Re: IO on guest is 20 times slower than host

Avi Kivity
2009-03-25T16:13:28Z

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?

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