source: postlfs/filesystems/aboutlvm.xml@ f716ef4

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<sect1 id="aboutlvm">
  <?dbhtml filename="aboutlvm.html"?>

  <sect1info>
    <othername>$LastChangedBy$</othername>
    <date>$Date$</date>
  </sect1info>

  <title>About Logical Volume Management (LVM)</title>

  <para>
    LVM manages disk drives.  It allows multiple drives and partitions
    to be combined into larger <emphasis>volume groups</emphasis>, assists in
    making backups through a <emphasis>snapshot</emphasis>, and allows for
    dynamic volume resizing. It can also provide mirroring similar to
    a RAID 1 array.
  </para>

  <para>
    A complete discussion of LVM is beyond the scope of this introduction,
    but basic concepts are presented below.
  </para>

  <para>
    To run any of the commands presented here, the <xref linkend='lvm2'/>
    package must be installed.  All commands must be run as the <systemitem
    class="username">root</systemitem> user.
  </para>

  <para>
    Management of disks with lvm is accomplished using the following concepts: 
  </para>

  <variablelist>
    <varlistentry>
      <term>physical volumes</term>
      <listitem>
        <para>
          These are physical disks or partitions such as
          /dev/sda3 or /dev/sdb.
        </para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>volume groups</term>
      <listitem>
        <para>
          These are named groups of physical volumes that
          can be manipulated by the administrator.  The number of physical
          volumes that make up a volume group is arbitrary.  Physical volumes
          can be dynamically added or removed from a volume group.
        </para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>logical volumes</term>
      <listitem>
        <para>
          Volume groups may be subdivided into logical volumes. Each logical
          volume can then be individually formatted as if it were a regular
          Linux partition. Logical volumes may be dynamically resized by
          the administrator according to need.
        </para>
      </listitem>

    </varlistentry>
  </variablelist>

  <para>
    To give a concrete example, suppose that you have two 2 TB disks. Also
    suppose a really large amount of space is required for a very large
    database, mounted on <filename class='directory'>/srv/mysql</filename>.
    This is what the initial set of partitions would look like:
  </para>

  <screen><literal>Partition  Use    Size      Partition Type
/dev/sda1  /boot  100MB     83 (Linux)
/dev/sda2  /       10GB     83 (Linux)
/dev/sda3  swap     2GB     82 (Swap)
/dev/sda4  LVM    remainder 8e (LVM)
/dev/sdb1  swap     2GB     82 (Swap)
/dev/sdb2  LVM    remainder 8e (LVM)</literal></screen>

  <para>
    First initialize the physical volumes:
  </para>

  <screen><userinput>pvcreate /dev/sda4 /dev/sdb2</userinput></screen>

  <note>
    <para>
      A full disk can be used as part of a physical volume, but
      beware that the <command>pvcreate</command> command will destroy any
      partition information on that disk.
    </para>
  </note>

  <para>
    Next create a volume group named lfs-lvm:
  </para>

  <screen><userinput>vgcreate lfs-lvm /dev/sda4  /dev/sdb2</userinput></screen>

  <para>
    The status of the volume group can be checked by running the command
    <command>vgscan</command>. Now create the logical volumes.  Since there
    is about 3900 GB available, leave about 900 GB free for expansion. Note
    that the logical volume named <emphasis>mysql</emphasis> is larger than
    any physical disk.
  </para>

  <screen><userinput>lvcreate --name mysql --size 2500G lfs-lvm
lvcreate --name home  --size  500G lfs-lvm</userinput></screen>

  <para>
    Finally the logical volumes can be formatted and mounted.  In this
    example, the jfs file system (<xref linkend='jfsutils'/>) is used for
    demonstration purposes.
  </para>

  <screen><userinput>mkfs -t ext4 /dev/lfs-lvm/home
mkfs -t jfs  /dev/lfs-lvm/mysql
mount /dev/lfs-lvm/home /home
mkdir -p /srv/mysql
mount /dev/lfs-lvm/mysql /srv/mysql</userinput></screen>

  <para>
    The LFS boot scripts automatically make these file systems available to
    the system in the checkfs script. Edit the <filename>/etc/fstab</filename>
    file as required to automatically mount them.
  </para>

  <para>
    A LVM logical volume can host a root filesystem, but requires the use
    of an initramfs (initial RAM file system) and is not discussed here.
  </para>

  <para>
    For a more information about LVM, see the <ulink
    url="http://www.tldp.org/HOWTO/LVM-HOWTO/">LVM HOWTO</ulink> and
    the lvm man pages.
  </para>

</sect1>