From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:40607) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Qbttr-0004Wd-Kb for Qemu-devel@nongnu.org; Wed, 29 Jun 2011 08:29:48 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Qbttq-0004MQ-0d for Qemu-devel@nongnu.org; Wed, 29 Jun 2011 08:29:47 -0400 Received: from mx1.redhat.com ([209.132.183.28]:54178) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Qbttp-0004MI-DC for Qemu-devel@nongnu.org; Wed, 29 Jun 2011 08:29:45 -0400 Message-ID: <4E0B1B62.9080309@redhat.com> Date: Wed, 29 Jun 2011 14:32:34 +0200 From: Kevin Wolf MIME-Version: 1.0 References: <4E0B1399.4050807@redhat.com> <4E0B152B.1000201@codemonkey.ws> <4E0B1787.30709@redhat.com> <4E0B1943.4090400@codemonkey.ws> In-Reply-To: <4E0B1943.4090400@codemonkey.ws> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] Default cache mode List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: Avi Kivity , Christoph Hellwig , Qemu-devel@nongnu.org, Stefan Hajnoczi Am 29.06.2011 14:23, schrieb Anthony Liguori: > 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? Honestly, I don't know. Usually the problem is resolved with setting a different cache option, so nobody bothers to ask for details. I'd guess that it's ext4 in most cases. > 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. Okay, I think that's a fair requirement. So let's plan to implement this in 0.16. Christoph, it looks like we're back to your WCE patches then. Do you still work on them? Kevin