https://raid.wiki.kernel.org/index.php?title=Devices&feed=atom&action=historyDevices - Revision history2024-03-28T14:49:11ZRevision history for this page on the wikiMediaWiki 1.19.24https://raid.wiki.kernel.org/index.php?title=Devices&diff=5417&oldid=prevAnthony Youngman: /* Devices */2016-09-26T21:49:47Z<p><span dir="auto"><span class="autocomment">Devices</span></span></p>
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<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>build your RAID devices - this gives you a lot of freedom in designing</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>build your RAID devices - this gives you a lot of freedom in designing</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>your RAID solution. For example, you can build a RAID from a mix of</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>your RAID solution. For example, you can build a RAID from a mix of</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div>SATA <del class="diffchange diffchange-inline">and USB devices</del>, and <del class="diffchange diffchange-inline">you can even build a RAID from </del>other RAID</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div>SATA, <ins class="diffchange diffchange-inline">network </ins>and other RAID devices (this is useful for RAID-0+1, where you simply construct two</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div>devices (this is useful for RAID-0+1, where you simply construct two</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>RAID-1 devices from ordinary disks, and finally construct a RAID-0</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>RAID-1 devices from ordinary disks, and finally construct a RAID-0</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div>device from those two RAID-1 devices).</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div>device from those two RAID-1 devices)<ins class="diffchange diffchange-inline">. It is not adviseable to use USB</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins class="diffchange diffchange-inline">devices, however, as these go to sleep and interact badly with the raid code</ins>.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>Therefore, in the following text, we will use the word "device" as</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>Therefore, in the following text, we will use the word "device" as</div></td></tr>
</table>Anthony Youngmanhttps://raid.wiki.kernel.org/index.php?title=Devices&diff=5313&oldid=prevAnthony Youngman: Minor edits to bring the page up to date2016-09-19T21:56:51Z<p>Minor edits to bring the page up to date</p>
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<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>ordinary disks, or from two disk partitions (on separate disks -</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>ordinary disks, or from two disk partitions (on separate disks -</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>please see the description of RAID-1 for details on this).</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>please see the description of RAID-1 for details on this).</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins style="color: red; font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins style="color: red; font-weight: bold; text-decoration: none;">(It is recommended not build a RAID array directly on a disk. It's</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins style="color: red; font-weight: bold; text-decoration: none;">not a problem with RAID, but some disk utilities assume that a drive </ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins style="color: red; font-weight: bold; text-decoration: none;">without a GPT or MBR is blank and will happily stomp all over it.)</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>There are no other special requirements to the devices from which you</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>There are no other special requirements to the devices from which you</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>build your RAID devices - this gives you a lot of freedom in designing</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>build your RAID devices - this gives you a lot of freedom in designing</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>your RAID solution. For example, you can build a RAID from a mix of</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>your RAID solution. For example, you can build a RAID from a mix of</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div><del class="diffchange diffchange-inline">IDE </del>and <del class="diffchange diffchange-inline">SCSI </del>devices, and you can even build a RAID from other RAID</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins class="diffchange diffchange-inline">SATA </ins>and <ins class="diffchange diffchange-inline">USB </ins>devices, and you can even build a RAID from other RAID</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>devices (this is useful for RAID-0+1, where you simply construct two</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>devices (this is useful for RAID-0+1, where you simply construct two</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>RAID-1 devices from ordinary disks, and finally construct a RAID-0</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>RAID-1 devices from ordinary disks, and finally construct a RAID-0</div></td></tr>
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<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>meaning "disk", "partition", or even "RAID device". A "device" in the</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>meaning "disk", "partition", or even "RAID device". A "device" in the</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>following text simply refers to a "Linux block device". It could be</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>following text simply refers to a "Linux block device". It could be</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div>anything from a <del class="diffchange diffchange-inline">SCSI </del>disk to a network block device. We will commonly</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div>anything from a <ins class="diffchange diffchange-inline">SATA </ins>disk to a network block device. We will commonly</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>refer to these "devices" simply as "disks", because that is what they</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>refer to these "devices" simply as "disks", because that is what they</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>will be in the common case.</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>will be in the common case.</div></td></tr>
