From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:44873)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <stefanha@redhat.com>) id 1VGlRl-00019r-TJ
	for qemu-devel@nongnu.org; Tue, 03 Sep 2013 03:54:50 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <stefanha@redhat.com>) id 1VGlRg-0006i4-8M
	for qemu-devel@nongnu.org; Tue, 03 Sep 2013 03:54:45 -0400
Received: from mx1.redhat.com ([209.132.183.28]:2583)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <stefanha@redhat.com>) id 1VGlRg-0006hx-07
	for qemu-devel@nongnu.org; Tue, 03 Sep 2013 03:54:40 -0400
Date: Tue, 3 Sep 2013 09:54:31 +0200
From: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20130903075431.GA1610@stefanha-thinkpad.redhat.com>
References: <20130902125722.GG5073@irqsave.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
In-Reply-To: <20130902125722.GG5073@irqsave.net>
Content-Transfer-Encoding: quoted-printable
Subject: Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup
 command
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: =?iso-8859-1?Q?Beno=EEt?= Canet <benoit.canet@irqsave.net>
Cc: pbonzini@redhat.com, dietmar@proxmox.com, qemu-devel@nongnu.org

On Mon, Sep 02, 2013 at 02:57:23PM +0200, Beno=EEt Canet wrote:
>=20
> I don't see the point of using hashes.
> Using hashes means that at least one extra read will be done on the tar=
get to
> compute the candidate target hash.
> It's bad for a cloud provider where IOs count is a huge cost.
>=20
> Another structure to replace a bitmap (smaller on the canonical case) w=
ould be
> a block table as described in the Hystor paper:
> www.cse.ohio-state.edu/~fchen/paper/papers/ics11.pdf

This is similar to syncing image formats that use a revision number for
each cluster instead of a hash.

The problem with counters is overflow.  In the case of Hystor it is not
necessary to preserve exact counts.  A dirty bitmap must mark a block
dirty if it has been modified, otherwise there is a risk of data loss.

A bit more than just counters are necessary to implement a persistent
dirty bitmap, but maybe it's possible with some additional state.

Stefan