From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:33037) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QbvS1-0006Eq-9z for Qemu-devel@nongnu.org; Wed, 29 Jun 2011 10:09:10 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1QbvRz-0000Qr-21 for Qemu-devel@nongnu.org; Wed, 29 Jun 2011 10:09:08 -0400 Received: from mx1.redhat.com ([209.132.183.28]:19778) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QbvRy-0000Qf-K3 for Qemu-devel@nongnu.org; Wed, 29 Jun 2011 10:09:06 -0400 Message-ID: <4E0B32A9.3010809@redhat.com> Date: Wed, 29 Jun 2011 16:11:53 +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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] Default cache mode List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Frediano Ziglio Cc: Qemu-devel@nongnu.org, Stefan Hajnoczi , Avi Kivity , Christoph Hellwig Am 29.06.2011 15:53, schrieb Frediano Ziglio: > 2011/6/29 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? >> >> 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 >> >> >> > > Personally I think default lead to poor performance but currently > people know that even critical application are handled correctly. > Assuming a client connect to a database server on a qemu guest when > server reply "transaction ok" you know that even a host crash lead to > a successful transaction. At least the fact that this assumption won't > be true has to be stated. If the guest is acting correctly, then this assumption is definitely true with cache=none/writeback. What needs to happen is that the guest issues a disk flush before sending your "transaction ok" message. If it doesn't do that it can fail even on real hardware. cache=writethrough makes things safe that aren't even safe on real hardware, that's why I consider it extremely conservative. Kevin