source: postlfs/filesystems/aboutlvm.xml@ ec64d28

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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>

  <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>