Re: [Qemu-block] [Qemu-devel] [PATCH 00/10] qcow2: Implement image locking

[Kevin Wolf]

Am 23.12.2015 um 11:47 hat Daniel P. Berrange geschrieben:
> On Wed, Dec 23, 2015 at 11:14:12AM +0800, Fam Zheng wrote:
> > On Tue, 12/22 17:46, Kevin Wolf wrote:
> > > Enough innocent images have died because users called 'qemu-img snapshot' 
> > > while
> > > the VM was still running. Educating the users doesn't seem to be a working
> > > strategy, so this series adds locking to qcow2 that refuses to access the 
> > > image
> > > read-write from two processes.
> > > 
> > > Eric, this will require a libvirt update to deal with qemu crashes which 
> > > leave
> > > locked images behind. The simplest thinkable way would be to 
> > > unconditionally
> > > override the lock in libvirt whenever the option is present. In that case,
> > > libvirt VMs would be protected against concurrent non-libvirt accesses, 
> > > but not
> > > the other way round. If you want more than that, libvirt would have to 
> > > check
> > > somehow if it was its own VM that used the image and left the lock 
> > > behind. I
> > > imagine that can't be too hard either.
> > 
> > The motivation is great, but I'm not sure I like the side-effect that an
> > unclean shutdown will require a "forced" open, because it makes using qcow2 
> > in
> > development cumbersome, and like you said, management/user also needs to 
> > handle
> > this explicitly. This is a bit of a personal preference, but it's strong 
> > enough
> > that I want to speak up.
> 
> Yeah, I am also not really a big fan of locking mechanisms which are not
> automatically cleaned up on process exit. On the other hand you could
> say that people who choose to run qemu-img manually are already taking
> fate into their own hands, and ending up with a dirty image on unclean
> exit is still miles better than loosing all your data.
> 
> > As an alternative, can we introduce .bdrv_flock() in protocol drivers, with
> > similar semantics to flock(2) or lockf(3)? That way all formats can benefit,
> > and a program crash will automatically drop the lock.
> 
> FWIW, the libvirt locking daemon (virtlockd) will already attempt to take
> out locks using fcntl()/lockf() on all disk images associated with a VM.

Does this actually mean that if QEMU did try to use flock(), it would
fail because libvirt is already holding the lock?

I considered adding both locking schemes (the qcow2 flag for qcow2 on
any backend; flock() for anything else on local files), but if this is
true, that's game over for any flock() based patches.

Kevin