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 18:13:25 +0200 Message-ID: <49CA5825.7030201@redhat.com> References: 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]:49636 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759946AbZCYQN2 (ORCPT ); Wed, 25 Mar 2009 12:13:28 -0400 In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: Kurt Yoder wrote: > Hello list, > > I'm puzzled as to why my KVM guest has IO that is 20 times slower than > the KVM host: > > me@guest:~$ dd if=/dev/zero of=/tmp/bigfile count=100000 > 100000+0 records in > 100000+0 records out > 51200000 bytes (51 MB) copied, 8.36612 s, 6.1 MB/s > me@guest:~$ df -h > Filesystem Size Used Avail Use% Mounted on > /dev/vda1 9.2G 587M 8.2G 7% / > tmpfs 123M 0 123M 0% /lib/init/rw > varrun 123M 28K 123M 1% /var/run > varlock 123M 0 123M 0% /var/lock > udev 123M 2.6M 120M 3% /dev > tmpfs 123M 0 123M 0% /dev/shm > > > > me@host:~$ dd if=/dev/zero of=/tmp/bigfile count=100000 > 100000+0 records in > 100000+0 records out > 51200000 bytes (51 MB) copied, 0.47188 s, 109 MB/s > > > > The VM is started via libvirt and is the only VM running on an > otherwise-unused system: > > root 19915 1 0 Mar24 ? 00:00:00 /usr/sbin/libvirtd -d > nobody 19937 19915 0 Mar24 ? 00:00:00 dnsmasq > --keep-in-foreground --strict-order --bind-interfaces --pid-file > --conf-file --listen-address 192.168.122.1 --except-interface lo > --dhcp-leasefile=/var/lib/libvirt/dhcp-default.leases --dhcp-range > 192.168.122.2,192.168.122.254 > root 24354 19915 2 09:52 ? 00:02:33 /usr/bin/kvm -S -M > pc -m 256 -smp 1 -name guest -monitor pty -boot c -drive > file=/dev/HW_RAID/Guest,if=virtio,index=0,boot=on -net > nic,macaddr=52:54:00:ba:e6:db,vlan=0,model=virtio -net > tap,fd=11,script=,vlan=0,ifname=vnet1 -serial none -parallel none -usb > -vnc 127.0.0.1:1 > > > > Can someone suggest where to start looking? Thanks, I get 141 MB/s on the same test (it only copies to memory anyway). Is your VM under memory pressure? -- error compiling committee.c: too many arguments to function