acronym>5 but is designed to prevent data
      write corruption.</para>

    <sect2>
      <title>ZFS Tuning</title>

      <para>The <acronym>ZFS</acronym> subsystem utilizes much of
	the system resources, so some tuning may be required to provide
	maximum efficiency during every-day use.  As an experimental
	feature in &os; this may change in the near future; however,
	at this	time, the following steps are recommended.</para>

      <sect3>
	<title>Memory</title>

	<para>The total system memory should be at least one gigabyte,
	  with two gigabytes or more recommended.  In all of the
	  examples here, the system has one gigabyte of memory with
	  several other tuning mechanisms in place.</para>

	<para>Some people have had luck using fewer than one gigabyte
	  of memory, but with such a limited amount of physical memory,
	  when the system is under heavy load, it is very plausible
	  that &os; will panic due to memory exhaustion.</para>
      </sect3>

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

	<para>It is recommended that unused drivers and options
	  be removed from the kernel configuration file.  Since most
	  devices are available as modules, they may simply be loaded
	  using the <filename>/boot/loader.conf</filename> file.</para>

	<para>Users of the &i386; architecture should add the following
	  option to their kernel configuration file, rebuild their
	  kernel, and reboot:</para>

	<programlisting>options 	KVA_PAGES=512</programlisting>

	<para>This option will expand the kernel address space, thus
	  allowing the <varname>vm.kvm_size</varname> tunable to be
	  pushed beyond the currently imposed limit of 1&nbsp;GB
	  (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 (4).  In this case, it is
	  <literal>512</literal> for 2&nbsp;GB.</para>
      </sect3>

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

	<para>The <devicename>kmem</devicename> address space should be
	  increased on all &os; architectures.  On the test system with
	  one gigabyte of physical memory, success was achieved with the
	  following options which should be placed in
	  the <filename>/boot/loader.conf</filename> file 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 ZFS-related
	  tuning, see
	  <ulink url="http://wiki.freebsd.org/ZFSTuningGuide"></ulink>.</para>
      </sect3>
    </sect2>

    <sect2>
      <title>Using <acronym>ZFS</acronym></title>

      <para>There is a start up mechanism that allows &os; to
	mount <acronym>ZFS</acronym> pools during system
	initialization.  To set it, issue the following
	commands:</para>

	<screen>&prompt.root; <userinput>echo 'zfs_enable="YES"' &gt;&gt; /etc/rc.conf</userinput>
&prompt.root; <userinput>/etc/rc.d/zfs start</userinput></screen>

	<para>The remainder of this document assumes three
	  <acronym>SCSI</acronym> disks are available, and their device names
	  are <devicename><replaceable>da0</replaceable></devicename>,
	  <devicename><replaceable>da1</replaceable></devicename>
	  and <devicename><replaceable>da2</replaceable></devicename>.
	  Users of <acronym>IDE</acronym> hardware may use the
	  <devicename><replaceable>ad</replaceable></devicename>
	  devices in place of <acronym>SCSI</acronym> hardware.</para>

      <sect3>
	<title>Single Disk Pool</title>

	<para>To create a simple, non-redundant <acronym>ZFS</acronym> pool using a
	  single disk device, use the <command>zpool</command> command:</para>

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

	<para>To view the new pool, review the output of the
	  <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 clearly shows the <literal>example</literal>
	  pool has not only been created but
	  <emphasis>mounted</emphasis> as well.  It is also accessible
	  just like a normal file system, files may be created on it
	  and users are able to browse it as in the
	  following example:</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>Unfortunately this pool is not taking advantage of
	  any <acronym>ZFS</acronym> features.  Create a file system
	  on this pool, and enable compression on it:</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> is now a
	  <acronym>ZFS</acronym> compressed file system.  Try copying
	  some large files to it by copying them to
	  <filename class="directory">/example/compressed</filename>.</para>

	<para>The compression may now be disabled with:</para>

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

	<para>To unmount the file system, issue the following command
	  and then verify by using the <command>df</command>
	  utility:</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>Re-mount the file system to make it accessible
	  again, 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/data on /example/data (zfs, local)
example/compressed on /example/compressed (zfs, local)</screen>

