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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 21:36:31 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:7.0) Gecko/20110927 Thunderbird/7.0 |
Am 14.11.2011 21:12, schrieb Anthony Liguori:
> On 11/14/2011 02:11 PM, Kevin Wolf wrote:
>> 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
>> :-)).
>
> Michael basically was trying to avoid having a VM's state change after you
> stopped the guest.
>
> With something like cache=unsafe that periodically flushes based on a timer
> (I
> think), you want to make sure that that doesn't happen after stop occurs.
This is a good point, but neither does cache=unsafe use a timer nor can
I see how invalidating the cache would avoid such behaviour. And
throwing away any unwritten changes doesn't really make it better.
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, 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),
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
[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
Re: [Qemu-devel] [PATCH 1/7] migrate: add migration blockers, Kevin Wolf, 2011/11/14
Re: [Qemu-devel] [PATCH 1/7] migrate: add migration blockers, Juan Quintela, 2011/11/14