source: postlfs/filesystems/aboutraid.xml

<?xml version="1.0" encoding="UTF-8"?>
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   "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
  <!ENTITY % general-entities SYSTEM "../../general.ent">
  %general-entities;
]>

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


  <title>About RAID</title>

  <para>
    The storage technology known as RAID (Redundant Array of
    Independent Disks) combines multiple physical disks into a logical
    unit.  The drives can generally be combined to provide data
    redundancy or to extend the size of logical units beyond the
    capability of the physical disks or both.  The technology
    also allows for providing hardware maintenance without powering
    down the system.
  </para>

  <para>
    The types of RAID organization are described in the <ulink
    url="https://raid.wiki.kernel.org/index.php/Overview#The_RAID_levels">
    RAID Wiki</ulink>.
  </para>

  <para>
    Note that while RAID provides protection against disk
    failures, it is not a substitute for backups.  A file deleted
    is still deleted on all the disks of a RAID array.  Modern backups
    are generally done via <xref linkend='rsync'/>.
  </para>

  <para>
    There are three major types of RAID implementation:
    Hardware RAID, BIOS-based RAID, and Software RAID.
  </para>

  <sect2 id="hwraid">
    <title>Hardware RAID</title>
    <para>
      Hardware based RAID provides capability through proprietary
      hardware and data layouts.  The control and configuration is generally
      done via firmware in conjunction with executable programs made
      available by the device manufacturer.  The capabilities are
      generally supplied via a PCI card, although there are some
      instances of RAID components integrated in to the motherboard.
      Hardware RAID may also be available in a stand-alone enclosure.
    </para>

    <para>
      One advantage of hardware-based RAID is that the drives
      are offered to the operating system as a logical drive and no
      operating system dependent configuration is needed.
    </para>

    <para>
      Disadvantages include difficulties in transferring drives
      from one system to another, updating firmware, or replacing
      failed RAID hardware.
    </para>

  </sect2>

  <sect2 id="biosraid">
    <title>BIOS-based RAID</title>

    <para>
      Some computers offer a hardware-like RAID implementation in the
      system BIOS.  Sometime this is referred to as 'fake' RAID as the
      capabilities are generally incorporated into firmware without any hardware
      acceleration.
    </para>

    <para>
      The advantages and disadvantages of BIOS-based RAID are generally
      the same as hardware RAID with the additional disadvantage that there
      is no hardware acceleration.
    </para>

    <para>
      In some cases, BIOS-based RAID firmware is enabled by default (e.g.
      some DELL systems).  If software RAID is desired, this option must be
      explicitly disabled in the BIOS.
    </para>

  </sect2>

  <sect2 id="swraid">
  <title>Software RAID</title>
    <para>
      Software based RAID is the most flexible form of RAID.  It is
      easy to install and update and provides full capability on all or
      part of any drives available to the system.  In BLFS, the RAID software
      is found in <xref linkend='mdadm'/>.
    </para>

    <para>
      Configuring a RAID device is straightforward using
      <application>mdadm</application>.  Generally devices are created in the
      <filename class='directory'>/dev</filename> directory as
      <filename>/dev/mdx</filename> where <emphasis>x</emphasis> is an integer.
    </para>

    <para>
      The first step in creating a RAID array is to use partitioning software
      such as <userinput>fdisk</userinput> or <xref linkend='parted'/> to
      define the partitions needed for the array.  Usually, there will be
      one partition on each drive participating in the RAID array, but that
      is not strictly necessary.  For this example, there will be four disk
      drives:
      <filename>/dev/sda</filename>,
      <filename>/dev/sdb</filename>,
      <filename>/dev/sdc</filename>, and
      <filename>/dev/sdd</filename>.  They will be partitioned as follows:
    </para>

