It should be possible to execute the QMP "drive-mirror" command in "none" sync mode and "absolute-paths" mode even for block devices lacking a backing file.
"absolute-paths" does in fact not require a backing file to be present, as can be seen from the "top" sync mode code path. "top" basically states that the device should indeed have a backing file - however, the current code catches the case if it doesn't and then simply treats it as "full" sync mode, creating a target image without a backing file (in "absolute-paths" mode). Thus, "absolute-paths" does not imply the target file must indeed have a backing file. Therefore, the target file may be left unbacked in case of "none" sync mode as well, if the specified device is not backed either. Currently, qemu will crash trying to dereference the backing file pointer since it assumes that it will always be non-NULL in that case ("none" with "absolute-paths"). The first patch in this series adds a check whether the specified block device is backed or not (creating an unbacked target image, if required); the second patch adds a test case for mirroring unbacked block devices. v2: - patch 1: Reuse an already existing codepath to create an unbacked target image instead of introducing a new one (based on Fam's comment). - patch 2: Incorporated test case into 041 instead of creating a new file (according to Xia's and Paolo's comments). Max Reitz (2): block/drive-mirror: Check for NULL backing_hd qemu-iotests: Extend 041 for unbacked mirroring blockdev.c | 4 +++- tests/qemu-iotests/041 | 25 +++++++++++++++++++++++++ tests/qemu-iotests/041.out | 4 ++-- 3 files changed, 30 insertions(+), 3 deletions(-) -- 1.8.4.2