Re: Poor Write I/O Performance on KVM-79

[Thomas Mueller <thomas@chaschperli.ch>]

On Sat, 03 Jan 2009 22:03:20 -0800, 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.
> 
> ...
> 
> KVM command to launch guest:
> 
> # /usr/bin/kvm -S -M pc -m 1024 -smp 1 -name demo4 \ -uuid
> 5b474147-f581-9a21-ac7d-cdd0ce881c5c -monitor pty -boot c \ -drive
> file=/iso/debian-testing-i386-netinst.iso,if=ide,media=cdrom,index=2 \
> -drive file=/srv/demo/demo4.img,if=virtio,index=0,boot=on -net \
> nic,macaddr=00:16:16:64:e6:de,vlan=0 -net \
> tap,fd=15,script=,vlan=0,ifname=vnet1 -serial pty -parallel none \ -usb
> -vnc 0.0.0.0:1 -k en-us
>

if you use qcow2 image with kvm-79 you have to use cache=writeback 
parameter to get some speed.

see also this post from Anthony Liguori:
http://thread.gmane.org/gmane.comp.emulators.kvm.devel/24765/focus=24811

- Thomas