From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:42423) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UkRJJ-0005TY-5Y for qemu-devel@nongnu.org; Wed, 05 Jun 2013 23:56:29 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UkRJG-0006xx-Kl for qemu-devel@nongnu.org; Wed, 05 Jun 2013 23:56:25 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35243) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UkRJG-0006xh-Cg for qemu-devel@nongnu.org; Wed, 05 Jun 2013 23:56:22 -0400 Date: Thu, 6 Jun 2013 11:56:18 +0800 From: Fam Zheng Message-ID: <20130606035618.GA24375@localhost.nay.redhat.com> References: <1369917299-5725-1-git-send-email-stefanha@redhat.com> <1369917299-5725-4-git-send-email-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1369917299-5725-4-git-send-email-stefanha@redhat.com> Subject: Re: [Qemu-devel] [PATCH v5 03/11] block: add basic backup support to block driver List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Kevin Wolf , qemu-devel@nongnu.org, dietmar@proxmox.com, imain@redhat.com, Paolo Bonzini , xiawenc@linux.vnet.ibm.com 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