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

From: Max Reitz
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 00/10] qcow2: Implement image locking
Date: Mon, 4 Jan 2016 18:02:52 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0

On 24.12.2015 06:41, Denis V. Lunev wrote:
> On 12/24/2015 02:19 AM, Max Reitz wrote:
>> So the benefits of a qcow2 flag are only minor ones. However, I
>> personally believe that automatic unlock on crash is a very minor
>> benefit as well. That should never happen in practice anyway, and a
>> crashing qemu is such a great inconvenience that I as a user wouldn't
>> really mind having to unlock the image afterwards.
> IMHO you are wrong. This is VERY important. The situation would be exactly
> the same after node poweroff, which could happen and really happens in
> the real life from time to time.
> In this cases VMs should start automatically and ASAP if configured this
> way. Any manual interaction here is a REAL pain.

Thanks, that's a good example.

However, I don't know much about management at that layer, so this is
probably where I'm out of the discussion.

(For instance, I don't know which kind of node you are talking about; I
presume it is a physical node, because if it was a virtual node, you'd
just kill the qemu instance in question by sending a QMP quit command.)

>> In fact, libvirt could even do that manually, couldn't it? If qemu
>> crashes, it just invokes qemu-img force-unlock on any qcow2 image which
>> was attached R/W to the VM.
> in the situation above libvirt does not have the information or this
> information could be unreliable.

Well, then s/libvirt/any of the management layers/. As far as I know,
qemu-img commands are still used pretty high up in the stack.

>>> 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.
>> Making other formats benefit from addressing this issue is a good point,
>> but it too is a minor point. Formats other than qcow2 and raw are only
>> supported for compatibility anyway, and we don't need this feature for
>> raw.
> I would like to have this covered by flock and this indeed working for
> years with Parallels.
>> I feel like most of the question which approach to take revolves around
>> "But what if qemu crashes?". You (and others) are right in that having
>> to manually unlock the image then is cumbersome, however, I think that:
>> (1) qemu should never crash anyway.
>> (2) If qemu does crash, having to unlock the image is probably the
>>      least of your worries.
>> (3) If you are using libvirt, it should actually be possible for
>>      libvirt to automatically force-unlock images on qemu crash.
>> This is why I don't think that keeping a locked image behind on qemu
>> crash is actually an issue.
>> Max
> pls see above. Node failure and unexpected power loss really matters.

Good points indeed (maybe, I can't actually judge, but I'll trust you).


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