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[libvirt] RFC [0/3]: A proposal for lock manager plugins



A few weeks back David introduce sync-manager as a means for protecting
against the same VM being started on multiple hosts:

  https://www.redhat.com/archives/libvir-list/2010-August/msg00179.html

This is obviously a desirable feature for libvirt, but we don't want to
have a hardcoded dependancy on a particular implementation. We can
achieve isolation between libvirt & sync-manager, allowing for alternative
implementations by specifying a formal plugin for supervisor processes.

What follows is my mockup of what a plugin API would look like, its
internal libvirt API and an outline of the usage scenarios in psuedo
code.

At the minimum level of compliance, the plugin implementation provides
for per-VM level exclusion. Optionally a plugin can declare that it

 - supports locks against individual resources (disks)
 - supports hotplug of individual resources
 - supports migration of the supervised process

Migration/hotplug will be forbid if those capabilities aren't declared
by the plugin.

In parallel with David's work on sync-manager, I intend to write a simple
plugin implementation based solely on fcntl() locking. It is important 
that we have 2 independant implementations to prove the validity of the
plugin API. The fcntl() impl should provide a zero-config impl we can 
deploy out of the box that will provide protection against 2 vms using
the same disk image on a host, and limited protection in multi-host
scenarios with shared filesystems (no protection for shared block devs). 
Perhaps we should have a 3rd impl based on libdlm too, for Red Hat 
ClusterSuite  scenarios, though sync-manager may well be general &
simple enough to easily cope with this already.

Daniel
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