[ovirt-users] [Qemu-block] qcow2 images corruption

[Kevin Wolf]

Am 07.02.2018 um 18:06 hat Nicolas Ecarnot geschrieben:
> TL; DR : qcow2 images keep getting corrupted. Any workaround?

Not without knowing the cause.

The first thing to make sure is that the image isn't touched by a second
process while QEMU is running a VM. The classic one is using 'qemu-img
snapshot' on the image of a running VM, which is instant corruption (and
newer QEMU versions have locking in place to prevent this), but we have
seen more absurd cases of things outside QEMU tampering with the image
when we were investigating previous corruption reports.

This covers the majority of all reports, we haven't had a real
corruption caused by a QEMU bug in ages.

> After having found (https://access.redhat.com/solutions/1173623) the right
> logical volume hosting the qcow2 image, I can run qemu-img check on it.
> - On 80% of my VMs, I find no errors.
> - On 15% of them, I find Leaked cluster errors that I can correct using
> "qemu-img check -r all"
> - On 5% of them, I find Leaked clusters errors and further fatal errors,
> which can not be corrected with qemu-img.
> In rare cases, qemu-img can correct them, but destroys large parts of the
> image (becomes unusable), and on other cases it can not correct them at all.

It would be good if you could make the 'qemu-img check' output available
somewhere.

It would be even better if we could have a look at the respective image.
I seem to remember that John (CCed) had a few scripts to analyse
corrupted qcow2 images, maybe we would be able to see something there.

> What I read similar to my case is :
> - usage of qcow2
> - heavy disk I/O
> - using the virtio-blk driver
> 
> In the proxmox thread, they tend to say that using virtio-scsi is the
> solution. Having asked this question to oVirt experts
> (https://lists.ovirt.org/pipermail/users/2018-February/086753.html) but it's
> not clear the driver is to blame.

This seems very unlikely. The corruption you're seeing is in the qcow2
metadata, not only in the guest data. If anything, virtio-scsi exercises
more qcow2 code paths than virtio-blk, so any potential bug that affects
virtio-blk should also affect virtio-scsi, but not the other way around.

> I agree with the answer Yaniv Kaul gave to me, saying I have to properly
> report the issue, so I'm longing to know which peculiar information I can
> give you now.

To be honest, debugging corruption after the fact is pretty hard. We'd
need the 'qemu-img check' output and ideally the image to do anything,
but I can't promise that anything would come out of this.

Best would be a reproducer, or at least some operation that you can link
to the appearance of the corruption. Then we could take a more targeted
look at the respective code.

> As you can imagine, all this setup is in production, and for most of the
> VMs, I can not "play" with them. Moreover, we launched a campaign of nightly
> stopping every VM, qemu-img check them one by one, then boot.
> So it might take some time before I find another corrupted image.
> (which I'll preciously store for debug)
> 
> Other informations : We very rarely do snapshots, but I'm close to imagine
> that automated migrations of VMs could trigger similar behaviors on qcow2
> images.

To my knowledge, oVirt only uses external snapshots and creates them
with QMP. This should be perfectly safe because from the perspective of
the qcow2 image being snapshotted, it just means that it gets no new
write requests.

Migration is something more involved, and if you could relate the
problem to migration, that would certainly be something to look into. In
that case, it would be important to know more about the setup, e.g. is
it migration with shared or non-shared storage?

> Last point about the versions we use : yes that's old, yes we're planning to
> upgrade, but we don't know when.

That would be helpful, too. Nothing is more frustrating that debugging a
bug in an old version only to find that it's already fixed in the
current version (well, except maybe debugging and finding nothing).

Kevin