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

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Jan 11, 2016 at 06:14:15PM +0100, Kevin Wolf wrote:
> 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?

It depends on the configuration of virtlockd, but out of the box
it will current uses fcntl() against the disk image directly. fcntl
uses a separate lockspace than flock() so their locks are invisible
to each other, except for NFS where linux apparently re-writes flock()
into fcntl(). So yeah, for at least NFS it would fail because libvirt
already holds the lock out of the box.

> 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.

Yeah if QEMU attempts to flock/fcntl that's going to cause trouble
unless it is disabled by default, in which case libvirt could simply
never enable it in order to avoid the clash.

Regards,
Daniel
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