[libvirt] RFC: xml-snapshot-data out-of-sync to .qcow2 data

Philipp Hahn hahn at univention.de
Fri Nov 16 06:10:35 UTC 2012

Hello Eric, hello List,

As QEMU stores the snapshot data inside the qcow2 files, modifing them 
directly might bring libvirts snapshot data out-of-sync. In my case a user 
replaced a qcow2 with a new file re-using the old name. libvirt still thinks 
that the snapshots are valid, while in reality the data is missing:

> # virsh snapshot-revert snap-test foo
> error: internal error Child process (/usr/bin/qemu-img snapshot -a 
foo /var/lib/libvirt/images/snap-test.qcow2) status unexpected: exit status 1

What would fix this scenario would be a validation mechanism within libvirt, 
which walks all snapshots and checks, if each referenced qcow2 file actually 
has the required snaphot data and mark the snapshot as invalid otherwise. 
This would require an addition to the xml data to add such a flag.

Or I could tell my users again not to re-use names still used by older 
snapshots, but that doesn't work as you see ;-)


Philipp Hahn           Open Source Software Engineer      hahn at univention.de
Univention GmbH        be open.                       fon: +49 421 22 232- 0
Mary-Somerville-Str.1  D-28359 Bremen                 fax: +49 421 22 232-99
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