From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: Poor Write I/O Performance on KVM-79 Date: Sun, 04 Jan 2009 15:24:26 +0200 Message-ID: <4960B88A.2010608@redhat.com> References: <66c93b820901032203yc735b1g48d5bf19deffbfc@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, Mark McLoughlin To: Alexander Atticus Return-path: Received: from mx2.redhat.com ([66.187.237.31]:51250 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750803AbZADNYb (ORCPT ); Sun, 4 Jan 2009 08:24:31 -0500 In-Reply-To: <66c93b820901032203yc735b1g48d5bf19deffbfc@mail.gmail.com> Sender: kvm-owner@vger.kernel.org List-ID: Alexander Atticus wrote: > Hello! > > I have been experimenting with KVM and have been experiencing poor write I/O > performance. I'm not sure whether I'm doing something wrong or if this is > just the current state of things. > > While writing to the local array on the node running the guests I get about > 200MB/s from dd (bs=1M count=1000) or about 90MB/s write performance from > iozone (sequencial) when I write to a 2G file with a 16M record length. The > node is an 8 disk system using 3ware in a RAID50 configuration. It has 8GB > of RAM. > > The guests get much slower disk access. The guests are using file based > backends (tried both qcow2 and raw) with virtio support. With no other > activity on the machine, I get about 6 to 7MB/s write performance from > iozone with the same test. Guests are running Debian lenny/sid with > 2.6.26-1-686. > qcow2 will surely lead to miserable performance. raw files are better. best is to use lvm. > I don't know whether this is because of context switching or what. Again, > I'm wondering how I can improve this performance or if there is something > I am doing wrong. As a side note, I have also noticed some weirdness with > qcow2 files; some windows installations freeze and some disk corruption > running iozone on Linux guests. All problems go away when I switch to raw > image files though. > > I realize I take a hit by running file-based backends, and that the tests > aren't altogether accurate because with 8GB of RAM, I'm not saturating the > cache but they are still very disparate in numbers which concerns me. > > Finally, does anyone know if KVM is now fully supporting SCSI pass-through > in KVM-79? Does this mean that I would vastly reduce context switching by > using an LVM backend device for guests or am I misunderstanding the benefits > of pass-through? > scsi is much improved in kvm-82 though it still needs a lot more testing. scsi pass-through is almost completely untested. What is the kernel version in the guest? IIRC there were some serious limitations on the request size that were recently removed. -- error compiling committee.c: too many arguments to function