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142 lines
9.0 KiB
XML
142 lines
9.0 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<refentry id="afs_cache5">
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<refmeta>
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<refentrytitle>afs_cache</refentrytitle>
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<manvolnum>5</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>afs_cache</refname>
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<refpurpose>Format of data stored in an AFS client disk cache</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Description</title>
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<para>The disk cache on a client machine is composed of multiple <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> files
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that contain the data, a <replaceable>CacheItems</replaceable> file that records index information
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for all of the <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> files, and a <replaceable>VolumeItems</replaceable> file that records the
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mapping between volume name and mount point for volumes.</para>
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<para>When it initializes, the Cache Manager creates the cache files in the
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configured cache location. The standard directory name is
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<replaceable>/usr/vice/cache</replaceable>, but it is acceptable to use a directory on a partition
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with more available space. To designate a different directory, change the
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value in the second field of the <replaceable>/usr/vice/etc/cacheinfo</replaceable> file before
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issuing the <emphasis role="bold">afsd</emphasis> command, or include the <emphasis role="bold">-cachedir</emphasis> argument to the
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<emphasis role="bold">afsd</emphasis> command.</para>
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<refsect2>
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<title><replaceable>CacheItems</replaceable></title>
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<para>The CacheItems file records information about each file in the disk cache
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on a client machine (each <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> file). The information includes the
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file ID number and associated volume version number of the AFS file
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currently stored in the <emphasis role="bold">V</emphasis><emphasis>n</emphasis> file, which enables the Cache Manager to
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determine which <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> file contains the AFS data it needs to present to
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an application.</para>
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<para>As it initializes, the Cache Manager creates the binary-format
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<replaceable>CacheItems</replaceable> file in the same local disk cache directory as the <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable>
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files that the <replaceable>CacheItems</replaceable> file describes, and it must always remain
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there.</para>
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</refsect2>
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<refsect2>
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<title><replaceable>VolumeItems</replaceable></title>
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<para>The <replaceable>VolumeItems</replaceable> file records the mapping between volume name and mount
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point for each volume that the Cache Manager has accessed since it
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initialized on a client machine using a disk cache. The Cache Manager uses
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the mappings to respond correctly to queries about the current working
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directory, which can come from the operating system or commands such as
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the UNIX <emphasis role="bold">pwd</emphasis> command.</para>
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<para>As it initializes, the Cache Manager creates the binary-format
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<replaceable>VolumeItems</replaceable> file in the local disk cache directory, and it must always
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remain there.</para>
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</refsect2>
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<refsect2>
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<title><replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable></title>
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<para>A <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> file can store a chunk of cached AFS data on a client machine
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that is using a disk cache. As the Cache Manager initializes, it verifies
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that the local disk cache directory houses a number of <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> files
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equal to the largest of the following:</para>
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<itemizedlist>
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<listitem>
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<para>100</para>
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</listitem>
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<listitem>
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<para>One and a half times the result of dividing the cache size by the chunk
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size (cachesize/chunksize * 1.5).</para>
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</listitem>
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<listitem>
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<para>The result of dividing the cache size by 10 MB (10,240).</para>
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</listitem>
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</itemizedlist>
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<para>The Cache Manager determines the cache size from the <emphasis role="bold">-blocks</emphasis> argument
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to the <emphasis role="bold">afsd</emphasis> command, or if the argument is not included, from the third
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field of the <replaceable>/usr/vice/etc/cacheinfo</replaceable> file. The default chunk size is
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64 KB; use the <emphasis role="bold">-chunksize</emphasis> argument to the <emphasis role="bold">afsd</emphasis> command to override
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it. To override the default number of chunks resulting from the
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calculation, include the <emphasis role="bold">-files</emphasis> argument to the <emphasis role="bold">afsd</emphasis>
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command. <link linkend="afsd8">afsd(8)</link> describes the restrictions on acceptable values for
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each of the arguments.</para>
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<para>If the disk cache directory houses fewer <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> files than necessary,
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the Cache Manager creates new ones, assigning each a unique integer <emphasis>n</emphasis>
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that distinguishes it from the other files; the integers start with 1 and
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increment by one for each <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> file created. The Cache Manager removes
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files if there are more than necessary. The Cache Manager also adds and
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removes <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> files in response to the <emphasis role="bold">fs setcachesize</emphasis> command,
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which can be used to alter the cache size between reboots.</para>
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<para><replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> files expand and contract to accommodate the size of the AFS
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directory listing or file they temporarily house. As mentioned, by default
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each <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> file holds up to 64 KB (65,536 bytes) of a cached AFS
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element. AFS elements larger than 64 KB are divided among multiple
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<emphasis role="bold">V</emphasis><emphasis>n</emphasis> files. If an element is smaller than 64 KB, the <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> file
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expands only to the required size. A <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> file accommodates only a
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single element, so if there many small cached elements, it is possible to
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exhaust the available <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> files without reaching the maximum cache
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size.</para>
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</refsect2>
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</refsect1>
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<refsect1>
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<title>Cautions</title>
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<para>Editing or removing the <replaceable>CacheItems</replaceable> or <replaceable>VolumeItems</replaceable> files or a
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<replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> file can cause a kernel panic. If the contents of <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> files
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seem out of date, clear the files by using the <emphasis role="bold">fs flush</emphasis> or <emphasis role="bold">fs
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flushvolume</emphasis> command. If any of the cache files are accidentally modified
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or deleted, rebooting the machine usually restores normal performance.</para>
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<para>To alter cache size (and thus the number of <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> files) between
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reboots, use the <emphasis role="bold">fs setcachesize</emphasis> command. Alternatively, alter the
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value of the <emphasis role="bold">-blocks</emphasis>, <emphasis role="bold">-files</emphasis> or <emphasis role="bold">-chunksize</emphasis> arguments to the
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<emphasis role="bold">afsd</emphasis> command invoked in the machine's AFS initialization file, and
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reboot. To refresh the contents of one or more <replaceable>V</replaceable><emphasis>n</emphasis><replaceable></replaceable> files, use the
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<emphasis role="bold">fs flush</emphasis> or <emphasis role="bold">fs flushvolume</emphasis> command.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para><link linkend="cacheinfo5">cacheinfo(5)</link>,
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<link linkend="afsd8">afsd(8)</link>,
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<link linkend="fs_checkvolumes1">fs_checkvolumes(1)</link>,
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<link linkend="fs_flush1">fs_flush(1)</link>,
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<link linkend="fs_flushvolume1">fs_flushvolume(1)</link>,
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<link linkend="fs_setcachesize1">fs_setcachesize(1)</link></para>
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</refsect1>
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<refsect1>
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<title>Copyright</title>
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<para>IBM Corporation 2000. <http://www.ibm.com/> All Rights Reserved.</para>
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<para>This documentation is covered by the IBM Public License Version 1.0. It was
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converted from HTML to POD by software written by Chas Williams and Russ
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Allbery, based on work by Alf Wachsmann and Elizabeth Cassell.</para>
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</refsect1>
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</refentry>
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