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<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>"good" disks. This can lead to a complete RAID failure and is the major</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>"good" disks. This can lead to a complete RAID failure and is the major</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>reason for using RAID-6 in preference to RAID-5 and a hot spare.</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>reason for using RAID-6 in preference to RAID-5 and a hot spare.</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins style="color: red; font-weight: bold; text-decoration: none;">Indeed, if using the wrong sort of disk it commonly leads to a complete</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins style="color: red; font-weight: bold; text-decoration: none;">raid failure. (It is usually possible to recover from this situation,</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins style="color: red; font-weight: bold; text-decoration: none;">however.)</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div>If you do frequent backups of the entire filesystem on the RAID array, then it</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div>If you do frequent backups of the entire filesystem on the RAID array<ins class="diffchange diffchange-inline">, </ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins class="diffchange diffchange-inline">or scrub the array regularly</ins>, then it</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>is highly unlikely that you would ever get in this situation - this is</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>is highly unlikely that you would ever get in this situation - this is</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>another very good reason for taking frequent backups. Remember, RAID</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>another very good reason for taking frequent backups. Remember, RAID</div></td></tr>
</table>Anthony Youngmanhttps://raid.wiki.kernel.org/index.php?title=Devices&diff=2412&oldid=prevViridian Development Corporation: add navigation2011-04-04T11:12:11Z<p>add navigation</p>
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<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins style="color: red; font-weight: bold; text-decoration: none;">|- padding:5px;padding-top:0.5em;font-size: 95%; </ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins style="color: red; font-weight: bold; text-decoration: none;">| Back to [[Why RAID?]] <span style="float:right; padding-left:5px;">Forward to [[Hardware issues]]</span></ins></div></td></tr>
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<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>=Devices=</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>=Devices=</div></td></tr>
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<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>The section on [[Detecting, querying and testing]] provides more information.</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>The section on [[Detecting, querying and testing]] provides more information.</div></td></tr>
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<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins style="color: red; font-weight: bold; text-decoration: none;">|- padding:5px;padding-top:0.5em;font-size: 95%; </ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins style="color: red; font-weight: bold; text-decoration: none;">| Back to [[Why RAID?]] <span style="float:right; padding-left:5px;">Forward to [[Hardware issues]]</span></ins></div></td></tr>
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</table>Viridian Development Corporationhttps://raid.wiki.kernel.org/index.php?title=Devices&diff=1420&oldid=prevDavidGreaves: develop spares text and added RAID-62006-10-03T12:49:52Z<p>develop spares text and added RAID-6</p>
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<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>"spare disks" and the "faulty disks".</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>"spare disks" and the "faulty disks".</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div><del class="diffchange diffchange-inline">To avoid confusion it </del>is worth mentioning the existence of [[<del class="diffchange diffchange-inline">MD Faulty</del>]] - this is a special debugging <del class="diffchange diffchange-inline">type </del>of RAID that uses a normal device and simulates faults.</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins class="diffchange diffchange-inline">It </ins>is worth mentioning the existence of <ins class="diffchange diffchange-inline">the</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div>[[<ins class="diffchange diffchange-inline">Overview#FAULTY|FAULTY RAID level</ins>]] <ins class="diffchange diffchange-inline">- don't get confused </ins>- this is a</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div>special debugging <ins class="diffchange diffchange-inline">level </ins>of RAID that uses a normal device and simulates faults.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>==Spare disks==</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>==Spare disks==</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div>Spare disks are disks that do not take part in the RAID set until one</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div>Spare disks <ins class="diffchange diffchange-inline">(often called hot spares) </ins>are disks that do not take part in the RAID set until one</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>of the active disks fail.  