Re: [Qemu-devel] [PATCH 5/7] block: allow migration to work with image files (v2)

[Kevin Wolf <kwolf@redhat.com>]

Am 14.11.2011 20:49, schrieb Anthony Liguori:
> On 11/14/2011 01:46 PM, Juan Quintela wrote:
>> Anthony Liguori<aliguori@us.ibm.com>  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/cover.1290613959.git.mst@redhat.com
>>>
>>> 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