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HTML
134 lines
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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="Volume, directory, and file considerations" />
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<meta name="abstract" content="Several special considerations will make your optical programming easier." />
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<title>Volume, directory, and file considerations</title>
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</head>
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<body id="considerations"><a name="considerations"><!-- --></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">Volume, directory, and file considerations</h1>
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<div><p>Several special considerations will make your optical programming
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easier.</p>
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<div class="section"><h4 class="sectiontitle">Volume considerations</h4><div class="p">Consider the following terms
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when referring to volumes:<ul><li><strong>Online</strong>. The volume is mounted in a drive under the read/write heads.</li>
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<li><strong>Near online</strong>. The volume is in the optical media library, but not
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online. The volume may be in a storage slot or the opposite side of an online
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volume.</li>
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<li><strong>Removed</strong>. The volume is not physically in an optical media library,
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but volume information for the volume was kept when the volume was removed.</li>
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<li><strong>Offline</strong>. The volume is in an optical device, but the device is
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either powered off, varied off, or no longer connected.</li>
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</ul>
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</div>
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<div class="p">Consider the following characteristics of optical volumes: <ul><li>An optical volume is one side of an optical cartridge.</li>
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<li>Some optical cartridges contain two volumes, others contain one.</li>
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<li>All volume names must be unique.</li>
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<li>Depending on the optical media density and type, the capacity of a volume
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can range from a few hundred megabytes to many gigabytes.</li>
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<li>Normally, a near online volume takes less than 10 seconds to become an
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online volume. This requires the volume to be mounted into a drive.</li>
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<li>The number of drives in the optical media library determines how many
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volumes can be online at any time. Only one volume can be mounted in a drive
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(online) at one time. The remaining volumes in the library are near online. </li>
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<li>Volumes are generally independent of each other, with one exception. The
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two volumes on the same cartridge can never be completely independent. Both
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volumes on a cartridge can never be online at the same time. Copying between
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two volumes on the same cartridge can be done, but it requires the cartridge
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to be <span class="q">"flipped"</span> several times to copy all of the requested files.</li>
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<li>There is no limit to the number of removed volumes that can exist.</li>
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</ul>
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</div>
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<p>How an application manages volumes depends almost entirely on
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the requirements of the application. Data should be written to volumes strategically,
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depending on the desired retrieval time in the future. If it is not desirable
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to wait for a near online volume to become online, the application might need
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to be set up so that the most likely volumes to be accessed are online.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Directory considerations</h4><p>The only limit to the number
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of directories that can be created on a volume is the capacity of the media.
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This restriction also applies to the number of files that can exist in an
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optical directory. Directories are not required to exist for files to be stored
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on a volume. If you want, all files can be stored in the root directory of
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a volume. The root directory is the ″/″ directory that is created when a volume
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is initialized. This root is not considered a directory in the traditional
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sense since it cannot be created or deleted like other directories. The root
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directory will always exist on initialized optical volumes.</p>
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<div class="p">Directories
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can be used to categorize optical files into more manageable subsets. Directories
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can contain files from a particular time period, subject, characteristic,
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or any combination of these. For example, there may be a directory <samp class="codeph">SPOOLFILES</samp> with
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subdirectories <samp class="codeph">YEAR_1994</samp> and <samp class="codeph">YEAR_1995</samp>.
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Taking this one step further, there could be subdirectories within these subdirectories
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named <samp class="codeph">MONTH_MARCH</samp> and <samp class="codeph">MONTH_APRIL</samp>. See the
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following for an example of this structure:<pre>/SPOOLFILES /YEAR_1994 /MONTH_MARCH
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86 Optical Support V5R3
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/MONTH_APRIL /YEAR_1995 /MONTH_MARCH /MONTH_APRIL</pre>
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In this example,
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the following are the fully qualified directory names:<pre>/SPOOLFILES
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/SPOOLFILES/YEAR_1994
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/SPOOLFILES/YEAR_1994/MONTH_MARCH
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/SPOOLFILES/YEAR_1994/MONTH_APRIL
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/SPOOLFILES/YEAR_1995
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/SPOOLFILES/YEAR_1995/MONTH_MARCH
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/SPOOLFILES/YEAR_1995/MONTH_APRIL</pre>
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</div>
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<p>Directories can be useful
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when categorizing files, but they are not necessary. Like volume names, directory
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names must be unique within the same volume. For example, volume VOL001 cannot
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have two directories named DIR001. Volume VOL001 can, however, have a DIR001
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directory and a DIR000/DIR001 directory. Also, a DIR001 directory can exist
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on volume VOL001 and volume VOL002. For information about directory
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naming conventions, see <a href="rzau8pathnames.htm">Path names</a>.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">File considerations</h4><p>The size of optical files depends
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almost entirely on the requirements of the application and the users of those
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files. The size of an optical file (accessible through HFS or the integrated
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file system) can range from 0 bytes to 4 294 705 152 bytes
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depending on the capacity of a volume. The physical size of the target piece
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of media is limited by the amount of free space available.</p>
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<div class="p">When selecting
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optimal file sizes for your application, pay special attention to the following
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considerations:<ul><li>The amount of system disk unit or main storage on the iSeries™ server</li>
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<li>How the data will be read (sequentially or randomly)</li>
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<li>Whether the entire file will typically be retrieved, or just a small portion</li>
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<li>Whether files will be updated once they are written to the volume</li>
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</ul>
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</div>
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<p>Generally, the larger the file, the better the performance and
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media use. When larger files are used, less media space is taken up by file
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directory information and more is used for actual data. Also, the performance
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related to file size is not a linear comparison. It does not take twice as
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long to write 20 KB of data as it does to write 10 KB of data. Performance
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(KB/second) improves as the amount of data being read or written increases.</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="rzau8concept.htm" title="Read this topic collection for basic concepts of optical device programming. Basic concepts that are described for optical device programming include the i5/OS optical storage system, integrated file system, hierarchical file system, and considerations for volumes.">Optical device programming concepts</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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