[libvirt] [PATCH 2/8] Basic framework for lock manager plugins

[Daniel P. Berrange]

On Mon, Nov 29, 2010 at 02:49:26PM -0500, David Teigland wrote:
> On Wed, Nov 24, 2010 at 03:08:41PM -0500, David Teigland wrote:
> > LOCK_MODE_NONE - the lock manager is unused, and it's up to the
> > application to do its own locking or coordination when accessing the
> > resource, e.g. if there's a clustered db or fs on the device that does its
> > own coordination.  You seem to be calling this SHARED, but there's
> > actually no lock or exclusion that a lock manager would implement for this
> > kind of usage.
> 
> I failed to mention that with a more elaborate set of lock modes like a
> dlm uses, the CW "concurrent write" mode fits the use case of leaving the
> coordination to the application.  Simpler lock managers only support
> Shared or Exclusive, of course, in which case "None" would apply.

This kind of difference in capabilities between lock managers is a good
example of why I only provide the disk sharing mode to the plugin API,
and leave choice of locking mode to the plugin.

Daniel