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From: "Denis V. Lunev" <den@openvz.org>
Message-ID: <5694F061.2000408@openvz.org>
Date: Tue, 12 Jan 2016 15:24:01 +0300
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Subject: Re: [Qemu-devel] [PATCH 1/5] block: added lock image option and
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To: Fam Zheng <famz@redhat.com>, Kevin Wolf <kwolf@redhat.com>
Cc: Olga Krishtal <okrishtal@virtuozzo.com>, qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>

On 01/12/2016 02:33 PM, Fam Zheng wrote:
> On Tue, 01/12 11:10, Kevin Wolf wrote:
>> The problem is that libvirt already takes a lock, as Dan mentioned in
>> another reply in this thread, so we can't enable locking in qemu by
>> default. It would always fail when run under libvirt.
>>
>> Unless I'm seriously mistaken, this means that flock() inside qemu is
>> dead.
> Yes, I see the problem with libvirt, but can we instead do these?
>
>    1) Do a soft flock() in QEMU invocation. If it fails, sliently ignore.
>    2) Do a hard flock() in qemu-img invocation. If it fails, report and exit.
>
> This way, if libvirt is holding flock, we can assume libvirt is actually
> "using" the image: 1) just works as before, but 2) will not break the qcow2.
> That is still a slight improvement, and does solve the reckless "qemu-img
> snapshot create" user's problem.
>
> Fam
assuming that we should lock "by default".

This looks good to me.

Den