When a device failure is detected, that</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>of the active disks fail.  When a device failure is detected, that</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div>device is marked as "<del class="diffchange diffchange-inline">bad</del>" and reconstruction is immediately started on</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div>device is marked as "<ins class="diffchange diffchange-inline">faulty</ins>" and reconstruction is immediately started on</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div>the first spare<del class="diffchange diffchange-inline">-</del>disk available.</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div>the first spare disk available.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>Thus, spare disks add a nice extra safety to especially RAID-5 systems</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>Thus, spare disks add a nice extra safety to especially RAID-5 systems</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>that perhaps are hard to get to (physically). One can allow the system</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>that perhaps are hard to get to (physically). One can allow the system</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div>to run for some time, with a faulty device, since all redundancy is</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div>to run for some time, with a faulty device, since <ins class="diffchange diffchange-inline">the spare disk takes </ins></div></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div><del class="diffchange diffchange-inline">preserved by means of the </del>spare <del class="diffchange diffchange-inline">disk</del>.</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins class="diffchange diffchange-inline">the place of the faulty device and </ins>all redundancy is <ins class="diffchange diffchange-inline">restored.</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div> </div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins class="diffchange diffchange-inline">It is also possible to have </ins>spare <ins class="diffchange diffchange-inline">disks spin-down</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins class="diffchange diffchange-inline">to save energy; obviously the spin-up time for these warm spares is</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins class="diffchange diffchange-inline">insignificant compared to the resync time</ins>.  </div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>You cannot be sure that your system will keep running after a disk</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>You cannot be sure that your system will keep running after a disk</div></td></tr>
<tr><td colspan="2" class="diff-lineno">Line 53:</td>
<td colspan="2" class="diff-lineno">Line 59:</td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>information. If multiple disks have built up bad blocks over time, the</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>information. If multiple disks have built up bad blocks over time, the</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>reconstruction itself can actually trigger a failure on one of the</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>reconstruction itself can actually trigger a failure on one of the</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div>"good" disks. This <del class="diffchange diffchange-inline">will </del>lead to a complete RAID failure. If you do</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div>"good" disks. This <ins class="diffchange diffchange-inline">can </ins>lead to a complete RAID failure <ins class="diffchange diffchange-inline">and is the major</ins></div></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div>frequent backups of the entire filesystem on the RAID array, then it</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins class="diffchange diffchange-inline">reason for using RAID-6 in preference to RAID-5 and a hot spare</ins>.</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div> </div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div>If you do frequent backups of the entire filesystem on the RAID array, then it</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>is highly unlikely that you would ever get in this situation - this is</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>is highly unlikely that you would ever get in this situation - this is</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>another very good reason for taking frequent backups. Remember, RAID</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>another very good reason for taking frequent backups. Remember, RAID</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>is not a substitute for backups.</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>is not a substitute for backups.</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins style="color: red; font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>==Faulty disks==</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>==Faulty disks==</div></td></tr>
<tr><td colspan="2" class="diff-lineno">Line 63:</td>
