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

[Richard W.M. Jones]

On Mon, Dec 07, 2009 at 10:31:28AM +0000, Jamie Lokier wrote:
> Anthony Liguori wrote:
> > I'm not sure whether it's best to enable it by default because, as I 
> > said earlier, I'm not comfortable with the lack of correctness wrt 
> > advisory vs. mandatory locking.
> 
> In my experience, disk images are accessed in one of five ways:
> 
>     qemu-img
>     qemu
>     qemu-nbd
>     mount -o loop
>     cp/rsync
> 
> If all but the last implement qemu's advisory locking, that's comforting.

It's not directly relevant to what you said above, but I'd just like
to interject my use case:

We'd like to prevent libguestfs from corrupting disk images.

If libguestfs is accessing a disk image read-only, then we would not
acquire any lock (ie. lock=none).  This gives reasonable results,
enough to grab a file or do 'virt-df' even against a running guest, in
all but the most pathological circumstances.  The only danger is that
libguestfs gets an error -- it can't corrupt the disk image of the
running VM.

If libguestfs is accessing a disk image read-write, then we would try
to acquire lock=exclusive.  At the same time, libvirt would be
modified so it sets lock=exclusive on all disk images, so libguestfs
would fail to acquire the lock when trying to write to a disk image
that is in use, and this would prevent disk corruption.

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