From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1Kas7M-0000Ts-SO for qemu-devel@nongnu.org; Wed, 03 Sep 2008 09:09:52 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1Kas7L-0000T3-Ce for qemu-devel@nongnu.org; Wed, 03 Sep 2008 09:09:52 -0400 Received: from [199.232.76.173] (port=46087 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1Kas7L-0000Sz-42 for qemu-devel@nongnu.org; Wed, 03 Sep 2008 09:09:51 -0400 Received: from ecfrec.frec.bull.fr ([129.183.4.8]:57782) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1Kas7L-0002CK-HQ for qemu-devel@nongnu.org; Wed, 03 Sep 2008 09:09:51 -0400 Subject: Re: [Qemu-devel][PATCH,RFC] Zero cluster dedup From: Laurent Vivier In-Reply-To: <48BE8754.8070202@suse.de> References: <48BE8754.8070202@suse.de> Content-Type: text/plain; charset=utf-8 Date: Wed, 03 Sep 2008 15:09:13 +0200 Message-Id: <1220447353.4189.17.camel@frecb07144> Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Reply-To: qemu-devel@nongnu.org List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Shahar Frank , qemu-devel@nongnu.org Le mercredi 03 septembre 2008 =C3=A0 14:47 +0200, Kevin Wolf a =C3=A9crit= : [...] > > For the > > cluster granularity issue, it is meaning less for dedup (zero or othe= r). > > The deduped clusters are rarely de-duped to sequential clusters and > > anyhow a dedup write operation is just changing the meta-data of the > > block (logical offset l2 entry + reference count of the physical > > cluster) so no large write optimization can be done. >=20 > This is still the L2 table which has to be written each time. Say we > have a 64k write, then you could save 15 L2 table writes if qcow_dedup > could handle zeroing multiple contiguous clusters at once. This could > matter for performance when wiping lots of data. >=20 > Laurent, do you have an opinion on this? You should know this stuff > better than me as you did the according patches. I agree with you. Shahar, I think you should make some performance benchmarks with "cache=3Doff" with and without your patch, just to see what happens. Laurent --=20 ----------------- Laurent.Vivier@bull.net ------------------ "La perfection est atteinte non quand il ne reste rien =C3=A0 ajouter mais quand il ne reste rien =C3=A0 enlever." Saint Exup=C3=A9ry