From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:47914) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cwSwS-0007Ql-Ut for qemu-devel@nongnu.org; Fri, 07 Apr 2017 08:24:41 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cwSwP-0006bU-RN for qemu-devel@nongnu.org; Fri, 07 Apr 2017 08:24:40 -0400 From: Alberto Garcia In-Reply-To: <20170407122021.GP13602@stefanha-x1.localdomain> References: <20170406150148.zwjpozqtale44jfh@perseus.local> <20170407122021.GP13602@stefanha-x1.localdomain> Date: Fri, 07 Apr 2017 14:24:35 +0200 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Subject: Re: [Qemu-devel] [RFC] Proposed qcow2 extension: subcluster allocation List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, Max Reitz , Kevin Wolf On Fri 07 Apr 2017 02:20:21 PM CEST, Stefan Hajnoczi wrote: >> Here are the results when writing to an empty 40GB qcow2 image with no >> backing file. The numbers are of course different but as you can see >> the patterns are similar: >> >> |-----------------+----------------+-----------------+-------------------| >> | cluster size | subclusters=on | subclusters=off | Max L2 cache size | >> |-----------------+----------------+-----------------+-------------------| >> | 2 MB (256 KB) | 1200 IOPS | 255 IOPS | 160 KB | >> | 512 KB (64 KB) | 3000 IOPS | 700 IOPS | 640 KB | >> | 64 KB (8 KB) | 7200 IOPS | 3300 IOPS | 5 MB | >> | 32 KB (4 KB) | 12300 IOPS | 4200 IOPS | 10 MB | >> | 4 KB (512 B) | 100 IOPS | 100 IOPS | 80 MB | >> |-----------------+----------------+-----------------+-------------------| > > I don't understand why subclusters=on performs so much better when > there's no backing file. Is qcow2 zeroing out the 64 KB cluster with > subclusters=off? It is, see https://lists.nongnu.org/archive/html/qemu-devel/2015-07/msg05877.html Berto