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