	<para>As observed, <acronym>ZFS</acronym> file systems, after
	  creation, may be used like ordinary file systems; however,
	  many other features are also available.  In the following
	  example, a new file system, <literal>data</literal> is
	  created.  Important files will be stored here, so the file
	  system is set 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 the <command>df</command> again:</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
	  the <command>df</command> through these examples, to show
	  that the file systems are using only the amount of space
	  they need and will all draw from the same pool.
	  The <acronym>ZFS</acronym> file system does away with concepts
	  such as volumes and partitions, and allows for several file
	  systems to occupy the same pool.  Destroy the file systems,
	  and then destroy the pool as they are 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>

	<para>Disks go bad and fail, an unavoidable trait.  When
	  this disk goes bad, the data will be lost.  One method of
	  avoiding data loss due to a failed hard disk is to implement
	  a <acronym>RAID</acronym>.  <acronym>ZFS</acronym> supports
	  this feature in its pool design which is covered in
	  the next section.</para>
      </sect3>

      <sect3>
	<title><acronym>ZFS</acronym> RAID-Z</title>

	<para>As previously noted, this section will assume that
	  three <acronym>SCSI</acronym> disks exist as devices
	  <devicename>da0</devicename>, <devicename>da1</devicename>
	  and <devicename>da2</devicename> (or <devicename>ad0</devicename>
	  and beyond in case IDE disks are being used).  To create a
	  <acronym>RAID</acronym>-Z pool, issue the following
	  command:</para>

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

	<note><para>&sun; recommends that the amount of devices used in a
	  <acronym>RAID</acronym>-Z configuration is between three and nine.  If your needs
	  call for a single pool to consist of 10 disks or more, consider
	  breaking it up into smaller <acronym>RAID</acronym>-Z groups.  If
	  you only have two disks and still require redundancy, consider using
	  a <acronym>ZFS</acronym> mirror instead.  See the &man.zpool.8;
	  manual page for more details.</para></note>

	<para>The <literal>storage</literal> zpool should have been
	  created.  This may be verified by using the &man.mount.8; and
	  &man.df.1; commands as before.  More disk devices may have
	  been allocated by adding them to the end of the list above.
	  Make a new file system in the pool, called
	  <literal>home</literal> where user files will eventually be
	  placed:</para>

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

	<para>It is now possible to enable compression and keep extra
	  copies of the user's home directories and files.  This may
	  be accomplished just as before using the following
	  commands:</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 should now have their data stored on the freshly
	  created <filename class="directory">/storage/home</filename>
	  file system.  Test by adding a new user and logging in as
	  that user.</para>

	<para>Try creating a snapshot which may be rolled back
	  later:</para>

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

	<para>Note that the snapshot option will only capture a real
	  file system, not a home directory or a file.  The
	  <literal>@</literal> character is a delimiter used between
	  the file system name or the volume name.  When a user's
	  home directory gets trashed, restore it with:</para>

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

	<para>To get a list of all available snapshots, run the
	  <command>ls</command> in the file system's
	  <filename class="directory">.zfs/snapshot</filename>
	  directory.  For example, to see the previously taken
	  snapshot, perform the following command:</para>

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

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

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

	<para>There is no reason, after all of this testing, we should
	  keep <filename class="directory">/storage/home</filename>
	  around in its present state.  Make it the real
	  <filename class="directory">/home</filename> file
	  system:</para>

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

	<para>Issuing the <command>df</command> and
	  <command>mount</command> commands will show that the system
	  now treats our file system as the real
	  <filename class="directory">/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</acronym>-Z
	  configuration.  To get status updates about the file systems
	  created during the nightly &man.periodic.8; runs, issue the
	  following command:</para>

	<screen>&prompt.root; <userinput>echo 'daily_status_zfs_enable="YES"' &gt;&gt; /etc/periodic.conf</userinput></screen>
      </sect3>

