From mboxrd@z Thu Jan 1 00:00:00 1970 From: john cooper Subject: Re: bad virtio disk performance Date: Mon, 27 Apr 2009 13:36:05 -0400 Message-ID: <49F5ED05.6020703@third-harmonic.com> References: <20090427151204.GA16943@xanadu.blop.info> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, john.cooper@redhat.com To: Lucas Nussbaum Return-path: Received: from dpc691978010.direcpc.com ([69.19.78.10]:39054 "EHLO anvil.third-harmonic.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751902AbZD0Rma (ORCPT ); Mon, 27 Apr 2009 13:42:30 -0400 In-Reply-To: <20090427151204.GA16943@xanadu.blop.info> Sender: kvm-owner@vger.kernel.org List-ID: Lucas Nussbaum wrote: > Hi, > > I'm experiencing bad disk I/O performance using virtio disks. > > I'm using Linux 2.6.29 (host & guest), kvm 84 userspace. > On the host, and in a non-virtio guest, I get ~120 MB/s when writing > with dd (the disks are fast RAID0 SAS disks). Could you provide detail of the exact type and size of i/o load you were creating with dd? Also the full qemu cmd line invocation in both cases would be useful. > In a guest with a virtio disk, I get at most ~32 MB/s. Which non-virtio interface was used for the comparison? > The rest of the setup is the same. For reference, I'm running kvm -drive > file=/tmp/debian-amd64.img,if=virtio. > > Is such performance expected? What should I check? Not expected, something is awry. blktrace(8) run on the host will shed some light on the type of i/o requests issued by qemu in both cases. -john -- john.cooper@third-harmonic.com