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<h1>Virtual machine lock manager</h1>
<ul id="toc"></ul>
<p>
Libvirt includes a framework for ensuring mutual exclusion
between virtual machines using host resources. Typically
this is used to prevent two VM processes from having concurrent
write access to the same disk image, as this would result in
data corruption if the guest was not using a cluster
aware filesystem.
</p>
<h2><a name="plugins">Lock manager plugins</a></h2>
<p>
The lock manager framework has a pluggable architecture,
to allow different locking technologies to be used.
</p>
<dl>
<dt>nop</dt>
<dd>This is a "no op" implementation which does absolutely
nothing. This can be used if mutual exclusion between
virtual machines is not required, or if it is being
solved at another level in the management stack.</dd>
<dt><a href="locking-lockd.html">lockd</a></dt>
<dd>This is the current preferred implementation shipped
with libvirt. It uses the <code>virtlockd</code> daemon
to manage locks using the POSIX fcntl() advisory locking
capability. As such it requires a shared filesystem of
some kind be accessible to all hosts which share the
same image storage.</dd>
<dt><a href="locking-sanlock.html">sanlock</a></dt>
<dd>This is an alternative implementation preferred by
the oVirt project. It uses a disk paxos algorithm for
maintaining continuously renewed leases. In the default
setup it requires some shared filesystem, but it is
possible to use it in a manual mode where the management
application creates leases in SAN storage volumes.
</dd>
</dl>
</body>
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