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

From: Roman Kagan
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH 00/10] qcow2: Implement image locking
Date: Wed, 23 Dec 2015 15:15:50 +0300
User-agent: Mutt/1.5.24 (2015-08-30)

On Wed, Dec 23, 2015 at 10:47:22AM +0000, Daniel P. Berrange wrote:
> On Wed, Dec 23, 2015 at 11:14:12AM +0800, Fam Zheng wrote:
> > 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.

Is it even possible without QEMU cooperating?  In particular in complex
cases with e.g. backing chains?

This was exactly the reason why we designed the "lock" option to take an
argument describing the locking mechanism to be used (see the tentative
patchset Denis posted in this thread).  The only one currently
implemented is flock()-based; however it can be extended to other
mechanisms like network / cluster / SAN lock managers, etc.  In
particular, it can be made to talk to virtlockd.


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