From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:34786) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Qbtns-0002yZ-S1 for Qemu-devel@nongnu.org; Wed, 29 Jun 2011 08:23:37 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Qbtnq-0003Pe-Ro for Qemu-devel@nongnu.org; Wed, 29 Jun 2011 08:23:36 -0400 Received: from mail-yw0-f45.google.com ([209.85.213.45]:57219) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Qbtnq-0003PY-HW for Qemu-devel@nongnu.org; Wed, 29 Jun 2011 08:23:34 -0400 Received: by ywb3 with SMTP id 3so527432ywb.4 for ; Wed, 29 Jun 2011 05:23:33 -0700 (PDT) Message-ID: <4E0B1943.4090400@codemonkey.ws> Date: Wed, 29 Jun 2011 07:23:31 -0500 From: Anthony Liguori MIME-Version: 1.0 References: <4E0B1399.4050807@redhat.com> <4E0B152B.1000201@codemonkey.ws> <4E0B1787.30709@redhat.com> In-Reply-To: <4E0B1787.30709@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] Default cache mode List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Avi Kivity , Christoph Hellwig , Qemu-devel@nongnu.org, Stefan Hajnoczi On 06/29/2011 07:16 AM, Kevin Wolf wrote: > Am 29.06.2011 14:06, schrieb Anthony Liguori: >> On 06/29/2011 06:59 AM, Kevin Wolf wrote: >>> Hi, >>> >>> I think we have touched this topic before during some IRC discussions or >>> somewhere deep in a mailing list thread, but I think it hasn't been >>> discussed on the list. >>> >>> Our default cache mode of cache=writethrough is extremely conservative >>> and provides absolute safety at the cost of performance, >> >> But for the most part, we track bare metal fairly well in terms of block >> performance, no? >> >> Or are you really referring to qcow2 as a specific example? In the >> past, we used a different default caching mode for qcow2. I think that >> could be done again if there was a compelling reason. > > No, people are also complaining about bad performance with raw. Which > isn't really surprising when you do a flush after each single write > request. O_SYNC is really much more than is needed in the average case. Which file system on the host? At any rate, I'm a big fan of making wce tunable in the guest and then I think setting wce=1 is quite reasonable to do by default. Regards, Anthony Liguori > > Kevin