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

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Dec 07, 2009 at 11:19:54AM +0000, Jamie Lokier wrote:
> Daniel P. Berrange wrote:
> > > 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.
> > > 
> > > 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?
> > 
> > Isn't this what Richard has already done ? The patch implements 'shared'
> > as a 'F_RDLCK' lock and 'exclusive' as 'F_WRLCK':
> 
> No, the question is whether it makes sense to provide a 'shared'
> option on the command line, or simply to always map:
> 
>      image opened read only => F_FDLCK
>      image opened writable => F_WRLCK
> 
> and provide only a single command line option: 'lock'.

That doesn't work in the case of setting up a clustered filesystem
shared between guests. That requires that the disk be opened writable,
but with a shared (F_RDLOCK) lock.


Daniel
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