Re: [libvirt] [RFC] qcow2v3 features support

Since 1.1 [1], QEMU supports qcow2 version 3 images. These are still reffered to as qcow2 everywhere (qmp, qemu/qemu-img command line). This format (besides some internal improvements) includes a bitfield of compatible/incompatible/auto-clear features. [2] The only feature so far is delayed refcount updating (since 1.2 [3]), which is compatible with 1.1 if the file has been closed properly, if not it has the dirty flag set. For qcow2v3 images, -o compat=1.1 has to be passed to qemu-img. (compat=0.10 means qcow2 which is the default at the moment) For delayed refcount updates it's -o lazy_refcounts=on. Using these options would require parsing 'qemu-img create -f qcow2 -o ? /dev/null' output, or assuming they are supported. Do we need a new format type (qcow2v3) that translates to qcow2 for qemu, or would XML like this be enough? <target> <path>/var/lib/libvirt/images/image</path> <format type='qcow2'/> <features version='1.1'> <lazy_refcounts compatible="no"/> </features> ... </target>