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

[Jamie Lokier]

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?

Same when you have guests sharing a partitioned disk, with each guest
accessing a different partition.  E.g. a Windows/Linux dual boot, and
you decide to run both guest OSes at the same time as they should be
fine.  You'd have to use 'lock=shared' - and when you do that, it
doesn't protect you against running them again by accident.

In all those cases you need locking at a higher level to be effective
at stopping the wrong guests from being invoked simultaneously.

So - can anyone think of a situation where being able to ask for a
shared lock on an image opened for writing is useful?

-- Jamie