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Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks.
From: |
Richard W.M. Jones |
Subject: |
Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks. |
Date: |
Mon, 7 Dec 2009 11:31:47 +0000 |
User-agent: |
Mutt/1.5.18 (2008-05-17) |
On Mon, Dec 07, 2009 at 11:30:14AM +0000, Daniel P. Berrange wrote:
> 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.
I think Jamie's point is that you might as well use no locking at all
in this configuration. It's hard to see what lock=shared is
protecting you against.
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-p2v converts physical machines to virtual machines. Boot with a
live CD or over the network (PXE) and turn machines into Xen guests.
http://et.redhat.com/~rjones/virt-p2v
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., (continued)
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., Jamie Lokier, 2009/12/07
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., Kevin Wolf, 2009/12/07
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., Daniel P. Berrange, 2009/12/07
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., Daniel P. Berrange, 2009/12/07
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., Jamie Lokier, 2009/12/07
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., Daniel P. Berrange, 2009/12/07
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks.,
Richard W.M. Jones <=
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., Jamie Lokier, 2009/12/07
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., Daniel P. Berrange, 2009/12/07
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., Richard W.M. Jones, 2009/12/07
- [Qemu-devel] Re: [PATCH] Disk image shared and exclusive locks., Paolo Bonzini, 2009/12/07
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., Anthony Liguori, 2009/12/07
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., Daniel P. Berrange, 2009/12/07
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., Anthony Liguori, 2009/12/07
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., Daniel P. Berrange, 2009/12/07
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., Anthony Liguori, 2009/12/07
- Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks., Kevin Wolf, 2009/12/08