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Date: Mon, 11 Jan 2016 18:31:06 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/5] block: added lock image option and
	callback
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To: "Denis V. Lunev" <den@openvz.org>
Cc: Fam Zheng <famz@redhat.com>, Olga Krishtal <okrishtal@virtuozzo.com>, qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>

Am 23.12.2015 um 08:46 hat Denis V. Lunev geschrieben:
> From: Olga Krishtal <okrishtal@virtuozzo.com>
> 
> While opening the image we want to be sure that we are the
> one who works with image, anf if it is not true -
> opening the image for writing should fail.
> 
> There are 2 ways at the moment: no lock at all and lock the file
> image.
> 
> Signed-off-by: Olga Krishtal <okrishtal@virtuozzo.com>
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Max Reitz <mreitz@redhat.com>
> CC: Eric Blake <eblake@redhat.com>
> CC: Fam Zheng <famz@redhat.com>

As long as locking is disabled by default, it's useless and won't
prevent people from corrupting their images. These corruptions happen
exactly because people don't know how to use qemu properly. You can't
expect them to enable locking manually.

Also, you probably need to consider bdrv_reopen() and live migration.
I think live migration would be blocked if source and destination both
see the lock; which is admittedly less likely than with the qcow2 patch
(and generally a problem of this series), but with localhost migration
and potentially with some NFS setups it can be the case.

Kevin