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

From: Denis V. Lunev
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 00/10] qcow2: Implement image locking
Date: Wed, 13 Jan 2016 12:11:10 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0

On 01/13/2016 11:56 AM, Markus Armbruster wrote:
Kevin Wolf <address@hidden> writes:

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
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.
"Game over" for any patches that use the same locking mechanism as
libvirt without coordinating with libvirt.

Of course, patches that use a new locking mechanism will almost
certainly need some libvirt work, too.

Can we come up with a more integrated solution where QEMU cooperates
with libvirt on locking when libvirt is in play, and does a more limited
job itself when libvirt isn't in play?

for me this seems viable. But this requires a serious efforts to
do and serious coordination.

How this could be done and how this could be coordinated
with the current state of uncertainty.


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