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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta name="generator" content="HTML Tidy, see www.w3.org" /><title>数据的完整性</title><meta name="GENERATOR" content="Modular DocBook HTML Stylesheet Version 1.7" /><link rel="HOME" title="FreeBSD 使用手册" href="index.html" /><link rel="UP" title="Vinum 卷管理程序" href="vinum-vinum.html" /><link rel="PREVIOUS" title="访问瓶颈" href="vinum-access-bottlenecks.html" /><link rel="NEXT" title="Vinum 目标" href="vinum-objects.html" /><link rel="STYLESHEET" type="text/css" href="docbook.css" /><meta http-equiv="Content-Type" content="text/html; charset=GB2312" /></head><body class="SECT1" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#840084"alink="#0000FF"><div class="NAVHEADER"><table summary="Header navigation table" width="100%" border="0" cellpadding="0"cellspacing="0"><tr><th colspan="3" align="center">FreeBSD 使用手册</th></tr><tr><td width="10%" align="left" valign="bottom"><a href="vinum-access-bottlenecks.html"accesskey="P">后退</a></td><td width="80%" align="center" valign="bottom">章 17. Vinum 卷管理程序</td><td width="10%" align="right" valign="bottom"><a href="vinum-objects.html"accesskey="N">前进</a></td></tr></table><hr align="LEFT" width="100%" /></div><div class="SECT1"><h1 class="SECT1"><a id="VINUM-DATA-INTEGRITY" name="VINUM-DATA-INTEGRITY">17.4.数据的完整性</a></h1><p>T当前磁盘的最后一个问题是它们不太可靠。虽然磁盘驱动器的可靠性在过去几年有了很大的提高，它们仍然是会损坏的服务器的最核心组件。当它们发生故障的时候, 结果可能是灾难性的:替换一个坏的磁盘驱动器，然后恢复数据可能要花费几天时间。</p><p>解决这个问题的传统方法是建立镜象 <span class="emphasis"><iclass="EMPHASIS">镜象</i></span>, 在不同的物理硬件上对数据做两个拷贝。 由于<acronymclass="ACRONYM">RAID</acronym> 的出现levels, 所以这个技术也被叫做 <acronymclass="ACRONYM">第一级RAID</acronym> 或者 <acronym class="ACRONYM">RAID-1</acronym>.任何写到卷的数据也会被写到镜象上，所以可以从任何一个拷贝读取数据，如果其中有一个失败了，数据就可以 在其他驱动器上访问到。</p><p>镜象有两个问题：</p><ul><li><p>价格. 它需要两倍的存储容量。</p></li><li><p>性能影响。 写入操作必须在两个驱动器上执行，所以它们花费两倍的带宽。读取数据并不会影响性能：看起来它们会更快。</p></li></ul><p>一个 可选的方案采用 <span class="emphasis"><i class="EMPHASIS">parity</i></span>,其实现了 <acronym class="ACRONYM">RAID</acronym> 2, 3, 4 和 5. 当然, <acronymclass="ACRONYM">RAID-5</acronym> 是最有趣的。 As implemented in Vinum, it is a variant ona striped organization which dedicates one block of each stripe to parity of the otherblocks. As implemented by Vinum, a <acronym class="ACRONYM">RAID-5</acronym> plex issimilar to a striped plex, except that it implements <acronymclass="ACRONYM">RAID-5</acronym> by including a parity block in each stripe. As requiredby <acronym class="ACRONYM">RAID-5</acronym>, the location of this parity block changesfrom one stripe to the next. The numbers in the data blocks indicate the relative blocknumbers.</p><p></p><div class="FIGURE"><a id="VINUM-RAID5-ORG" name="VINUM-RAID5-ORG"></a><p><b>图形 17-3. RAID-5 Organization</b></p><p><img src="vinum/vinum-raid5-org.png" /></p></div><br /><br /><p>Compared to mirroring, <acronym class="ACRONYM">RAID-5</acronym> has the advantage ofrequiring significantly less storage space. Read access is similar to that of stripedorganizations, but write access is significantly slower, approximately 25% of the readperformance. If one drive fails, the array can continue to operate in degraded mode: aread from one of the remaining accessible drives continues normally, but a read from thefailed drive is recalculated from the corresponding block from all the remainingdrives.</p></div><div class="NAVFOOTER"><hr align="LEFT" width="100%" /><table summary="Footer navigation table" width="100%" border="0" cellpadding="0"cellspacing="0"><tr><td width="33%" align="left" valign="top"><a href="vinum-access-bottlenecks.html"accesskey="P">后退</a></td><td width="34%" align="center" valign="top"><a href="index.html"accesskey="H">起点</a></td><td width="33%" align="right" valign="top"><a href="vinum-objects.html"accesskey="N">前进</a></td></tr><tr><td width="33%" align="left" valign="top">访问瓶颈</td><td width="34%" align="center" valign="top"><a href="vinum-vinum.html"accesskey="U">上一级</a></td><td width="33%" align="right" valign="top">Vinum 目标</td></tr></table></div></body></html>⌨️ 快捷键说明
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