From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: IO on guest is 20 times slower than host Date: Wed, 25 Mar 2009 19:55:08 +0200 Message-ID: <49CA6FFC.2030807@redhat.com> References: <49CA5825.7030201@redhat.com> <4AC00930-1EBB-4704-94CF-29478D07F03A@yoderhome.com> <49CA5D53.1080401@redhat.com> <49CA61E2.3050400@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org To: Kurt Yoder Return-path: Received: from mx2.redhat.com ([66.187.237.31]:39281 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753446AbZCYRyd (ORCPT ); Wed, 25 Mar 2009 13:54:33 -0400 In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: Kurt Yoder wrote: > > What do you mean about the cache? Is my test fundamentally flawed? I > *thought* I was testing write speed on the disk... 'dd', without further arguments, will write to the page cache and let the kernel write the data back at a later time. If you increase the block size (bs=1M count=1000) you should see much faster times on the host, I wouldn't be surprised to see 1GB/s. If you want to test disk speed, use of=/dev/blah oflag=direct. Beware of destroying your data disk. Something weird is happening with your system. If you extend the test, what does 'top' show? On both guest and host. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.