152 lines
8.9 KiB
HTML
152 lines
8.9 KiB
HTML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<meta name="DC.Title" content="Device parity protection" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Device parity protection</title>
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</head>
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<body id="hdrdpp"><a name="hdrdpp"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Device parity protection</h1>
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<div><p>Device parity protection is a hardware availability function that protects
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data from being lost because of disk unit failure or because of damage to
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a disk. To protect data, the disk input/output adapter (IOA) calculates and
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saves a parity value for each bit of data. Conceptually, the IOA computes
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the parity value from the data at the same location on each of the other disk
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units in the device parity set. When a disk failure occurs, the data can be
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reconstructed by using the parity value and the values of the bits in the
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same locations on the other disks. The system continues to run while the data
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is being reconstructed. The overall goal of device parity protection is to
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provide high availability and to protect data as inexpensively as possible.</p>
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<div class="section"><h4 class="sectiontitle">Two types of device parity protection</h4><p>RAID 5 and
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RAID 6 are the two types of device parity protection. </p>
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</div>
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<div class="section"><h4 class="sectiontitle">RAID 5</h4><p>If more than one disk fails, you must restore
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the data from the backup media. Logically, the capacity of one disk unit is
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dedicated to storing parity data in a parity set. However, in practice the
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parity data is spread among multiple disk units. Restoring data to a disk
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pool that has disk units with device parity protection may take longer than
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a disk pool that contains only unprotected disk units.</p>
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<div class="p">Systems with
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IOAs released after V5R2 can have a minimum number of 3 disk units in a parity
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set. The maximum number of disk units in a parity set is 18.<div class="note"><span class="notetitle">Note:</span> Systems
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with IOAs released before V5R2 of i5/OS™, the minimum number of disk units
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in a parity set is 4. The maximum number of disk units in a parity set is
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10.</div>
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</div>
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<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" width="100%" frame="border" border="1" rules="all"><thead align="left"><tr><th class="oddrowblue" align="left" valign="bottom" width="50%" id="d0e38">Number of disk
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units in a parity set</th>
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<th class="oddrowblue" align="left" valign="bottom" width="50%" id="d0e40">Number of disk
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units that store parity</th>
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</tr>
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</thead>
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<tbody><tr><td align="center" valign="top" width="50%" headers="d0e38 ">3</td>
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<td align="center" valign="top" width="50%" headers="d0e40 ">2</td>
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</tr>
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<tr><td align="center" valign="top" width="50%" headers="d0e38 ">4-7</td>
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<td align="center" valign="top" width="50%" headers="d0e40 ">4</td>
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</tr>
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<tr><td align="center" valign="top" width="50%" headers="d0e38 ">8-15</td>
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<td align="center" valign="top" width="50%" headers="d0e40 ">8</td>
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</tr>
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<tr><td align="center" valign="top" width="50%" headers="d0e38 ">16-18</td>
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<td align="center" valign="top" width="50%" headers="d0e40 ">16</td>
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</tr>
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</tbody>
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</table>
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</div>
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</div>
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<div class="section"><h4 class="sectiontitle">RAID 6</h4><p id="hdrdpp__raid6p1"><a name="hdrdpp__raid6p1"><!-- --></a>If more than two disk units
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fail, you must restore the data from the backup media. Logically, the capacity
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of two disk units is dedicated to storing parity data in a parity set. However,
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in practice the parity data is spread among multiple disk units.</p>
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<p>The
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minimum number of disk units in a parity set is 4. The maximum number of disk
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units in a parity set is 18. </p>
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<p>When a RAID 6 parity set is started, all
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of the disk units contain parity. Restoring data to a disk pool that has disk
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units with device parity protection may take longer than a disk pool that
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contains only unprotected disk units.</p>
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<div class="note"><span class="notetitle">Note:</span> It is recommended that you
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use more than four disk units in a RAID 6 device parity set, because the capacity
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of two disk units is dedicated to storing parity data in a parity set.</div>
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<p><img src="./delta.gif" alt="Start of change" />Raid 6 requires a new storage adapter that supports this new
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function. The 571B is the first adapter to support Raid 6.<img src="./deltaend.gif" alt="End of change" /></p>
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</div>
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<div class="section"><h4 class="sectiontitle">Device parity protection is not a substitute for a backup
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and recovery strategy</h4><p>Device parity protection is not a substitute
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for a backup and recovery strategy. Device parity protection can prevent your
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system from stopping when certain types of failures occur. It can speed up
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your recovery process for certain types of failures. But device parity protection
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does not protect you from many types of failures, such as a site disaster
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or an operator or programmer error. It does not protect against system outages
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that are caused by failures in other disk-related hardware (such as IOAs,
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disk I/O processors, or a system bus).</p>
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<p>If possible, you should protect
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all the disk units on your system with either device parity protection or <a href="rzalymirror.htm">Work with mirrored protection</a>. This prevents the loss of information when
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disk failure occurs. In many cases, you can also keep your system operational
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while a disk unit is being repaired or replaced.</p>
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<p>For information about
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how to start using device parity protection, see <a href="../books/sc415304.pdf" target="_blank">Backup
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and Recovery</a><img src="wbpdf.gif" alt="Link to PDF" />.</p>
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</div>
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</div>
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<div>
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<ul class="ullinks">
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<li class="ulchildlink"><strong><a href="rzalydeviceparityben.htm">Benefits of device parity protection</a></strong><br />
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</li>
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<li class="ulchildlink"><strong><a href="rzalydeviceparitycost.htm">Costs and limitations of device parity protection</a></strong><br />
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</li>
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<li class="ulchildlink"><strong><a href="rzalydppiop.htm">How device parity protection works</a></strong><br />
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</li>
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<li class="ulchildlink"><strong><a href="rzalymigrating.htm">Migrating to a new input/output adapter</a></strong><br />
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</li>
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<li class="ulchildlink"><strong><a href="rzalyelmdasdpp.htm">Elements of device parity protection</a></strong><br />
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</li>
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<li class="ulchildlink"><strong><a href="rzalydppprf.htm">How device parity protection affects performance</a></strong><br />
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</li>
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</ul>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rzalydpconcepts.htm">Disk protection</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="rzalyexamplegeomirror.htm">Example: Independent disk pools with geographic mirroring</a></div>
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</div>
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<div class="relinfo"><strong>Related information</strong><br />
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<div><a href="rzalygeobenefits.htm">Benefits of geographic mirroring</a></div>
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<div><a href="rzalymirrorconcept.htm">Mirrored protection</a></div>
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<div><a href="rzalypoolthreshold.htm">Set the threshold of a disk pool</a></div>
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</div>
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</div>
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</body>
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</html>
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