[libvirt] image locking?

[Daniel P. Berrange]

On Wed, Oct 22, 2008 at 10:21:38PM +0200, Guido G?nther wrote:
> On Tue, Oct 21, 2008 at 12:16:01PM -0700, Itamar Heim wrote:
> > While this might work for SBC (although most enterprises have the
> > datacenter on remote sites as well, so not always that easy).  I don't
> > think the solution is viable for CBC though (I am not sure CBC would
> > use iSCSI, probably NFS is a more relevant option, but the leased
> > locking is required there as well, just as a collaborative effort to
> > notate to the non-responding node to stop writing to the image).
> What about using dlm? This gives fencing for free. See my other post
> - allowing libvirt to use cluster wide storage locks would solve your
> problem?

Yes it all comes back to clustering. If you have clustering you can 
build a reliable image locking system across all types of storage
backend. If you don't have clustering you can make a best effort,
likely only enforcing safety within the scope of a single host. 

We need libvirt to enforce safety per host using standard POSIX locking
because we cann't demand that everyone using clustering.

We should optionally use some cluster based locking scheme if it is 
available, as a 2nd layer ontop of the basic single-host view locking

Daniel
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