85 lines
5.3 KiB
HTML
85 lines
5.3 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.Type" content="task" />
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<meta name="DC.Title" content="Expanding buffer I/O through HFS" />
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<meta name="abstract" content="Read about tailoring read requests for performance and to expand buffer I/O in HFS." />
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<meta name="description" content="Read about tailoring read requests for performance and to expand buffer I/O in HFS." />
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<meta name="copyright" content="(C) Copyright IBM Corporation 2000, 2006" />
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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>Expanding buffer I/O through HFS</title>
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</head>
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<body id="bufferio"><a name="bufferio"><!-- --></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">Expanding buffer I/O through HFS</h1>
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<div><p>Read about tailoring read requests for performance and to expand
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buffer I/O in HFS.</p>
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<div class="p"><div class="p">An alternative method of opening a stream file through HFS can
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improve performance for applications that typically read portions, but not
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all, of the data in large optical files. This alternate method of input/output
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is referred to as <dfn class="term">expanding buffer I/O</dfn>. Expanding buffer I/O
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is available only to HFS API applications when accessing High Performance
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Optical File System (HPOFS) or ISO 9660 formatted media. This attribute is
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ignored when the media format is UDF.<div class="note"><span class="notetitle">Note:</span> Using the HFS APIs, optical file
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data is buffered into a virtual optical file in i5/OS™ main storage. If expanding buffer
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I/O is not selected as an option, the size of this buffer is equal to the
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size of the actual optical file. For example, a 100 MB file on optical media
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has a 100 MB buffer when the file is opened through the HFS API Open Stream
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File. The performance cost for overhead operations involving the optical buffer
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is proportional to the buffer size. The time it would take to read one byte
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of a 100 MB file is substantially greater than reading one byte of a 50 KB
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file.</div>
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</div>
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<p>When an optical file is opened for expanding buffer I/O,
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the size of the buffer begins at zero and expands as data is read into the
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buffer as requested by the application. The minimum amount of the size expansion
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is 256 KB. The buffer expands only if the requested data is contained within
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a logical 256 KB page that is not yet contained in the buffer. For these reasons,
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the amount of time it would take to read one byte of a 100 MB file opened
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for expanding buffer I/O should be roughly identical to the time to read one
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byte of a 50 KB file opened in the same manner.</p>
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</div>
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<div class="section"><strong>Situations in which expanding buffer I/O is useful</strong><div class="p">Expanding
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buffer I/O should be considered as an option for improving the performance
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of reads if any of the following conditions are met:<ul><li>The typical size of an optical file to be read is greater than 256 KB.</li>
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<li>The amount of data read from the optical file between the open and close
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stream file is a fraction of the total file data. The exact fraction would
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be impossible to specify, but the performance improvements that are achieved
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will be greater the smaller the fraction. For example, an application that
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used expanding buffer I/O to read 25 KB of a 50 MB file would experience much
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greater performance improvements than an application that read 45 MB of the
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same file. An application that reads the entire 50 MB example file 40 KB at
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a time through multiple reads probably would not experience any performance
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improvement using expanding buffer I/O.</li>
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<li>The application will not issue the Set Stream File Size, Lock-Unlock Byte
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Range, or Write Stream file APIs while the file is open for expanding buffer
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I/O. </li>
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</ul>
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</div>
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<p>For specific details on how to implement expanding buffer I/O,
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see <a href="specialattributes.htm">Using special attributes</a>.</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="tipsandtechniques.htm" title="The following techniques are often helpful in designing custom optical programs for your business.">Tips: Optical programming</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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