Re: [Qemu-devel] qemu and qemu.git -> Migration + disk stress introduces qcow2 corruptions

[Kevin Wolf <kwolf@redhat.com>]

Am 09.11.2011 22:01, schrieb Anthony Liguori:
> On 11/09/2011 03:00 PM, Michael S. Tsirkin wrote:
>> On Wed, Nov 09, 2011 at 02:22:02PM -0600, Anthony Liguori wrote:
>>> On 11/09/2011 02:18 PM, Michael S. Tsirkin wrote:
>>>> On Wed, Nov 09, 2011 at 11:35:54AM -0600, Anthony Liguori wrote:
>>>>> On 11/09/2011 11:02 AM, Avi Kivity wrote:
>>>>>> On 11/09/2011 06:39 PM, Anthony Liguori wrote:
>>>>>>>
>>>>>>> Migration with qcow2 is not a supported feature for 1.0.  Migration is
>>>>>>> only supported with raw images using coherent shared storage[1].
>>>>>>>
>>>>>>> [1] NFS is only coherent with close-to-open which right now is not
>>>>>>> good enough for migration.
>>>>>>
>>>>>> Say what?
>>>>>
>>>>> Due to block format probing, we read at least the first sector of
>>>>> the disk during start up.
>>>>
>>>> A simple solution is not to do any probing before the VM is first
>>>> started on the incoming path.
>>>>
>>>> Any issues with this?
>>>>
>>>
>>> http://mid.gmane.org/1284213896-12705-4-git-send-email-aliguori@us.ibm.com
>>> I think Kevin wanted open to get delayed.
>>>
>>> Regards,
>>>
>>> Anthony Liguori
>>
>> So, this patchset just needs to be revived and polished up?
> 
> What I took from the feedback was that Kevin wanted to defer open until the 
> device model started.  That eliminates the need to reopen or have a invalidation 
> callback.
> 
> I think it would be good for Kevin to comment here though because I might have 
> misunderstood his feedback.

Your approach was to delay reads, but still keep the image open. I think
I worried that we might have additional reads somewhere that we don't
know about, and this is why I proposed delaying the open as well, so
that any read would always fail.

I believe just reopening the image is (almost?) as good and it's way
easier to do, so I would be inclined to do that for 1.0.

I'm not 100% sure about cases like iscsi, where reopening doesn't help.
I think delaying the open doesn't help there either if you migrate from
A to B and then back from B to A, you could still get old data. So for
iscsi probably cache=none remains the only safe choice, whatever we do.

Kevin