[Qemu-devel] [BUG] Qcow2 corruption on snapshot revert

[Qemu-devel] [BUG] Qcow2 corruption on snapshot revert

[Philipp Hahn]

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Hello,

it seems like I have discovered a bug in qemu-0.14.1 which corrupts Qcow2 
image files when using internal snapshots.
I tied this both on an amd64 host running our Debian bases UCS distribution 
(using pure qemu-kvm_0.14 and pure qemu-kvm_0.14.1) and also on a pure Debian 
i386 sid installation (patched qemu-kvm_0.14.1).
I can reproducte this every time doing the following steps:
0. Create and install VM
1. Shut down VM
2. Create offline snapshot using "qemu-img snapshot -c Off image.qcow2"
3. Start VM
4. Create online snapshot using "savevm On"
5. Shut down VM
6. Revert to snapshot using "qemu-img snapshot -a Off image.qcow2"
Repeat step 6 three or more times and the Image is destroyed.

My guess is that this is a but in qemu-img, which is related to having an 
online snapshot (VM running) while reverting to an offline snapshot (VM 
stopped), because I wasn't able to reproduce this using only offline 
snapshots or only online snapshots.

I have attached a shell script to reproduce this bug on all my runs.
* It both happens with kvm-0.14.1 and also with qemu-0.14.1.
* The cache stragtegy "writethrough" vs. "writeback" is irrelevant.

The script needs an image nameded "pmh_squeeze-0.qcow.bak2" of an VM where you 
can login via serial console using "root" for login and "univention" for 
password. You can use other images as well, as long as you modify the block 
device from within the VM. You get get it from 
<http://download.univention.de/download/temp/qemu-0.14-qcow2/>

There you will also find the beginning of a small Python program 
called "qcow2.py" which can be used to dump the structure of an qcow2 file in 
some more human readable format with some consistency checking.

Related links:
<http://git.kernel.org/?p=virt/kvm/qemu-kvm.git;a=commit;h=e11480db7ff15a9e878f6b3cc1199b439bf7c825>
<http://git.kernel.org/?p=virt/kvm/qemu-kvm.git;a=commit;h=f0aa7a8b2d518c54430e4382309281b93e51981a>
<http://lists.gnu.org/archive/html/qemu-devel/2011-04/msg01376.html>
<https://forge.univention.org/bugzilla/show_bug.cgi?id=22221>

Any help is appreciated.

Sincerely
Philipp Hahn
-- 
Philipp Hahn           Open Source Software Engineer      hahn@univention.de
Univention GmbH        Linux for Your Business        fon: +49 421 22 232- 0
Mary-Somerville-Str.1  D-28359 Bremen                 fax: +49 421 22 232-99
                                                   http://www.univention.de/

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Re: [Qemu-devel] [BUG] Qcow2 corruption on snapshot revert

[Kevin Wolf]

Am 04.08.2011 16:20, schrieb Philipp Hahn:
> Hello,
> 
> it seems like I have discovered a bug in qemu-0.14.1 which corrupts Qcow2 
> image files when using internal snapshots.
> I tied this both on an amd64 host running our Debian bases UCS distribution 
> (using pure qemu-kvm_0.14 and pure qemu-kvm_0.14.1) and also on a pure Debian 
> i386 sid installation (patched qemu-kvm_0.14.1).
> I can reproducte this every time doing the following steps:
> 0. Create and install VM
> 1. Shut down VM
> 2. Create offline snapshot using "qemu-img snapshot -c Off image.qcow2"
> 3. Start VM
> 4. Create online snapshot using "savevm On"
> 5. Shut down VM
> 6. Revert to snapshot using "qemu-img snapshot -a Off image.qcow2"
> Repeat step 6 three or more times and the Image is destroyed.
> 
> My guess is that this is a but in qemu-img, which is related to having an 
> online snapshot (VM running) while reverting to an offline snapshot (VM 
> stopped), because I wasn't able to reproduce this using only offline 
> snapshots or only online snapshots.
> 
> I have attached a shell script to reproduce this bug on all my runs.
> * It both happens with kvm-0.14.1 and also with qemu-0.14.1.
> * The cache stragtegy "writethrough" vs. "writeback" is irrelevant.
> 
> The script needs an image nameded "pmh_squeeze-0.qcow.bak2" of an VM where you 
> can login via serial console using "root" for login and "univention" for 
> password. You can use other images as well, as long as you modify the block 
> device from within the VM. You get get it from 
> <http://download.univention.de/download/temp/qemu-0.14-qcow2/>
> 
> There you will also find the beginning of a small Python program 
> called "qcow2.py" which can be used to dump the structure of an qcow2 file in 
> some more human readable format with some consistency checking.
> 
> Related links:
> <http://git.kernel.org/?p=virt/kvm/qemu-kvm.git;a=commit;h=e11480db7ff15a9e878f6b3cc1199b439bf7c825>
> <http://git.kernel.org/?p=virt/kvm/qemu-kvm.git;a=commit;h=f0aa7a8b2d518c54430e4382309281b93e51981a>
> <http://lists.gnu.org/archive/html/qemu-devel/2011-04/msg01376.html>
> <https://forge.univention.org/bugzilla/show_bug.cgi?id=22221>
> 
> Any help is appreciated.

Thanks for the report. This small script seems to be enough to reproduce
the corruption:

qemu-img create -f qcow2 /tmp/sn.qcow2 64M
qemu-img snapshot -c foo /tmp/sn.qcow2
qemu-io -c 'write -b 0 4M' /tmp/sn.qcow2
qemu-img snapshot -a foo /tmp/sn.qcow2

I'll look into the details tomorrow.

Kevin