91 lines
5.2 KiB
HTML
91 lines
5.2 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="Write cache and auxiliary write cache IOA" />
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<link rel="stylesheet" type="text/css" href="./ibmdita.css" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Write cache and auxiliary write cache IOA </title>
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</head>
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<body id="cache"><a name="cache"><!-- --></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">Write cache and auxiliary write cache IOA </h1>
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<div><div class="section"><h4 class="sectiontitle">Write cache</h4>The write cache provides greater data integrity
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and improved performance. When the iSeries(TM) server sends a write operation,
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the data is written to the cache. Then, a write-completion message is sent
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back to the server. Later, the data is written to the disk. The cache provides
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a faster write capability and ensures data integrity.<p>The following actions
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occur during a write request from the server:</p>
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<ol><li>Data is committed to a nonvolatile battery-backed cache in the IOA.</li>
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<li>A write completion message is sent from the server.</li>
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<li>The following actions occur after the write completion message is sent.<ol type="a"><li>A write operation is sent from the IOA cache to the disk unit:<ul><li>A write operation is sent from the IOA cache to the disk unit:<ul><li>Reads the original data.</li>
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<li>Calculates delta parity by comparing new and original data.</li>
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<li>Writes the new data.</li>
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</ul>
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</li>
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<li>Write operations for parity data:<ul><li>Reads the original parity information.</li>
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<li>Calculates the new parity by comparing the delta parity and the original
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parity.</li>
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<li>Writes the new parity information.</li>
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</ul>
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</li>
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</ul>
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</li>
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<li>Data is marked as committed data when it is successfully written to both
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the data disk unit and the parity disk unit.</li>
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</ol>
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</li>
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</ol>
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<p>The performance for this type of write operation is dependent on disk
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contention and the time that is needed to calculate the parity information.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Auxiliary cache IOA</h4>The auxiliary cache
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IOA mirrors the write cache on a storage IOA. Protection is enhanced because
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two copies of data are stored onto two separate IOAs. If a failure occurs
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to the write cache, then the auxiliary cache IOA serves as a back up during
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the recovery of the failed IOA.<p>When an iSeries(TM) server sends a write
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operation, the data is written to the write cache on the storage IOA. The
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storage IOA mirrors the write cache data to the auxiliary write cache IOA.
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Then, a write-completion message is sent back to the server and the data is
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then written to a disk.</p>
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<div class="note"><span class="notetitle">Note:</span> In order for write cache mirroring to occur,
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the storage IOAs connected to supported auxiliary write cache IOA. The storage
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IOA and the auxiliary write cache IOA must also be in the same enclosure and
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under the same partition.</div>
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<p id="cache__acinfo"><a name="cache__acinfo"><!-- --></a>The auxiliary write cache is
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an additional IOA that has a one-to-one relationship with a disk IOA. The
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auxiliary write cache protects against extended outages due to the failure
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of a disk IOA or its cache by providing a copy of the write cache which can
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be recovered following the repair of the disk IOA. This avoids a potential
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system reload and gets the system back on line as soon as the disk IOA is
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replaced and the recovery procedure completes. Note, however, that this IOA
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is not a failover device that can keep the system operational if the disk
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IOA, or it’s cache, fails.</p>
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<p>For more information about auxiliary write
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cache see the <a href="http://www.redbooks.ibm.com/redpapers/abstracts/redp4003.html" target="_blank">Planning for IBM<sup>®</sup> i5 Data Protection with Auxiliary Write
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Cache Solutions</a> redbook.</p>
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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="rzalydppprf.htm">How device parity protection affects performance</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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