<screen><literal>Partition Size     Type                Use
sda1:     100 MB   fd Linux raid auto  /boot    (RAID 1) /dev/md0
sda2:      10 GB   fd Linux raid auto  /        (RAID 1) /dev/md1
sda3:       2 GB   83 Linux swap       swap
sda4      300 GB   fd Linux raid auto  /home    (RAID 5) /dev/md2

sdb1:     100 MB   fd Linux raid auto  /boot    (RAID 1) /dev/md0
sdb2:      10 GB   fd Linux raid auto  /        (RAID 1) /dev/md1
sdb3:       2 GB   83 Linux swap       swap
sdb4      300 GB   fd Linux raid auto  /home    (RAID 5) /dev/md2

sdc1:      12 GB   fd Linux raid auto  /usr/src (RAID 0) /dev/md3
sdc2:     300 GB   fd Linux raid auto  /home    (RAID 5) /dev/md2

sdd1:      12 GB   fd Linux raid auto  /usr/src (RAID 0) /dev/md3
sdd2:     300 GB   fd Linux raid auto  /home    (RAID 5) /dev/md2 </literal></screen>

    <para>
      In this arrangement, a separate boot partition is created as the
      first small RAID array and a root filesystem as the secong RAID array,
      both mirrored.  The third partition is a large (about 1TB) array for the
      <filename class='directory'>/home</filename> directory.  This provides
      an ability to stripe data across multiple devices, improving speed for
      both reading and writing large files.  Finally, a fourth array is created
      that concatenates two partitions into a larger device.
    </para>

    <note>
      <para>
        All <application>mdadm</application> commands must be run
        as the <systemitem class="username">root</systemitem> user.
      </para>
    </note>

    <para>
      To create these RAID arrays the commands are:
    </para>

<screen><userinput>/sbin/mdadm -Cv /dev/md0 --level=1 --raid-devices=2 /dev/sda1 /dev/sdb1
/sbin/mdadm -Cv /dev/md1 --level=1 --raid-devices=2 /dev/sda2 /dev/sdb2
/sbin/mdadm -Cv /dev/md3 --level=0 --raid-devices=2 /dev/sdc1 /dev/sdd1
/sbin/mdadm -Cv /dev/md2 --level=5 --raid-devices=4 \
        /dev/sda4 /dev/sdb4 /dev/sdc2 /dev/sdd2 </userinput></screen>

    <para>
      The devices created can be examined by device.  For example,
      to see the details of <filename>/dev/md1</filename>, use
      <userinput>/sbin/mdadm --detail /dev/md1</userinput>:
    </para>

<screen><literal>        Version : 1.2
  Creation Time : Tue Feb  7 17:08:45 2012
     Raid Level : raid1
     Array Size : 10484664 (10.00 GiB 10.74 GB)
  Used Dev Size : 10484664 (10.00 GiB 10.74 GB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Tue Feb  7 23:11:53 2012
          State : clean
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           Name : core2-blfs:0  (local to host core2-blfs)
           UUID : fcb944a4:9054aeb2:d987d8fe:a89121f8
         Events : 17

    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1</literal></screen>

    <para>
      From this point, the partitions can be formatted with the filesystem of
      choice (e.g. ext3, ext4, <xref linkend='xfsprogs'/>, etc).  The formatted 
      partitions can then be
      mounted.  The <filename>/etc/fstab</filename> file can use the devices
      created for mounting at boot time and the linux command line in
      <filename>/boot/grub/grub.cfg</filename> can specify
      <option>root=/dev/md1</option>.
    </para>

    <note>
      <para>
        The swap devices should be specified in the
        <filename>/etc/fstab</filename> file as normal.  The kernel normally
        stripes swap data across multiple swap files and should not be made
        part of a RAID array.
      </para>
    </note>

    <para>
      For further options and management details of RAID devices, refer to
      <userinput>man mdadm</userinput>.
    </para>

    <para>
      Additional details for monitoring RAID arrays and dealing with
      problems can be found at the <ulink
      url="https://raid.wiki.kernel.org/index.php/Linux_Raid">Linux RAID
      Wiki</ulink>.
    </para>

  </sect2>

</sect1>