On Thu, 05/30 14:34, Stefan Hajnoczi wrote:
> +
> +static int coroutine_fn backup_before_write_notify(
> + NotifierWithReturn *notifier,
> + void *opaque)
> +{
> + BdrvTrackedRequest *req = opaque;
> +
> + return backup_do_cow(req->bs, req->sector_num, req->nb_sectors, NULL);
> +}
I'm wondering if we can see the logic here with a backing hd
relationship? req->bs is a backing file of job->target, but guest is
going to write to it, so we need to COW down the data to job->target
before overwritting (i.e. cluster is not allocated in child).
I think if we do this in block layer, there's not much necessity for a
before-write notifier here (although it may be useful for other cases):
in bdrv_write:
for child in req->bs->open_children
if not child->is_allocated(req->sectors)
do COW to child
The advantage of this is that we won't need to start block-backup job in
sync mode "none" to do point-in-time snapshot (image fleecing), and we
get writable snapshot (possibility to open backing file writable and
write to it safely) as a by-product.
But we will need to keep track of parent<->child of block states, and we
still need to take care of overlapping writing between block job and
guest request.
--
Fam