      <sect3>
	<title>Recovering <acronym>RAID</acronym>-Z</title>

	<para>Every software <acronym>RAID</acronym> has a method of
	  monitoring their <literal>state</literal>.
	  <acronym>ZFS</acronym> is no exception.  The status of
	  <acronym>RAID</acronym>-Z devices may be viewed with the
	  following command:</para>

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

	<para>If all pools are healthy and everything is normal, the
	  following message will be returned:</para>

	<screen>all pools are healthy</screen>

	<para>If there is an issue, perhaps a disk has gone offline,
	  the pool state will be returned and 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 states that the device was taken offline by the
	  administrator.  This is true for this particular example.
	  To take the disk offline, the following command was
	  used:</para>

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

	<para>It is now possible to replace the
	  <devicename>da1</devicename> after the system has been
	  powered down.  When the system is back online, the following
	  command may issued to replace the disk:</para>

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

	<para>From here, the status may be checked again, this time
	  without the <option>-x</option> flag to get state
	  information:</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>As shown from this example, everything appears to be
	  normal.</para>
      </sect3>

      <sect3>
	<title>Data Verification</title>

	<para>As previously mentioned, <acronym>ZFS</acronym> uses
	  <literal>checksums</literal> to verify the integrity of
	  stored data.  They are enabled automatically upon creation
	  of file systems and may be disabled using the following
	  command:</para>

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

	<para>This is not a wise idea; however, as checksums take
	  very little storage space and are more useful enabled.  There
	  also appear to be no noticeable costs having them enabled.
	  While enabled, it is possible to have <acronym>ZFS</acronym>
	  check data integrity using checksum verification.  This
	  process is known as <quote>scrubbing.</quote>  To verify the
	  data integrity of the <literal>storage</literal> pool, issue
	  the following command:</para>

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

	<para>This process may take considerable time depending on
	  the amount of data stored.  It is also very
	  <acronym>I/O</acronym> intensive, so much that only one
	  of these operations may be run at any given time.  After
	  the scrub has completed, the status is updated and may be
	  viewed by issuing a status request:</para>

	<screen>&prompt.root; <userinput>zpool status storage</userinput>
 pool: storage
 state: ONLINE
 scrub: scrub completed with 0 errors on Sat Aug 30 19:57:37 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>The completion time is in plain view in this example.
	  This feature helps to ensure data integrity over a long
	  period of time.</para>

	<para>There are many more options for the Z&nbsp;file system,
	  see the &man.zfs.8; and &man.zpool.8; manual
	  pages.</para>
      </sect3>
    </sect2>
  </sect1>


  <!--
      XXXTR: stub sections (added later, as needed, as desire,
      after I get opinions from -doc people):

      Still need to discuss native and foreign file systems.

  <sect1>
    <title>Device File System</title>
  </sect1>

  <sect1>
    <title>DOS and NTFS File Systems</title>
    <para>This is a good section for those who transfer files, using
      USB devices, from Windows to FreeBSD and vice-versa.  My camera,
      and many other cameras I have seen default to using FAT16.  There
      is (was?) a kde utility, I think called kamera, that could be used
      to access camera devices.  A section on this would be useful.</para>

    <para>XXXTR: Though!  The disks chapter, covers a bit of this and
      devfs under it's USB devices.  It leaves a lot to be desired though,
      see:
http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/usb-disks.html
      It may be better to flesh out that section a bit more.  Add the
      word "camera" to it so that others can easily notice.</para>
  </sect1>

  <sect1>
    <title>Linux EXT File System</title>

    <para>Probably NOT as useful as the other two, but it requires
      knowledge of the existence of the tools.  Which are hidden in
      the ports collection.  Most Linux guys would probably only use
      Linux, BSD guys would be smarter and use NFS.</para>
  </sect1>

  <sect1>
    <title>HFS</title>

    <para>I think this is the file system used on Apple OSX.  There are
      tools in the ports collection, and with Apple being a big
      FreeBSD supporter and user of our technologies, surely there
      is enough cross over to cover this?</para>
  </sect1>
  -->

</chapter>