From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: kvm-83 write performance raw Date: Mon, 02 Mar 2009 16:21:07 -0600 Message-ID: <49AC5BD3.9050206@codemonkey.ws> References: <49AC42F4.3030101@codemonkey.ws> <49AC4E05.4030805@codemonkey.ws> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Mailing Lists To: AelMalinka@gmail.com Return-path: Received: from qw-out-2122.google.com ([74.125.92.26]:11764 "EHLO qw-out-2122.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751107AbZCBW1e (ORCPT ); Mon, 2 Mar 2009 17:27:34 -0500 Received: by qw-out-2122.google.com with SMTP id 5so3883037qwi.37 for ; Mon, 02 Mar 2009 14:27:31 -0800 (PST) In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: Malinka Rellikwodahs wrote: > On Mon, Mar 2, 2009 at 16:22, Anthony Liguori wrote: > >> Malinka Rellikwodahs wrote: >> >>>>> What version of kvm is this? Is it kvm-68? You'll have better luck >>>>> with >>>>> something newer than that. >>>>> >>>>> >>> kvm-83 is the one with the problem, kvm-68 is working correctly. >>> >>> >> kvm-68 and qcow2 both use cache=writeback by default which is less safe than >> cache=writethrough which is now the default. >> >> But performance shouldn't be as bad as your seeing. >> > > Running the kvm-84 install on the qcow image as kvm -m 1024 -drive > file=qcow,boot=on,cache=writethrough, I get similar performance to the > raw performance. > > So it looks like with cache set to writethrough there's a big > performance hit on this setup, any ideas where to look for that? > cache=writeback is "fake" performance. In theory, it should be better than native because it's being loose with data consistency. So the real question is why is performance so bad. It's probably got something to do with IDE. It could be that kvm-84's IDE is pre-Avi's AIO implementation such that you're splitting up IDE requests into something small. They should be submitted in parallel but maybe that's not working out well with your disk setup. Regards, Anthony Liguori >> Regards, >> >> Anthony Liguori >>