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<!-- #include VIRTUAL="/includes/global_format.inc.asp" --> <html> <head> <link rel="stylesheet" type="text/css" href="dell.css"> <meta HTTP-EQUIV="Content-Type" content="text/html; charset=iso-8859-1"> <meta name="GENERATOR" content="Microsoft FrontPage 4.0"> <meta name="ProgId" content="FrontPage.Editor.Document"> <title>Read, Write, and Cache Policies</title> </head> <body bgcolor="#FFFFFF"> <!-- #include VIRTUAL="/includes/global_header.inc.asp" --> <!-- @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ --> <h3><a name="top">Read, Write, and Cache Policies</a></h3> <p>This module defines the different types of read, write, and cache that can be configured for the storage devices that the Server Administrator storage management service can view.</p> <h4><a name="read_policies">Read Policies</a></h4> <p>Read policies indicate whether or not the controller should read sequential sectors of the logical drive when seeking data. The read policies are as follows:</p> <p><b>Read-Ahead</b>. When using read-ahead policy, the controller reads sequential sectors of the logical drive when seeking data. Read-ahead policy may improve system performance if the data is actually written to sequential sectors of the logical drive.</p> <p><b>No-Read-Ahead</b>. Selecting no-read-ahead policy indicates that the controller should not use read-ahead policy.</p> <p><b>Adaptive Read-Ahead</b>. When using adaptive read-ahead policy, the controller initiates read-ahead only if the two most recent read requests accessed sequential sectors of the disk. If subsequent read requests access random sectors of the disk, the controller reverts to no-read-ahead policy. The controller continues to evaluate whether read requests are accessing sequential sectors of the disk, and can initiate read-ahead if necessary.</p> <p>[<a href="#top">Back to Top</a>]</p> <h4><a name="write_policies">Write Policies</a></h4> <p>Write policies specify whether the controller sends a write-request completion signal as soon as the data is in the cache or after it has been written to disk. The write policies are as follows:</p> <p><b>Write-Back Caching</b>. When using write-back caching, the controller sends a write-request completion signal as soon as the data is in the controller cache but has not yet been written to disk. Write-back caching may provide improved performance since subsequent read requests can more quickly retrieve data from the controller cache than they could from the disk. Write-back caching also entails a data security risk, however, since a system failure could prevent the data from being written to disk even though the controller has sent a write-request completion signal. In this case, data may be lost. Other applications may also experience problems when taking actions that assume the data is available on the disk.</p> <p>[<a href="#top">Back to Top</a>]</p> <p><b>Write-Through Caching</b>. When using write-through caching, the controller sends a write-request completion signal only after the data is written to the disk. Write-through caching provides better data security than write-back caching, since the system assumes the data is available only after it has been safely written to the disk.</p> <h4><a name="cache_policies">Cache Policies</a></h4> <p>The Direct I/O and Cache I/O cache policies apply to reads on a specific logical drive. These settings do not affect the read-ahead policy. The Direct I/O and Cache I/O cache policies are: </p> <p><b>Cache I/O.</b> Specifies that all reads are buffered in cache memory.</p> <p><b>Direct I/O.</b> Specifies that reads are not buffered in cache memory. When using direct I/O, data is transferred to the controller cache and the host system simultaneously during a read request. If a subsequent read request requires data from the same data block, it can be read directly from the controller cache. The direct I/O setting does not override the cache policy settings. Direct I/O is also the default setting. </p> <p>[<a href="#top">Back to Top</a>]</p> <p> </p> <!-- @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ --> <!-- #include VIRTUAL="/includes/global_footer.inc.asp" --> </body> </html>