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

[Kevin Wolf <kwolf@redhat.com>]

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