Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks.

[Daniel P. Berrange]

On Mon, Dec 07, 2009 at 11:28:34AM +0000, Jamie Lokier wrote:
> Kevin Wolf wrote:
> > Am 07.12.2009 11:31, schrieb Jamie Lokier:
> > > So the distinction read/write makes more sense.  Can anyone think of a
> > > situation where a shared lock on an image opened for writing is useful?
> > 
> > I think there are people using shared writable images with cluster file
> > systems.
> 
> Yes, they are.  Please the following again:
> 
> > > Sometimes shared access to a raw image (partitioned or whole disk
> > > filesystem) is ok, and sometimes it is not ok.  Only the user knows
> > > the difference, because only the user knows if the guests they are
> > > running use distinct partitions in the same raw image, or cooperative
> > > access to a shard image.
> > >
> > > But does it make sense to request a shared lock in that case?  Not
> > > really.  If you have a group of guests correctly sharing an image, you
> > > still want to prevent running the same group a second time - and a
> > > shared lock wouldn't do that, because each group would be requesting
> > > shared locks.
> 
> If you run each guest in the disk-sharing cluster with 'lock=shared',
> it reflects that they are sharing - but that doesn't actually prevent
> mistakes, does it?  If you run any of those guests a second time, it
> won't prevent that.  So what's the point in the shared lock?

In the case of the clustered scenario, your guest will likely have
two disks. Its exclusive root disk, and the shared cluster disk. The
lock on the former will ensure you can't run the guest itself twice.
The lock on the latter will prevent other apps thinking they've got
exclusive access to the cluster disk

Regards,
Daniel
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