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[libvirt] using sync_manager with libvirt


We've been working on a program called sync_manager that implements
shared-storage-based leases to protect shared resources.  One way we'd like
to use it is to protect vm images that reside on shared storage,
i.e. preventing two vm's on two hosts from using the same image at once.

It's functional, and the next big step is using it through libvirt.

sync_manager "wraps" a process, i.e. acquires a lease, forks&execs a
process, renews the lease wile the process runs, and releases the lease
when the process exits.  While the process runs, it has exclusive access
to whatever resource was named in the lease that was acquired.

A command would be something like this:

sync_manager daemon -i <host_id> -n <vm_id> -l <lease> -c <command> <args>

  <host_id>	is integer between 1 and 2000 that is statically
		assigned to each host.

  <vm_id>	is a unique identifer for the process that will be run,
		e.g. the vm name or uuid.

  <lease>	defines the shared storage area that sync_manager should
		use for performing the disk-paxos based synchronization.
		It consists of <resource_name>:<path>:<offset>, where
		<resource_name> is likely to be the vm name/uuid (or the
		name of the vm's disk image), and <path>:<offset> is an
		area of shared storage that has been allocated for
		sync_manager to use (a separate area for each resource/vm).

  <command> <args>
		would be the qemu command line that is currently used.

We expect these new config values will need a place to live in the libvirt
xml config file, and libvirt will need to fork sync_manager -c qemu rather
than qemu directly.  At least those are the most obvious things that need
doing, there are sure to be other api or functional issues.

sync_manager only forks when running the command, and doesn't change any
fd's, so any command output should appear unchanged to libvirt.  Would
there be any problem with sync_manager also printing its own warnings and
errors to stderr?

While the main point of sync_manager is the disk leases to synchronize
access among hosts, it also uses posix locks to synchronize access among
local processes.



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