On Tue, Aug 20, 2013 at 11:33:36AM +0200, Ján Tomko wrote:

Hello!

QEMU is switching the default QCOW2 version from v2 (compat=0.10) to v3 (compat=1.1) [1]

Currently, libvirt only specifies the compat=0.10 option if it was explicitly requested (to avoid parsing qemu-img help output [2]) and assumes the format to be v2 when it calls qemu-img without the compat option.

With this change in qemu-img a volume with no <features> or <compat> elements will be created as qcow2v3 with the new qemu-img (but the compat level won't be reflected in volume XML until refresh).

According to the IRC conversation with Eric Blake and Kevin Wolf (bug I filed: [3]), it seems we should:

* always specify the compat option if it's supported by qemu-img (which would solve the problem mentioned above)

This is definitely desired.

* provide an option in qemu.conf to set the default compatibility level, defaulting to 1.1 to make it easier to use the new format This would probably require a new storage.conf file, since the storage driver doesn't have access to the qemu driver config, but: does this seem reasonable? Should we add a default feature list (for the only feature) as well?

I'm not really convinced by this. If we allowing change the default to be v3, this may well break applications / harm portability of the qcow files they create. IMHO we should only ever use v3 if the app requested v3. Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|