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    usually involve physical processes, and network access is limited by
    your available bandwidth. Memory access on the other hand can take mere
    nano-seconds.</p>

    <p>System memory isn't cheap though, byte for byte it's by far the most
    expensive type of storage and it's important to ensure that it is used
    efficiently. By caching files in memory you decrease the amount of 
    memory available on the system. As we'll see, in the case of operating
    system caching, this is not so much of an issue, but when using
    Apache's own in-memory caching it is important to make sure that you
    do not allocate too much memory to a cache. Otherwise the system
    will be forced to swap out memory, which will likely degrade 
    performance.</p>

    <h3>Operating System Caching</h3>
      

      <p>Almost all modern operating systems cache file-data in memory managed
      directly by the kernel. This is a powerful feature, and for the most
      part operating systems get it right. For example, on Linux, let's look at
      the difference in the time it takes to read a file for the first time
      and the second time;</p>

      <div class="example"><pre>
colm@coroebus:~$ time cat testfile &gt; /dev/null
real    0m0.065s
user    0m0.000s
sys     0m0.001s
colm@coroebus:~$ time cat testfile &gt; /dev/null
real    0m0.003s
user    0m0.003s
sys     0m0.000s</pre></div>

      <p>Even for this small file, there is a huge difference in the amount
      of time it takes to read the file. This is because the kernel has cached
      the file contents in memory.</p>

      <p>By ensuring there is "spare" memory on your system, you can ensure 
      that more and more file-contents will be stored in this cache. This 
      can be a very efficient means of in-memory caching, and involves no 
      extra configuration of Apache at all.</p>

      <p>Additionally, because the operating system knows when files are 
      deleted or modified, it can automatically remove file contents from the 
      cache when neccessary. This is a big advantage over Apache's in-memory 
      caching which has no way of knowing when a file has changed.</p>
    

    <p>Despite the performance and advantages of automatic operating system
    caching there are some circumstances in which in-memory caching may be 
    better performed by Apache.</p>

    <p>Firstly, an operating system can only cache files it knows about. If 
    you are running Apache as a proxy server, the files you are caching are
    not locally stored but remotely served. If you still want the unbeatable
    speed of in-memory caching, Apache's own memory caching is needed.</p>

    <h3>MMapStatic Caching</h3>
      

      <p><code class="module"><a href="./mod/mod_file_cache.html">mod_file_cache</a></code> provides the 
      <code class="directive"><a href="./mod/mod_file_cache.html#mmapstatic">MMapStatic</a></code> directive, which
      allows you to have Apache map a static file's contents into memory at
      start time (using the mmap system call). Apache will use the in-memory 
      contents for all subsequent accesses to this file.</p>

      <div class="example"><pre>MMapStatic /usr/local/apache2/htdocs/index.html</pre></div>

      <p>As with the
      <code class="directive"><a href="./mod/mod_file_cache.html#cachefile">CacheFile</a></code> directive, any
      changes in these files will not be picked up by Apache after it has
      started.</p>

      <p> The <code class="directive"><a href="./mod/mod_file_cache.html#mmapstatic">MMapStatic</a></code> 
      directive does not keep track of how much memory it allocates, so
      you must ensure not to over-use the directive. Each Apache child
      process will replicate this memory, so it is critically important
      to ensure that the files mapped are not so large as to cause the
      system to swap memory.</p>
    

    <h3>mod_mem_cache Caching</h3>
      

      <p><code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_cache</a></code> provides a HTTP-aware intelligent
      in-memory cache. It also uses heap memory directly, which means that
      even if <var>MMap</var> is not supported on your system, 
      <code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_cache</a></code> may still be able to perform caching.</p>

      <p>Caching of this type is enabled via;</p>

      <div class="example"><pre>
# Enable memory caching
CacheEnable mem /

# Limit the size of the cache to 1 Megabyte
MCacheSize 1024</pre></div>
    
  </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
<div class="section">
<h2><a name="disk" id="disk">Disk-based Caching</a></h2>
    

     <table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_cache.html#cacheenable">CacheEnable</a></code></li><li><code class="directive"><a href="./mod/mod_cache.html#cachedisable">CacheDisable</a></code></li></ul></td></tr></table>
       
    <p><code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> provides a disk-based caching mechanism 
    for <code class="module"><a href="./mod/mod_cache.html">mod_cache</a></code>. As with <code class="module"><a href="./mod/mod_mem_cache.html">mod_mem_cache</a></code>
    this cache is intelligent and content will be served from the cache only
    as long as it is considered valid.</p>

