Not also, that there is another benefit of such thing: we'll
implement this callback in qcow2 driver, so that backup will read
clusters not in guest cluster order, but in host cluster order, to
read more sequentially, which should bring better performance on
rotating disks.
I’m not exactly sure how you envision this to work, but block_status
also already gives you the host offset in *map.
But block-status doesn't give a possibility to read sequentially. For
this, user should call block-status several times until the whole disk
covered, then sort the segments by host offset. I wonder, could it be
implemented as some iterator, like
read_iter = bdrv_get_sequential_read_iter(source)
while (extents = bdrv_read_next(read_iter)):
for ext in extents:
start_writing_task(target, ext.offset, ext.bytes, ext.qiov)
where bdrv_read_next will read guest data in host-cluster-sequence..