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