Re: [Qemu-devel] Re: [et-mgmt-tools] Image Corruption Possible with qemu and qemu-kvm

[Chris Wilson]

Hi all,

On Wed, 29 Aug 2007, Alexander Graf wrote:
> Paul Jakma wrote:
> > On Mon, 27 Aug 2007, Anthony Liguori wrote:
> >
> > > I think this is the right level myself.  Advisory locks work okay but 
> > > not all filesystems support them... I think it would do more harm than 
> > > good to have a feature like that was supposed to provide a safe-guard 
> > > but then frequently didn't work.
> >                ^^^^^^^^^^^^^^^^^^^^^^
> >
> > Are you trying to say that any kind of significant portion of QEMU 
> > users have clustered file-systems?
> >
> > I think that's unlikely, and it'd be nice if QEMU by default did a 
> > fcntl() on writeable image files to lock itself from multiple access, 
> > that'd benefit the vast majority of users.
>
> I'm usually running one "main" qemu instance that has read/write access 
> to the disk file and several others for tests that use the -snapshot 
> option, so I think it's very important to have an easy means of 
> switching this check off.

Would it make sense to take a read lock (preventing concurrent writes) on 
read-only snapshot images, and a write lock on all writable files? That 
should allow one to run as many instances as you want off a single 
snapshot in snapshot mode, but still prevent one from (accidentally) 
running a non-snapshot QEMU on the same image, or from (accidentally) 
running two instances writing to the same COW file.

By removing the need to add the lock-override command line switch to qemu 
in this case, it would reduce the risk of accidental image corruption by 
accidentally overriding the lock.

Cheers, Chris.
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