Re: IO on guest is 20 times slower than host

Avi Kivity
2009-03-26T10:16:22Z

Kurt Yoder wrote:
>
> I see. I looked up another test: using hdparm -t. It doesn't show the
> situation as quite so bad, but the guest is still a little over half
> the speed of the host:
>
> me@host:~$ sudo hdparm -t /dev/mapper/HW_RAID-ROOT
>
> /dev/mapper/HW_RAID-ROOT:
> Timing buffered disk reads: 282 MB in 3.00 seconds = 93.92 MB/sec
>
> me@guest:~# hdparm -t /dev/vda
>
> /dev/vda:
> Timing buffered disk reads: 156 MB in 3.03 seconds = 51.56 MB/sec

This is reasonable. IDE emulation is not expected to be as fast as the
host.

>> Something weird is happening with your system. If you extend the
>> test, what does 'top' show? On both guest and host.
>
> If I extend the test thusly on the guest:
>
> dd if=/dev/zero of=/bigfile count=10000000
>
> I see 100% CPU utilization on the guest, and 100% CPU utilization on
> one of the host cores.

Can you post 'vmstat 1' snippets on both guest and host while this is
running?

Oh, and try switching to the deadline I/O scheduler on the host instead
of the default cfq.

--
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