Hi, the patch is self-explanatory, but I'm using the cover letter to raise a couple of related questions.
Since commit c8bb23cbdbe / QEMU 4.1.0 (and if the storage backend allows it) writing to an image created with preallocation=metadata can be slower (20% in my tests) than writing to an image with no preallocation at all. So: a) shall we include a warning in the documentation ("note that this preallocation mode can result in worse performance")? b) why don't we also initialize preallocated clusters with QCOW_OFLAG_ZERO? (at least when there's no subclusters involved, i.e. no backing file). This would make reading from them (and writing to them, after this patch) faster. Berto Alberto Garcia (1): qcow2: Skip copy-on-write when allocating a zero cluster include/block/block.h | 2 +- block/commit.c | 2 +- block/io.c | 20 +++++++++++++++++--- block/mirror.c | 3 ++- block/qcow2.c | 26 ++++++++++++++++---------- block/replication.c | 2 +- block/stream.c | 2 +- qemu-img.c | 2 +- 8 files changed, 40 insertions(+), 19 deletions(-) -- 2.20.1