115 lines
7.4 KiB
HTML
115 lines
7.4 KiB
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="How device parity protection works" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>How device parity protection works</title>
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</head>
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<body id="hdrdppiop"><a name="hdrdppiop"><!-- --></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">How device parity protection works</h1>
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<div><div class="section"><h4 class="sectiontitle">RAID 5</h4><p>The input/output adapter determines how parity
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sets are formed. For V5R2 and later input/output adapters, you do have the
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ability to choose how you want the parity set to be <a href="rzalychgdpy.htm">optimized</a>. You can optimize according to <em>availability</em>, <em>capacity</em>, <em>performance</em>,
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or a <em>balanced</em> version. A parity set optimized for availability offers
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a greater level of protection, because it allows a parity set to remain functional
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in the event of a single SCSI bus failure on the IOA. The parity set is formed
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from at least three disk units of equal capacity each attached to a separate
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SCSI bus on the input/output adapter (IOA). If you optimize by capacity, the
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IOA tends to create parity sets with a greater number of disk units. You will
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increase space used for storing user data, but performance may not be as high.
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If you optimize for performance, the IOA tends to create a parity set with
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fewer disk units. This should contribute to faster read and write operations,
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but might also dedicate slightly more disk capacity to storing parity data.</p>
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<p>It is possible to include additional disk units of the same capacity
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in a device parity set after device parity protection is initially started.
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You can include up to two disk units at the same time; however, if three or
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more disk units are present and eligible for device parity protection, the
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system requires that you start a new parity set, rather than include them
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in an existing parity set. In iSeries™ Navigator you can view the properties
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of each disk unit. If the protection status of a disk unit is <em>unprotected</em>,
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it is not protected by device parity protection or mirroring and may be eligible
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to be included in a parity set or to be started in a new parity set. This
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will also be indicated by the model number which should be 050 (or 060 if
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it is a compressed disk unit). You can also exclude disks that do not store
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parity data from a parity set without stopping device parity protection. You
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can exclude a <em>protected</em> unit with a model number, for example 070 (or
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080 if it is a compressed disk unit), because it is a disk unit that does
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not store parity data.</p>
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<p>When a device parity set grows you may want to
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consider redistributing the parity data. For example you may begin with seven
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or fewer disk units, but expand to eight or more by including more disk units.
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When this happens, you can improve the performance on the device parity set
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by stopping parity protection and starting it again. This redistributes the
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parity data across eight disks rather than four. In general, spreading the
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parity data across more disk units improves performance.</p>
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<p>A write cache
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is included in the input/output adapter (IOA) for each parity set to improve
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performance of interactive write workloads.</p>
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<div class="note"><span class="notetitle">Note:</span> If possible, start device
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parity protection before adding disk units to a disk pool. This significantly
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reduces the time it takes to start device parity and configure the disk units.</div>
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</div>
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<div class="section"><h4 class="sectiontitle">RAID 6</h4><p>The input/output adapter determines
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how parity sets are formed. RAID 6 protection gives you optimal performance,
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capacity, and balance, so selecting any of these parity set optimizations
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is meaningless and will not affect the outcome of the parity set. If you choose
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to optimize by availability, a greater level of protection is achieved, because
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it allows a parity set to remain functional in the event of a single SCSI
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bus failure on the IOA. The parity set is formed from at least four disk units
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of equal capacity, with no more than two disk units attached to an individual
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SCSI bus on the input/output adapter (IOA). Device parity protection reduces
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the potential number of objects that could be damaged when a disk fails.</p>
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<p>It
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is possible to include additional disk units of the same capacity in a device
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parity set after device parity protection is initially started. You can include
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up to two disk units at the same time; however, if three or more disk units
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are present and eligible for device parity protection, the system requires
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that you start a new parity set, rather than include them in an existing parity
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set. In iSeries Navigator
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you can view the properties of each disk unit. If the protection status of
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a disk unit is <em>unprotected</em>, it is not protected by device parity protection
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or mirroring and may be eligible to be included in a parity set or to be started
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in a new parity set. This will also be indicated by the model number which
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should be 050. You can also exclude disks that do not store parity data from
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a parity set without stopping device parity protection. You can exclude a <em>protected</em> unit
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with a model number of 090 because it is a disk unit that does not store parity
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data.</p>
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<p>When a device parity set grows, you may want to consider redistributing
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the parity data. For example you may begin with seven or fewer disk units,
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but expand to ten or more by including more disk units. When this happens,
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you can improve the performance on the device parity set by stopping parity
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protection and starting it again.</p>
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<p>A write cache is included in the input/output
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adapter (IOA) for each parity set to improve performance of interactive write
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workloads.</p>
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<div class="note"><span class="notetitle">Note:</span> If possible, start device parity protection before adding
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disk units to a disk pool. This significantly reduces the time it takes to
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start device parity protection and configure the disk units.</div>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rzalydpp.htm">Device parity protection</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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