Re: IO on guest is 20 times slower than host

Kurt Yoder
2009-03-25T17:11:50Z

On Mar 25, 2009, at 12:54 PM, Avi Kivity wrote:

> Kurt Yoder wrote:
>>
>> I upped the memory on the VM to 1 GB and ran the same test. It
>> actually got *slower*:
>>
>> me@guest:~$ free
>> total used free shared buffers
>> cached
>> Mem: 1024224 131324 892900 0 1816
>> 18328
>> -/+ buffers/cache: 111180 913044
>> Swap: 488248 0 488248
>> me@guest:~$ sudo -s
>> me@guest:~# dd if=/dev/zero of=/bigfile count=100000
>> 100000+0 records in
>> 100000+0 records out
>> 51200000 bytes (51 MB) copied, 22.2141 s, 2.3 MB/s
>>
>
>
> Wierd. Can you change the disk interface to ide to rule out virtio
> issues?
>
> (though dd should be running entirely in cache)
>
> Is your disk a raw volume or qcow file?

It was actually set as an ide disk before it was a virtio disk. I
switched it to virtio to try to get rid of this problem, but no luck
so far.

The disk lives on an LVM logical volume.

What do you mean about the cache? Is my test fundamentally flawed? I
*thought* I was testing write speed on the disk...

-Kurt