From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: how to tweak kernel to get the best out of kvm? Date: Sat, 13 Mar 2010 10:54:12 +0200 Message-ID: <4B9B52B4.9040505@redhat.com> References: <4B912156.5020707@darkharri.de> <4B94D947.3060303@redhat.com> <4B979776.1000701@aixigo.de> <4B979B81.5060508@redhat.com> <4B98EEFA.2090208@aixigo.de> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Harald Dunkel , KVM Mailing List To: Harald Dunkel Return-path: Received: from mx1.redhat.com ([209.132.183.28]:1029 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932875Ab0CMIyZ (ORCPT ); Sat, 13 Mar 2010 03:54:25 -0500 In-Reply-To: <4B98EEFA.2090208@aixigo.de> Sender: kvm-owner@vger.kernel.org List-ID: On 03/11/2010 03:24 PM, Harald Dunkel wrote: > Hi Avi, > > I had missed to include some important syslog lines from the > host system. See attachment. > > On 03/10/10 14:15, Avi Kivity wrote: > >> You have tons of iowait time, indicating an I/O bottleneck. >> >> > Is this disk IO or network IO? disk. > The rsync session puts a > high load on both, but actually I do not see how a high > load on disk or block IO could make the virtual hosts > unresponsive, as shown by the hosts syslog? > > qcow2 is still not fully asynchronous, so sometimes when it waits, a vcpu waits as well. >> Here the problem is likely the host filesystem and/or I/O scheduler. >> >> The optimal layout is placing guest disks in LVM volumes, and accessing >> them with -drive file=...,cache=none. However, file-based access should >> also work. >> >> > I will try LVM tomorrow, when the test with reiserfs is completed. > > If the slowdown is indeed due to I/O, LVM (with cache=off) should eliminate it completely. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.