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Re: [Qemu-devel] [PATCH 5/7] block: allow migration to work with image f
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH 5/7] block: allow migration to work with image files (v2) |
Date: |
Mon, 14 Nov 2011 21:11:01 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:7.0) Gecko/20110927 Thunderbird/7.0 |
Am 14.11.2011 20:49, schrieb Anthony Liguori:
> On 11/14/2011 01:46 PM, Juan Quintela wrote:
>> Anthony Liguori<address@hidden> wrote:
>>> On 11/14/2011 07:11 AM, Juan Quintela wrote:
>>>>
>>>>> diff --git a/cpus.c b/cpus.c
>>>>> index 82530c4..ae5ec99 100644
>>>>> --- a/cpus.c
>>>>> +++ b/cpus.c
>>>>> @@ -398,6 +398,7 @@ static void do_vm_stop(RunState state)
>>>>> vm_state_notify(0, state);
>>>>> qemu_aio_flush();
>>>>> bdrv_flush_all();
>>>>> + bdrv_invalidate_cache_all();
>>>>> monitor_protocol_event(QEVENT_STOP, NULL);
>>>>> }
>>>>
>>>> This is too much. Reopening all qcow2 images each time that we stop the
>>>> vm looks excesive, no?
>>>
>>> This general code came in via:
>>>
>>> http://mid.gmane.org/address@hidden
>>>
>>> That series made migration stable after issuing a stop operation. I
>>> believe the justification was for debugging purposes or something like
>>> that.
>>>
>>> At any rate, invalidating the cache is part of what's required to make
>>> things stable. If you look at something like cache=unsafe, the only
>>> way the metadata will get flushed if via a bdrv_close since bdrv_flush
>>> is a nop.
>>>
>>> So this is needed as long as we care about supporting this use-case.
>>
>> Then we need a "proper" qcow2 invalidate call. Doing in qemu toplevel:
>>
>> (qemu)stop
>>
>> And now all your qcow2 block devices are closed, or perhaps failing to
>> re-open() looks too much to me (TM).
>>
>> Kevin?
>
> Look closely at the patch. It doesn't actually close()/open() anything.
>
> It just invokes the bdrv_close() routine which calls the free functions on
> the
> l1/l2 caching functions. bdrv_open() doesn't actually open anything (it
> assumes
> the file is already open. It just reads the header and metadata over again.
>
> For something that's basically a hack, it turned out to work very cleanly :-)
But why do we need to do it on stop?
I don't think it makes even sense logically: bdrv_invalidate_cache()
means "throw all your caches away and refetch everything from disk".
What do we gain from doing this on stop? To some degree I could
understand if you did it on cont, so that you can modify an image on the
host while the VM is stopped (though I would still consider it criminal
:-)).
Kevin
- Re: [Qemu-devel] [PATCH 2/7] ivshmem: use migration blockers to prevent live migration in peer mode, (continued)
[Qemu-devel] [PATCH 3/7] qcow2: add a migration blocker, Anthony Liguori, 2011/11/12
[Qemu-devel] [PATCH 5/7] block: allow migration to work with image files (v2), Anthony Liguori, 2011/11/12
- Re: [Qemu-devel] [PATCH 5/7] block: allow migration to work with image files (v2), Juan Quintela, 2011/11/14
- Re: [Qemu-devel] [PATCH 5/7] block: allow migration to work with image files (v2), Anthony Liguori, 2011/11/14
- Re: [Qemu-devel] [PATCH 5/7] block: allow migration to work with image files (v2), Juan Quintela, 2011/11/14
- Re: [Qemu-devel] [PATCH 5/7] block: allow migration to work with image files (v2), Anthony Liguori, 2011/11/14
- Re: [Qemu-devel] [PATCH 5/7] block: allow migration to work with image files (v2),
Kevin Wolf <=
- Re: [Qemu-devel] [PATCH 5/7] block: allow migration to work with image files (v2), Anthony Liguori, 2011/11/14
- Re: [Qemu-devel] [PATCH 5/7] block: allow migration to work with image files (v2), Juan Quintela, 2011/11/14
- Re: [Qemu-devel] [PATCH 5/7] block: allow migration to work with image files (v2), Kevin Wolf, 2011/11/14
- Re: [Qemu-devel] [PATCH 5/7] block: allow migration to work with image files (v2), Anthony Liguori, 2011/11/14
Re: [Qemu-devel] [PATCH 5/7] block: allow migration to work with image files (v2), Juan Quintela, 2011/11/14
[Qemu-devel] [PATCH 6/7] qcow2: implement bdrv_invalidate_cache, Anthony Liguori, 2011/11/12
[Qemu-devel] [PATCH 4/7] qed: add migration blocker, Anthony Liguori, 2011/11/12
[Qemu-devel] [PATCH 7/7] qcow2: relax migration blocker, Anthony Liguori, 2011/11/12
Re: [Qemu-devel] [PATCH 1/7] migrate: add migration blockers, Kevin Wolf, 2011/11/14