87 lines
5.2 KiB
HTML
87 lines
5.2 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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<html lang="en-us" xml:lang="en-us">
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<meta name="DC.Title" content="Mirrored protection costs and limitations" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Mirrored protection costs and limitations</title>
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</head>
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<body id="hdrmirrorcost"><a name="hdrmirrorcost"><!-- --></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">Mirrored protection costs and limitations</h1>
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<div><div class="section"><h4 class="sectiontitle">Costs</h4><p>The main cost of using mirrored protection
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is in additional hardware. To achieve high availability and prevent data loss
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when a disk unit fails, you need mirrored protection for all the disk pools.
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This normally requires twice as many disk units. If you want continuous operation
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and prevention of data loss when a disk unit, I/O adapter, or I/O processor
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fails, you need duplicate I/O adapter and I/O processors. A model upgrade
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can be done to get nearly continuous operation and to prevent data loss when
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any of these failures occur, as well as the failure of a bus. If bus 1 fails,
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the system cannot continue to operate. Because bus failures are rare, and
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bus-level protection is not significantly greater than I/O processor-level
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protection, you may not find a model upgrade to be cost-effective for your
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protection needs.</p>
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<p>Mirrored protection has a minimal effect on performance.
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If the buses, I/O processors, and I/O adapter are no more heavily loaded on
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a system with mirrored protection than they are on an equivalent system without
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mirrored protection, then the performance of the two systems should be approximately
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the same.</p>
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<p>In deciding whether to use mirrored protection on your system,
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you must evaluate the cost of potential downtime against the cost of additional
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hardware, over the life of the system. The additional cost in performance
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or system complexity is typically negligible. You should also
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consider other availability and recovery alternatives, such as device parity
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protection. Mirrored protection normally requires twice as many storage units.
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For <a href="rzalycmaintdef.htm">concurrent maintenance</a> and
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higher availability on systems with mirrored protection, other disk-related
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hardware may be required.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Limitations</h4><p>Although mirrored protection can keep
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the system available after disk-related hardware failures occur, it is not
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a replacement for save procedures. There can be multiple types of disk-related
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hardware failures, or disasters (such as flood or sabotage) that require backup
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media.</p>
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<p>Mirrored protection cannot keep your system available if the
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remaining storage unit in the mirrored pair fails before the first failing
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storage unit is repaired and mirrored protection is resumed. If two failed
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storage units are in different mirrored pairs, the system is still available
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and normal mirrored protection recovery is done because the mirrored pairs
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are not dependent on each other for recovery. If a second storage unit of
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the same mirrored pair fails, the failure may not result in a data loss. If
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the failure is limited to the disk electronics, or if the service representative
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can successfully use the save disk unit data function to recover all of the
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data, no data is lost.</p>
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<p>If both storage units in a mirrored pair fail
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causing data loss, the entire disk pool is lost and all units in the disk
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pool are cleared. You must be prepared to restore your disk pool from the
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backup media and <a href="../rzaki/rzakiappyjrn.htm">apply
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any journal changes</a>.</p>
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<p>When starting the mirrored protection operation,
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objects that are created on a preferred unit may be moved to another unit.
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The preferred unit may no longer exist after mirror protection is started.</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="rzalydpconcepts.htm">Disk 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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