man/sd_readahead.html

   1 <html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>sd_readahead</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><style>
   2     a.headerlink {
   3       color: #c60f0f;
   4       font-size: 0.8em;
   5       padding: 0 4px 0 4px;
   6       text-decoration: none;
   7       visibility: hidden;
   8     }
   9
  10     a.headerlink:hover {
  11       background-color: #c60f0f;
  12       color: white;
  13     }
  14
  15     h1:hover > a.headerlink, h2:hover > a.headerlink, h3:hover > a.headerlink, dt:hover > a.headerlink {
  16       visibility: visible;
  17     }
  18   </style><a href="index.html">Index </a>·
  19   <a href="systemd.directives.html">Directives </a>·
  20   <a href="../python-systemd/index.html">Python </a>·
  21   <a href="../libudev/index.html">libudev </a>·
  22   <a href="../libudev/index.html">gudev </a><span style="float:right">systemd 208</span><hr><div class="refentry"><a name="sd_readahead"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>sd_readahead — Control ongoing disk boot-time read-ahead operations</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><div class="funcsynopsis"><pre class="funcsynopsisinfo">#include "sd-readahead.h"</pre><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef">int <b class="fsfunc">sd_readahead</b>(</code></td><td>const char *<var class="pdparam">action</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1"><a name="idm274682558800"></a><h2 id="Description">Description<a class="headerlink" title="Permalink to this headline" href="#Description">¶</a></h2><p><code class="function">sd_readahead()</code> may be
  23                 called by programs involved with early boot-up to
  24                 control ongoing boot-time disk read-ahead operations. It may be
  25                 used to terminate read-ahead operations in case an
  26                 uncommon disk access pattern is to be expected and
  27                 hence read-ahead replay or collection is unlikely to
  28                 have the desired speed-up effect on the current or
  29                 future boot-ups.</p><p>The <em class="parameter"><code>action</code></em> should be one
  30                 of the following strings:</p><div class="variablelist"><dl class="variablelist"><dt id="cancel"><span class="term">cancel</span><a class="headerlink" title="Permalink to this term" href="#cancel">¶</a></dt><dd><p>Terminates read-ahead
  31                                 data collection, and drops all
  32                                 read-ahead data collected during this
  33                                 boot-up.</p></dd><dt id="done"><span class="term">done</span><a class="headerlink" title="Permalink to this term" href="#done">¶</a></dt><dd><p>Terminates read-ahead
  34                                 data collection, but keeps all
  35                                 read-ahead data collected during this
  36                                 boot-up around for use during
  37                                 subsequent boot-ups.</p></dd><dt id="noreplay"><span class="term">noreplay</span><a class="headerlink" title="Permalink to this term" href="#noreplay">¶</a></dt><dd><p>Terminates read-ahead
  38                                 replay.</p></dd></dl></div></div><div class="refsect1"><a name="idm274682550656"></a><h2 id="Return Value">Return Value<a class="headerlink" title="Permalink to this headline" href="#Return%20Value">¶</a></h2><p>On failure, these calls return a negative
  39                 errno-style error code. It is generally recommended to
  40                 ignore the return value of this call.</p></div><div class="refsect1"><a name="idm274682549296"></a><h2 id="Notes">Notes<a class="headerlink" title="Permalink to this headline" href="#Notes">¶</a></h2><p>This function is provided by the reference
  41                 implementation of APIs for controlling boot-time
  42                 read-ahead and distributed with the systemd
  43                 package. The algorithm it implements is simple, and
  44                 can easily be reimplemented in daemons if it is
  45                 important to support this interface without using the
  46                 reference implementation.</p><p>Internally, this function creates a file in
  47                 <code class="filename">/run/systemd/readahead/</code> which is
  48                 then used as flag file to notify the read-ahead
  49                 subsystem.</p><p>For details about the algorithm check the
  50                 liberally licensed reference implementation sources:
  51                 <a class="ulink" href="http://cgit.freedesktop.org/systemd/systemd/plain/src/readahead/sd-readahead.c" target="_top">http://cgit.freedesktop.org/systemd/systemd/plain/src/readahead/sd-readahead.c</a>
  52                 and <a class="ulink" href="http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-readahead.h" target="_top">http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-readahead.h</a></p><p><code class="function">sd_readahead()</code> is
  53                 implemented in the reference implementation's drop-in
  54                 <code class="filename">sd-readahead.c</code> and
  55                 <code class="filename">sd-readahead.h</code> files. It is
  56                 recommended that applications consuming this API copy
  57                 the implementation into their source tree. For more
  58                 details about the reference implementation see
  59                 <a href="sd-readahead.html"><span class="citerefentry"><span class="refentrytitle">sd-readahead</span>(3)</span></a></p><p>If -DDISABLE_SYSTEMD is set during compilation,
  60                 this function will always return 0 and otherwise
  61                 become a NOP.</p></div><div class="refsect1"><a name="idm274686496128"></a><h2 id="Examples">Examples<a class="headerlink" title="Permalink to this headline" href="#Examples">¶</a></h2><div class="example"><a name="idm274686495456"></a><p class="title"><b>Example 1. Cancelling all read-ahead operations</b></p><div class="example-contents"><p>During boots where SELinux has to
  62                         relabel the file system hierarchy, it will
  63                         create a large amount of disk accesses that
  64                         are not necessary during normal boots. Hence
  65                         it is a good idea to disable both read-ahead replay and read-ahead collection.
  66                         </p><pre class="programlisting">sd_readahead("cancel");
  67 sd_readahead("noreplay");</pre></div></div><br class="example-break"></div><div class="refsect1"><a name="idm274686493088"></a><h2 id="See Also">See Also<a class="headerlink" title="Permalink to this headline" href="#See%20Also">¶</a></h2><p>
  68                         <a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>,
  69                         <a href="sd-readahead.html"><span class="citerefentry"><span class="refentrytitle">sd-readahead</span>(3)</span></a>,
  70                         <a href="daemon.html"><span class="citerefentry"><span class="refentrytitle">daemon</span>(7)</span></a>
  71                 </p></div></div></body></html>