static/freebsd/man4/syncer.4 3.html


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<table class="head">
  <tr>
    <td class="head-ltitle">SYNCER(4)</td>
    <td class="head-vol">Device Drivers Manual</td>
    <td class="head-rtitle">SYNCER(4)</td>
  </tr>
</table>
<div class="manual-text">
<section class="Sh">
<h1 class="Sh" id="NAME"><a class="permalink" href="#NAME">NAME</a></h1>
<p class="Pp"><code class="Nm">syncer</code> &#x2014; <span class="Nd">file
    system synchronizer kernel process</span></p>
</section>
<section class="Sh">
<h1 class="Sh" id="SYNOPSIS"><a class="permalink" href="#SYNOPSIS">SYNOPSIS</a></h1>
<table class="Nm">
  <tr>
    <td><code class="Nm">syncer</code></td>
    <td></td>
  </tr>
</table>
</section>
<section class="Sh">
<h1 class="Sh" id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">DESCRIPTION</a></h1>
<p class="Pp">The <code class="Nm">syncer</code> kernel process helps protect
    the integrity of disk volumes by flushing volatile cached file system data
    to disk.</p>
<p class="Pp">The kernel places all <a class="Xr">vnode(9)</a>'s in a number of
    queues. The <code class="Nm">syncer</code> process works through the queues
    in a round-robin fashion, usually processing one queue per second. For each
    <a class="Xr">vnode(9)</a> on that queue, the <code class="Nm">syncer</code>
    process forces a write out to disk of its dirty buffers.</p>
<p class="Pp">The usual delay between the time buffers are dirtied and the time
    they are synced is controlled by the following <a class="Xr">sysctl(8)</a>
    tunable variables:</p>
<table class="Bl-column">
  <tr id="Variable">
    <td><a class="permalink" href="#Variable"><i class="Em">Variable</i></a></td>
    <td><a class="permalink" href="#Default"><i class="Em" id="Default">Default</i></a></td>
    <td><a class="permalink" href="#Description"><i class="Em" id="Description">Description</i></a></td>
  </tr>
  <tr id="kern.filedelay">
    <td><var class="Va">kern.filedelay</var></td>
    <td>30</td>
    <td>time to delay syncing files</td>
  </tr>
  <tr id="kern.dirdelay">
    <td><var class="Va">kern.dirdelay</var></td>
    <td>29</td>
    <td>time to delay syncing directories</td>
  </tr>
  <tr id="kern.metadelay">
    <td><var class="Va">kern.metadelay</var></td>
    <td>28</td>
    <td>time to delay syncing metadata</td>
  </tr>
</table>
</section>
<section class="Sh">
<h1 class="Sh" id="SEE_ALSO"><a class="permalink" href="#SEE_ALSO">SEE
  ALSO</a></h1>
<p class="Pp"><a class="Xr">sync(2)</a>, <a class="Xr">fsck(8)</a>,
    <a class="Xr">sync(8)</a>, <a class="Xr">sysctl(8)</a></p>
</section>
<section class="Sh">
<h1 class="Sh" id="HISTORY"><a class="permalink" href="#HISTORY">HISTORY</a></h1>
<p class="Pp">The <code class="Nm">syncer</code> process is a descendant of the
    &#x2018;update&#x2019; command, which appeared in
    <span class="Ux">Version&#x00A0;6 AT&amp;T UNIX</span>, and was usually
    started by <span class="Pa">/etc/rc</span> when the system went multi-user.
    A kernel initiated &#x2018;update&#x2019; process first appeared in
    <span class="Ux">FreeBSD 2.0</span>.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="BUGS"><a class="permalink" href="#BUGS">BUGS</a></h1>
<p class="Pp">It is possible on some systems that a <a class="Xr">sync(2)</a>
    occurring simultaneously with a crash may cause file system damage. See
    <a class="Xr">fsck(8)</a>.</p>
</section>
</div>
<table class="foot">
  <tr>
    <td class="foot-date">July 14, 2000</td>
    <td class="foot-os">FreeBSD 15.0</td>
  </tr>
</table>