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

[Kevin Wolf <kwolf@redhat.com>]

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<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
>> :-)).
> 
> 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