mirror of
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229 lines
8.2 KiB
XML
229 lines
8.2 KiB
XML
<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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<!ENTITY % general-entities SYSTEM "../../general.ent">
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%general-entities;
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]>
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<sect1 id="raid">
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<?dbhtml filename="raid.html"?>
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<title>About RAID</title>
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<para>
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The storage technology known as RAID (Redundant Array of
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Independent Disks) combines multiple physical disks into a logical
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unit. The drives can generally be combined to provide data
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redundancy or to extend the size of logical units beyond the
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capability of the physical disks or both. The technology
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also allows for providing hardware maintenance without powering
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down the system.
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</para>
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<para>
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The types of RAID organization are described in the <ulink
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url="https://raid.wiki.kernel.org/index.php/Overview#The_RAID_levels">
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RAID Wiki</ulink>.
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</para>
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<para>
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Note that while RAID provides protection against disk
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failures, it is not a substitute for backups. A file deleted
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is still deleted on all the disks of a RAID array. Modern backups
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are generally done via <xref linkend='rsync'/>.
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</para>
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<para>
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There are three major types of RAID implementation:
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Hardware RAID, BIOS-based RAID, and Software RAID.
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</para>
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<sect2 id="hwraid">
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<title>Hardware RAID</title>
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<para>
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Hardware based RAID provides capability through proprietary
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hardware and data layouts. The control and configuration is generally
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done via firmware in conjunction with executable programs made
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available by the device manufacturer. The capabilities are
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generally supplied via a PCI card, although there are some
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instances of RAID components integrated in to the motherboard.
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Hardware RAID may also be available in a stand-alone enclosure.
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</para>
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<para>
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One advantage of hardware-based RAID is that the drives
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are offered to the operating system as a logical drive and no
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operating system dependent configuration is needed.
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</para>
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<para>
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Disadvantages include difficulties in transferring drives
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from one system to another, updating firmware, or replacing
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failed RAID hardware.
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</para>
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</sect2>
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<sect2 id="biosraid">
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<title>BIOS-based RAID</title>
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<para>
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Some computers offer a hardware-like RAID implementation in the
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system BIOS. Sometime this is referred to as 'fake' RAID as the
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capabilities are generally incorporated into firmware without any hardware
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acceleration.
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</para>
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<para>
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The advantages and disadvantages of BIOS-based RAID are generally
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the same as hardware RAID with the additional disadvantage that there
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is no hardware acceleration.
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</para>
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<para>
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In some cases, BIOS-based RAID firmware is enabled by default (e.g.
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some DELL systems). If software RAID is desired, this option must be
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explicitly disabled in the BIOS.
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</para>
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</sect2>
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<sect2 id="swraid">
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<title>Software RAID</title>
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<para>
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Software based RAID is the most flexible form of RAID. It is
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easy to install and update and provides full capability on all or
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part of any drives available to the system. In BLFS, the RAID software
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is found in <xref linkend='mdadm'/>.
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</para>
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<para>
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Configuring a RAID device is straightforward using
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<application>mdadm</application>. Generally devices are created in the
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<filename class='directory'>/dev</filename> directory as
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<filename>/dev/mdx</filename> where <emphasis>x</emphasis> is an integer.