<td colspan="2" class="diff-lineno">Line 72:</td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>When the RAID layer handles device failures just fine, crashed disks</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>When the RAID layer handles device failures just fine, crashed disks</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>are marked as faulty, and reconstruction is immediately started on the</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>are marked as faulty, and reconstruction is immediately started on the</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div>first spare-disk available.</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div>first spare-disk available<ins class="diffchange diffchange-inline">. If no spare is available then the array runs</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins class="diffchange diffchange-inline">in 'degraded' mode</ins>.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>Faulty disks still appear and behave as members of the array. The RAID</div></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"><div>Faulty disks still appear and behave as members of the array. The RAID</div></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div>layer just <del class="diffchange diffchange-inline">treats crashed devices </del>as <del class="diffchange diffchange-inline">inactive parts of the filesystem</del>.</div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div>layer just <ins class="diffchange diffchange-inline">avoids reading/writing them.</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div> </div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins class="diffchange diffchange-inline">If a device needs to be removed from an array for any reason (eg pro-active</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins class="diffchange diffchange-inline">replacement due to SMART reports) then it must be marked </ins>as <ins class="diffchange diffchange-inline">faulty before it</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins class="diffchange diffchange-inline">can be removed</ins>.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td><td class='diff-marker'> </td><td style="background: #eee; color:black; font-size: smaller;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="background: #ffa; color:black; font-size: smaller;"><div><del class="diffchange diffchange-inline">(Need comments </del>on <del class="diffchange diffchange-inline">the UUU_ syntax </del>and <del class="diffchange diffchange-inline">removing them)</del></div></td><td class='diff-marker'>+</td><td style="background: #cfc; color:black; font-size: smaller;"><div><ins class="diffchange diffchange-inline">The section </ins>on <ins class="diffchange diffchange-inline">[[Detecting, querying </ins>and <ins class="diffchange diffchange-inline">testing]] provides more information.</ins></div></td></tr>
</table>DavidGreaveshttps://raid.wiki.kernel.org/index.php?title=Devices&diff=1400&oldid=prevDavidGreaves at 07:23, 3 October 20062006-10-03T07:23:20Z<p></p>
<p><b>New page</b></p><div>=Devices=<br />
<br />
Software RAID devices are so-called "block" devices, like ordinary<br />
disks or disk partitions. A RAID device is "built" from a number of<br />
other block devices - for example, a RAID-1 could be built from two<br />
ordinary disks, or from two disk partitions (on separate disks -<br />
please see the description of RAID-1 for details on this).<br />
<br />
There are no other special requirements to the devices from which you<br />
build your RAID devices - this gives you a lot of freedom in designing<br />
your RAID solution. For example, you can build a RAID from a mix of<br />
IDE and SCSI devices, and you can even build a RAID from other RAID<br />
devices (this is useful for RAID-0+1, where you simply construct two<br />
RAID-1 devices from ordinary disks, and finally construct a RAID-0<br />
device from those two RAID-1 devices).<br />
<br />
Therefore, in the following text, we will use the word "device" as<br />
meaning "disk", "partition", or even "RAID device". A "device" in the<br />
following text simply refers to a "Linux block device". It could be<br />
anything from a SCSI disk to a network block device. We will commonly<br />
refer to these "devices" simply as "disks", because that is what they<br />
will be in the common case.<br />
<br />
However, there are several roles that devices can play in your arrays.<br />
A device could be a "spare disk", it could have failed and thus be a<br />
"faulty disk", or it could be a normally working and fully functional<br />
device actively used by the array.<br />
<br />
In the following we describe two special types of devices; namely the<br />
"spare disks" and the "faulty disks".<br />
<br />
To avoid confusion it is worth mentioning the existence of [[MD Faulty]] - this is a special debugging type of RAID that uses a normal device and simulates faults.<br />
<br />
==Spare disks==<br />
<br />
Spare disks are disks that do not take part in the RAID set until one<br />
of the active disks fail. When a device failure is detected, that<br />
device is marked as "bad" and reconstruction is immediately started on<br />
the first spare-disk available.<br />
<br />
Thus, spare disks add a nice extra safety to especially RAID-5 systems<br />
that perhaps are hard to get to (physically). One can allow the system<br />
to run for some time, with a faulty device, since all redundancy is<br />
preserved by means of the spare disk.<br />
<br />
You cannot be sure that your system will keep running after a disk<br />
crash though. The RAID layer should handle device failures just fine,<br />
but SCSI drivers could be broken on error handling, or the IDE chipset<br />
could lock up, or a lot of other things could happen.<br />
<br />
Also, once reconstruction to a hot-spare begins, the RAID layer will<br />
start reading from all the other disks to re-create the redundant<br />
information. If multiple disks have built up bad blocks over time, the<br />
reconstruction itself can actually trigger a failure on one of the<br />
"good" disks. This will lead to a complete RAID failure. If you do<br />
frequent backups of the entire filesystem on the RAID array, then it<br />
is highly unlikely that you would ever get in this situation - this is<br />
another very good reason for taking frequent backups. Remember, RAID<br />
is not a substitute for backups.<br />
<br />
==Faulty disks==<br />
<br />
When the RAID layer handles device failures just fine, crashed disks<br />
are marked as faulty, and reconstruction is immediately started on the<br />
first spare-disk available.<br />
<br />
Faulty disks still appear and behave as members of the array. The RAID<br />
layer just treats crashed devices as inactive parts of the filesystem.<br />
<br />
(Need comments on the UUU_ syntax and removing them)</div>DavidGreaves