89 lines
4.3 KiB
HTML
89 lines
4.3 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
|
|
<HTML>
|
|
<HEAD>
|
|
<META NAME="GENERATOR" CONTENT="LinuxDoc-Tools 0.9.21">
|
|
<TITLE>The Software-RAID HOWTO: Devices</TITLE>
|
|
<LINK HREF="Software-RAID-HOWTO-4.html" REL=next>
|
|
<LINK HREF="Software-RAID-HOWTO-2.html" REL=previous>
|
|
<LINK HREF="Software-RAID-HOWTO.html#toc3" REL=contents>
|
|
</HEAD>
|
|
<BODY>
|
|
<A HREF="Software-RAID-HOWTO-4.html">Next</A>
|
|
<A HREF="Software-RAID-HOWTO-2.html">Previous</A>
|
|
<A HREF="Software-RAID-HOWTO.html#toc3">Contents</A>
|
|
<HR>
|
|
<H2><A NAME="s3">3.</A> <A HREF="Software-RAID-HOWTO.html#toc3">Devices</A></H2>
|
|
|
|
<P><B>This HOWTO is deprecated; the Linux RAID HOWTO is maintained as a wiki by the
|
|
linux-raid community at
|
|
<A HREF="http://raid.wiki.kernel.org/">http://raid.wiki.kernel.org/</A></B></P>
|
|
<P>Software RAID devices are so-called "block" devices, like ordinary
|
|
disks or disk partitions. A RAID device is "built" from a number of
|
|
other block devices - for example, a RAID-1 could be built from two
|
|
ordinary disks, or from two disk partitions (on separate disks -
|
|
please see the description of RAID-1 for details on this).</P>
|
|
<P>There are no other special requirements to the devices from which you
|
|
build your RAID devices - this gives you a lot of freedom in designing
|
|
your RAID solution. For example, you can build a RAID from a mix of
|
|
IDE and SCSI devices, and you can even build a RAID from other RAID
|
|
devices (this is useful for RAID-0+1, where you simply construct two
|
|
RAID-1 devices from ordinary disks, and finally construct a RAID-0
|
|
device from those two RAID-1 devices).</P>
|
|
<P>Therefore, in the following text, we will use the word "device" as
|
|
meaning "disk", "partition", or even "RAID device". A "device" in the
|
|
following text simply refers to a "Linux block device". It could be
|
|
anything from a SCSI disk to a network block device. We will commonly
|
|
refer to these "devices" simply as "disks", because that is what they
|
|
will be in the common case.</P>
|
|
<P>However, there are several roles that devices can play in your
|
|
arrays. A device could be a "spare disk", it could have failed and
|
|
thus be a "faulty disk", or it could be a normally working and fully
|
|
functional device actively used by the array.</P>
|
|
<P>In the following we describe two special types of devices; namely the
|
|
"spare disks" and the "faulty disks".</P>
|
|
|
|
|
|
<H2><A NAME="ss3.1">3.1</A> <A HREF="Software-RAID-HOWTO.html#toc3.1">Spare disks</A>
|
|
</H2>
|
|
|
|
<P>Spare disks are disks that do not take part in the RAID set until one
|
|
of the active disks fail. When a device failure is detected, that
|
|
device is marked as "bad" and reconstruction is immediately started
|
|
on the first spare-disk available.</P>
|
|
<P>Thus, spare disks add a nice extra safety to especially RAID-5 systems
|
|
that perhaps are hard to get to (physically). One can allow the system
|
|
to run for some time, with a faulty device, since all redundancy is
|
|
preserved by means of the spare disk.</P>
|
|
<P>You cannot be sure that your system will keep running after a disk
|
|
crash though. The RAID layer should handle device failures just fine,
|
|
but SCSI drivers could be broken on error handling, or the IDE chipset
|
|
could lock up, or a lot of other things could happen.</P>
|
|
<P>Also, once reconstruction to a hot-spare begins, the RAID layer will
|
|
start reading from all the other disks to re-create the redundant
|
|
information. If multiple disks have built up bad blocks over time, the
|
|
reconstruction itself can actually trigger a failure on one of the
|
|
"good" disks. This will lead to a complete RAID failure. If you do
|
|
frequent backups of the entire filesystem on the RAID array, then it
|
|
is highly unlikely that you would ever get in this situation - this is
|
|
another very good reason for taking frequent backups. Remember, RAID
|
|
is not a substitute for backups.</P>
|
|
|
|
|
|
<H2><A NAME="ss3.2">3.2</A> <A HREF="Software-RAID-HOWTO.html#toc3.2">Faulty disks</A>
|
|
</H2>
|
|
|
|
<P>When the RAID layer handles device failures just fine, crashed disks
|
|
are marked as faulty, and reconstruction is immediately started
|
|
on the first spare-disk available.</P>
|
|
<P>Faulty disks still appear and behave as members of the array. The RAID
|
|
layer just treats crashed devices as inactive parts of the filesystem.</P>
|
|
|
|
|
|
|
|
<HR>
|
|
<A HREF="Software-RAID-HOWTO-4.html">Next</A>
|
|
<A HREF="Software-RAID-HOWTO-2.html">Previous</A>
|
|
<A HREF="Software-RAID-HOWTO.html#toc3">Contents</A>
|
|
</BODY>
|
|
</HTML>
|