[libvirt] [RFC] QCOW2 version defaults in qemu-img and libvirt

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) * 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? Jan [1] http://lists.nongnu.org/archive/html/qemu-devel/2013-08/msg02549.html [2] https://www.redhat.com/archives/libvir-list/2013-February/msg00301.html [3] https://bugzilla.redhat.com/show_bug.cgi?id=997977