Re: [Qemu-devel] [PATCH v5 03/11] block: add basic backup support to block driver

[Fam Zheng]

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