Hello Kevin,

Le 13/02/2018 à 10:41, Kevin Wolf a écrit :
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.

Actually, my main concern is mostly about finding the cause rather than correcting my corrupted VMs.

Another way to say it : I prefer to help oVirt than help myself.

The first thing to make sure is that the image isn't touched by a second
process while QEMU is running a VM.

Indeed, I read some BZ about this issue : they were raised by a user who ran some qemu-img commands on a "mounted" image, thus leading to some corruption. In my case, I'm not playing with this, and the corrupted VMs were only touched by classical oVirt actions.

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.

May I ask after what QEMU version this kind of locking has been added.
As I wrote, our oVirt setup is 3.6 so not recent.


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.

See attachment.


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.

I just exported it like this :
qemu-img convert /dev/the_correct_path /home/blablah.qcow2.img

The resulting file is 32G and I need an idea to transfer this img to you.


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.

Are you saying:
- the corruption is in the metadata and in the guest data
OR
- the corruption is only in the metadata
?

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 get that.


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

Done.

and ideally the image to do anything,

I remember some Redhat people once gave me a temporary access to put heavy file on some dedicated server. Is it still possible?

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.

Sure.
Alas I find no obvious pattern leading to corruption :
From the guest side, it appeared with windows 2003, 2008, 2012, linux centOS 6 and 7. It appeared with virtio-blk; and I changed some VMs to used virtio-scsi but it's too soon to see appearance of corruption in that case. As I said, I'm using snapshots VERY rarely, and our versions are too old so we do them the cold way only (VM shutdown). So very safely. The "weirdest" thing we do is to migrate VMs : you see how conservative we are!

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?

I'm 99% sure the corrupted VMs have never see a snapshot, and 99% sure they have been migrated at most once.
For me *this* is the track to follow.

We have 2 main 3.6 oVirt DCs each having 4 dedicated LUNs, connected via iSCSI. Two SANs are serving those volumes. These are Equallogic and the setup of each volume contains a check saying :
Access type : "Shared"
http://psonlinehelp.equallogic.com/V5.0/Content/V5TOC/Allowing_or_disallowing_multi_ho.htm
(shared access to the iSCSI target from multiple initiators)

To be honest, I've never been comfortable with this point:
- In a complete different context, I'm using it to allow two files servers to publish an OCFS2 volume embedded in a clustered-LVM. It is absolutely reliable as *c*LVM and OCFS2 are explicitly written to manage concurrent access. - In the case of oVirt, we are here allowing tens of hosts to connect to the same LUN. This LUN is then managed by a classical LVM setup, but I see here no notion of concurrent access management. To date, I still haven't understood how was managed these concurrent access to the same LUN with no crash.

I hope I won't find no skeletons in the closet.

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

Exact, but as I wrote to Yaniv, it would be sad to setup a brand new 4.2 DC and to face the bad old issues. For the record, I just finished to setup another 4.2 DC, but it'll be long before I could apply to it a similar workload as the 3.6 production site.

--
Nicolas ECARNOT

Attachment: qemu-img_check.txt.gz
Description: application/gzip

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