Re: virtio disk slower than IDE?

[Gordan Bobic <gordan@bobich.net>]

Dor Laor wrote:
> On 11/15/2009 02:00 PM, Gordan Bobic wrote:
>> Dor Laor wrote:
>>> On 11/14/2009 04:23 PM, Gordan Bobic wrote:
>>>> I just tried paravirtualized virtio block devices, and my tests show
>>>> that they are approximately 30% slower than emulated IDE devices. I'm
>>>> guessing this isn't normal. Is this a known issue or am I likely to 
>>>> have
>>>> mosconfigured something? I'm using 64-bit RHEL/CentOS 5 (both host and
>>>> guest).
>>>
>>> Please try to change the io scheduler on the host to io scheduler, it
>>> should boost your performance back.
>>
>> I presume you mean the deadline io scheduler. I tried that (kernel
>> parameter elevator=deadline) and it made no measurable difference
>> compared to the cfq scheduler.
> 
> What version of kvm do you use? Is it rhel5.4?

It's RHEL 5.4.

$ rpm -qa | grep -i kvm
kmod-kvm-83-105.el5_4.9
kvm-83-105.el5_4.9



> Can you post the qemu cmdline and the perf test in the guest?

Here is what is in the libvirt log:

For IDE emulation:

LC_ALL=C PATH=/sbin:/usr/sbin:/bin:/usr/bin HOME=/root USER=root 
LOGNAME=root /usr/libexec/qemu-kvm -S -M pc -m 2048 -smp 4 -name 
RHEL_5_x86-64 -uuid cb44b2c5-e64b-848f-77af-f8e7f02fa2ca 
-no-kvm-pit-reinjection -monitor pty -pidfile 
/var/run/libvirt/qemu//RHEL_5_x86-64.pid -boot c -drive 
file=/var/lib/libvirt/images/RHEL_5_x86-64.img,if=ide,index=0,boot=on 
-net nic,macaddr=54:52:00:5a:67:4b,vlan=0,model=e1000 -net 
tap,fd=15,script=,vlan=0,ifname=vnet0 -serial pty -parallel none -usb 
-vnc 127.0.0.1:0 -k en-gb

For virtio:

LC_ALL=C PATH=/sbin:/usr/sbin:/bin:/usr/bin HOME=/root USER=root 
LOGNAME=root /usr/libexec/qemu-kvm -S -M pc -m 2048 -smp 4 -name 
RHEL_5_x86-64 -uuid cb44b2c5-e64b-848f-77af-f8e7f02fa2ca 
-no-kvm-pit-reinjection -monitor pty -pidfile 
/var/run/libvirt/qemu//RHEL_5_x86-64.pid -boot c -drive 
file=/var/lib/libvirt/images/CentOS_5_x86-64.img,if=virtio,index=0,boot=on 
-net nic,macaddr=54:52:00:5a:67:4b,vlan=0,model=e1000 -net 
tap,fd=15,script=,vlan=0,ifname=vnet0 -serial pty -parallel none -usb 
-vnc 127.0.0.1:0 -k en-gb

The test is building the Linux kernel (only taking the second run to 
give the test the benefit of local cache):

make clean; make -j8 all; make clean; sync; time make -j8 all

This takes about 10 minutes with IDE disk emulation and about 13 minutes 
with virtio. I ran the tests multiple time with most non-essential 
services on the host switched off (including cron/atd), and the guest in 
single-user mode to reduce the "noise" in the test to the minimum, and 
the results are pretty consistent, with virtio being about 30% behind.

> Lastly, do you use cache=wb on qemu? it's just a fun mode, we use 
> cache=off only.

I don't see the option being set in the logs, so I'd guess it's whatever 
qemu-kvm defaults to.

Gordan