FYI If cinder (or any part of Nova/Glance/etc) ever creates qcow2 files with backing files, it must also make sure to use the '-obacking_fmt=FORMATNAME' option when creating them, to stop QEMU probiing formats when opening backing files.
FYI If cinder (or any part of Nova/Glance/etc) ever creates qcow2 files with backing files, it must also make sure to use the '-obacking_ fmt=FORMATNAME' option when creating them, to stop QEMU probiing formats when opening backing files.