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

[Anthony Liguori <anthony@codemonkey.ws>]

On 11/11/2011 08:29 AM, Kevin Wolf wrote:
> Am 11.11.2011 15:03, schrieb Anthony Liguori:
>> On 11/11/2011 04:15 AM, Kevin Wolf wrote:
>>> Am 10.11.2011 22:30, schrieb Anthony Liguori:
>>>> Live migration with qcow2 or any other image format is just not going to work
>>>> right now even with proper clustered storage.  I think doing a block level flush
>>>> cache interface and letting block devices decide how to do it is the best approach.
>>>
>>> I would really prefer reusing the existing open/close code. It means
>>> less (duplicated) code, is existing code that is well tested and doesn't
>>> make migration much of a special case.
>>
>> Just to be clear, reopen only addresses image format migration.  It does not
>> address NFS migration since it doesn't guarantee close-to-open semantics.
>
> Yes. But image formats are the only thing that is really completely
> broken today. For NFS etc. we can tell users to use
> cache=none/directsync and they will be good. There is no such option
> that makes image formats safe.
>
>> The problem I have with the reopen patches are that they introduce regressions
>> and change at semantics for a management tool.  If you look at the libvirt
>> workflow with encrypted disks, it would break with the reopen patches.
>
> Yes, this is nasty. But on the other hand: Today migration is broken for
> all qcow2 images, with the reopen it's only broken for encrypted ones.
> Certainly an improvement, even though there's still a bug left.

This sounds like a good thing to work through in the next release.

>
>>> If you want to avoid reopening the file on the OS level, we can reopen
>>> only the topmost layer (i.e. the format, but not the protocol) for now
>>> and in 1.1 we can use bdrv_reopen().
>>
>> I don't view not supporting migration with image formats as a regression as it's
>> never been a feature we've supported.  While there might be confusion about
>> support around NFS, I think it's always been clear that image formats cannot be
>> used.
>>
>> Given that, I don't think this is a candidate for 1.0.
>
> Nobody says it's a regression, but it's a bad bug and you're blocking a
> solution for it for over a year now because the solution isn't perfect
> enough in your eyes. :-(

This patch was posted a year ago.  Feedback was provided and there was never any 
follow up[1].  I've never Nack'd this approach.  I can't see how I was blocking 
this since I never even responded in the thread.  If this came in before soft 
freeze, I wouldn't have objected if you wanted to go in this direction.

This is not a bug fix, this is a new feature.  We're long past feature freeze. 
It's not a simple and obvious fix either.  It only partially fixes the problem 
and introduces other problems.  It's not a good candidate for making an 
exception at this stage in the release.

[1] http://mid.gmane.org/cover.1294150511.git.quintela@redhat.com

Regards,

Anthony Liguori

>
> Kevin
>