    <p>Typically the module will be configured as so;</p>

    <div class="example"><pre>
CacheRoot   /var/cache/apache/
CacheEnable disk /
CacheDirLevels 2
CacheDirLength 1</pre></div>

    <p>Importantly, as the cached files are locally stored, operating system
    in-memory caching will typically be applied to their access also. So 
    although the files are stored on disk, if they are frequently accessed 
    it is likely the operating system will ensure that they are actually
    served from memory.</p>

    <h3>Understanding the Cache-Store</h3>
      

      <p>To store items in the cache, <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> creates
      a 22 character hash of the URL being requested. This hash incorporates
      the hostname, protocol, port, path and any CGI arguments to the URL,
      to ensure that multiple URLs do not collide.</p>

      <p>Each character may be any one of 64-different characters, which mean
      that overall there are 64^22 possible hashes. For example, a URL might
      be hashed to <code>xyTGxSMO2b68mBCykqkp1w</code>. This hash is used
      as a prefix for the naming of the files specific to that URL within
      the cache, however first it is split up into directories as per
      the <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlevels">CacheDirLevels</a></code> and
      <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code> 
      directives.</p>

      <p><code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlevels">CacheDirLevels</a></code> 
      specifies how many levels of subdirectory there should be, and
      <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code>
      specifies how many characters should be in each directory. With
      the example settings given above, the hash would be turned into
      a filename prefix as 
      <code>/var/cache/apache/x/y/TGxSMO2b68mBCykqkp1w</code>.</p>

      <p>The overall aim of this technique is to reduce the number of
      subdirectories or files that may be in a particular directory,
      as most file-systems slow down as this number increases. With
      setting of "1" for 
      <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code>
      there can at most be 64 subdirectories at any particular level. 
      With a setting of 2 there can be 64 * 64 subdirectories, and so on.
      Unless you have a good reason not to, using a setting of "1"
      for <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlength">CacheDirLength</a></code>
      is recommended.</p>

      <p>Setting 
      <code class="directive"><a href="./mod/mod_disk_cache.html#cachedirlevels">CacheDirLevels</a></code>
      depends on how many files you anticipate to store in the cache.
      With the setting of "2" used in the above example, a grand
      total of 4096 subdirectories can ultimately be created. With
      1 million files cached, this works out at roughly 245 cached 
      URLs per directory.</p>

      <p>Each URL uses at least two files in the cache-store. Typically
      there is a ".header" file, which includes meta-information about 
      the URL, such as when it is due to expire and a ".data" file
      which is a verbatim copy of the content to be served.</p>

      <p>In the case of a content negotiated via the "Vary" header, a
      ".vary" directory will be created for the URL in question. This 
      directory will have multiple ".data" files corresponding to the
      differently negotiated content.</p>
    

    <h3>Maintaining the Disk Cache</h3>
      
    
      <p>Although <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> will remove cached content
      as it is expired, it does not maintain any information on the total
      size of the cache or how little free space may be left.</p>

      <p>Instead, provided with Apache is the <a href="programs/htcacheclean.html">htcacheclean</a> tool which, as the name
      suggests, allows you to clean the cache periodically. Determining 
      how frequently to run <a href="programs/htcacheclean.html">htcacheclean</a> and what target size to 
      use for the cache is somewhat complex and trial and error may be needed to
      select optimal values.</p>

      <p><a href="programs/htcacheclean.html">htcacheclean</a> has two modes of 
      operation. It can be run as persistent daemon, or periodically from 
      cron. <a href="programs/htcacheclean.html">htcacheclean</a> can take up to an hour 
      or more to process very large (tens of gigabytes) caches and if you are 
      running it from cron it is recommended that you determine how long a typical 
      run takes, to avoid running more than one instance at a time.</p>

      <p class="figure">
      <img src="images/caching_fig1.gif" alt="" width="600" height="406" /><br />
      <a id="figure1" name="figure1"><dfn>Figure 1</dfn></a>: Typical
      cache growth / clean sequence.</p>

      <p>Because <code class="module"><a href="./mod/mod_disk_cache.html">mod_disk_cache</a></code> does not itself pay attention
      to how much space is used you should ensure that 
      <a href="programs/htcacheclean.html">htcacheclean</a> is configured to 
      leave enough "grow room" following a clean.</p>
    

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