source: postlfs/filesystems/aboutlvm.xml

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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
  <!ENTITY % general-entities SYSTEM "../../general.ent">
  %general-entities;
]>

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


  <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>
    It may be needed to activate those logical volumes, for them to
    appear in <filename class="directory">/dev</filename>. They can all
    be activated at the same time by issuing, as the
    <systemitem class="username">root</systemitem> user:
  </para>

<screen role="root"><userinput>vgchange -a y</userinput></screen>

  <para revision="sysv">
    The LFS boot scripts automatically make these logical volumes available to
    the system in the <command>udev</command> 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). The initramfs proposed in
    <xref linkend="initramfs"/> allows to pass the lvm volume in
    the <parameter>root=</parameter> switch of the kernel command line.
  </para>

  <para revision="systemd">
    If not using an initramfs, there is a race condition in <application>
    systemd</application> preventing mounting logical volumes through
    <filename>/etc/fstab</filename>. You must create a <quote>mount</quote>
    unit (see <ulink role='man'
    url='&man;systemd.mount.5'>systemd.mount(5)</ulink>) as in the following
    example, which mounts the <filename class="directory">/home</filename>
    directory automatically at boot:
  </para>

<screen role="root" revision="systemd"><userinput>cat &gt; /etc/systemd/system/home.mount &lt;&lt; EOF
<literal>[Unit]
Description=Mount the lvm volume /dev/lfs-lvm/home to /home

[Mount]
What=/dev/lfs-lvm/home
Where=/home
Type=ext4
Options=default

[Install]
WantedBy=multi-user.target</literal>
EOF</userinput></screen>

  <note revision="systemd">
    <para>
    The name of the unit must be the name of the mount point with the
    `/' character replaced by `-', omitting the leading one.
    </para>
  </note>

  <para revision="systemd">
    Next the unit must be enabled with:
  </para>

<screen role="root" revision="systemd"><userinput>systemctl enable home.mount</userinput></screen>

  <para>
    For more information about LVM, see the <ulink
    url="https://tldp.org/HOWTO/LVM-HOWTO/">LVM HOWTO</ulink> and
    the lvm man pages. A good in-depth
    <ulink url="https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/logical_volume_manager_administration/index">
    guide</ulink> is available from RedHat<superscript>&reg;</superscript>,
    although it makes sometimes reference to proprietary tools.
  </para>

</sect1>