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

[Anthony Liguori <anthony@codemonkey.ws>]

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.

Regards,

Anthony Liguori

>
> Kevin
>
>