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</para>
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<para>
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The first step in creating a RAID array is to use partitioning software
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such as <userinput>fdisk</userinput> or <xref linkend='parted'/> to
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define the partitions needed for the array. Usually, there will be
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one partition on each drive participating in the RAID array, but that
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is not strictly necessary. For this example, there will be four disk
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drives:
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<filename>/dev/sda</filename>,
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<filename>/dev/sdb</filename>,
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<filename>/dev/sdc</filename>, and
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<filename>/dev/sdd</filename>. They will be partitioned as follows:
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</para>
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<screen><literal>Partition Size Type Use
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sda1: 100 MB fd Linux raid auto /boot (RAID 1) /dev/md0
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sda2: 10 GB fd Linux raid auto / (RAID 1) /dev/md1
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sda3: 2 GB 83 Linux swap swap
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sda4 300 GB fd Linux raid auto /home (RAID 5) /dev/md2
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sdb1: 100 MB fd Linux raid auto /boot (RAID 1) /dev/md0
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sdb2: 10 GB fd Linux raid auto / (RAID 1) /dev/md1
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sdb3: 2 GB 83 Linux swap swap
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sdb4 300 GB fd Linux raid auto /home (RAID 5) /dev/md2
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sdc1: 12 GB fd Linux raid auto /usr/src (RAID 0) /dev/md3
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sdc2: 300 GB fd Linux raid auto /home (RAID 5) /dev/md2
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sdd1: 12 GB fd Linux raid auto /usr/src (RAID 0) /dev/md3
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sdd2: 300 GB fd Linux raid auto /home (RAID 5) /dev/md2 </literal></screen>
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<para>
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In this arrangement, a separate boot partition is created as the
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first small RAID array and a root filesystem as the secong RAID array,
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both mirrored. The third partition is a large (about 1TB) array for the
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<filename class='directory'>/home</filename> directory. This provides
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an ability to stripe data across multiple devices, improving speed for
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both reading and writing large files. Finally, a fourth array is created
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that concatenates two partitions into a larger device.
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</para>
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<note>
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<para>
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All <application>mdadm</application> commands must be run
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as the <systemitem class="username">root</systemitem> user.
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</para>
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</note>
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<para>
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To create these RAID arrays the commands are:
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</para>
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<screen><userinput>/sbin/mdadm -Cv /dev/md0 --level=1 --raid-devices=2 /dev/sda1 /dev/sdb1
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/sbin/mdadm -Cv /dev/md1 --level=1 --raid-devices=2 /dev/sda2 /dev/sdb2
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/sbin/mdadm -Cv /dev/md3 --level=0 --raid-devices=2 /dev/sdc1 /dev/sdd1
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/sbin/mdadm -Cv /dev/md2 --level=5 --raid-devices=4 \
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/dev/sda4 /dev/sdb4 /dev/sdc2 /dev/sdd2 </userinput></screen>
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<para>
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The devices created can be examined by device. For example,
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to see the details of <filename>/dev/md1</filename>, use
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<userinput>/sbin/mdadm --detail /dev/md1</userinput>:
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</para>
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<screen><literal> Version : 1.2
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Creation Time : Tue Feb 7 17:08:45 2012
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Raid Level : raid1
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Array Size : 10484664 (10.00 GiB 10.74 GB)
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Used Dev Size : 10484664 (10.00 GiB 10.74 GB)
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Raid Devices : 2
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Total Devices : 2
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Persistence : Superblock is persistent
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Update Time : Tue Feb 7 23:11:53 2012
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State : clean
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Active Devices : 2
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Working Devices : 2
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Failed Devices : 0
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Spare Devices : 0
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Name : core2-blfs:0 (local to host core2-blfs)
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UUID : fcb944a4:9054aeb2:d987d8fe:a89121f8
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Events : 17
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Number Major Minor RaidDevice State
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0 8 1 0 active sync /dev/sda1
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1 8 17 1 active sync /dev/sdb1</literal></screen>
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<para>
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From this point, the partitions can be formatted with the filesystem of
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choice (e.g. ext3, ext4, <xref linkend='xfsprogs'/>, etc). The formatted
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partitions can then be
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mounted. The <filename>/etc/fstab</filename> file can use the devices
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created for mounting at boot time and the linux command line in
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<filename>/boot/grub/grub.cfg</filename> can specify
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<option>root=/dev/md1</option>.
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</para>
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<note>
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<para>
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The swap devices should be specified in the
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<filename>/etc/fstab</filename> file as normal. The kernel normally
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stripes swap data across multiple swap files and should not be made
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part of a RAID array.
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</para>
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</note>
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<para>
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For further options and management details of RAID devices, refer to
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<userinput>man mdadm</userinput>.
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</para>
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<para>
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Additional details for monitoring RAID arrays and dealing with
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problems can be found at the <ulink
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url="https://raid.wiki.kernel.org/index.php/Linux_Raid">Linux RAID
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Wiki</ulink>.
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</para>
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</sect2>
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</sect1>
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