qemu-img: Use BDRV_REQ_NO_FALLBACK for pre-zeroing
If qemu-img convert sees that the target image isn't zero-initialised yet, it tries to do an efficient zero write for the whole image first to save the overhead of repeated explicit zero writes during the conversion. Obviously, this provides only an advantage if the pre-zeroing is actually efficient. Otherwise, we can end up writing zeroes slowly while zeroing out the whole image, and then overwrite the same blocks again with real data, potentially doubling the written data. Pass BDRV_REQ_NO_FALLBACK to blk_make_zero() to avoid this case. If we can't efficiently zero out, we'll instead write explicit zeroes only if there is no data to be written to a block. Signed-off-by: NKevin Wolf <kwolf@redhat.com> Acked-by: NEric Blake <eblake@redhat.com>
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