Re: [libvirt-users] Yet another disk I/O performance issue

Matteo Lanati
2014-01-20T16:09:30Z

Hi Daniel,

thank you very much for your explanation.
Now every fits into the picture. I run the tests you suggested and the numbers
make much more sense.
Thanks for stating the obvious but it was the first time I needed to compare
I/O.
Best regards,

Matteo


On 20 Jan 2014, at 15:19, Daniel P. Berrange <berrange@redhat.com> wrote:

> 'dd' is rarely a meaningful performance benchmark.
>
> In particular with the way you're invoking it, you're not
requesting
> that the OS flush data to disk - so the data will be building up in
> RAM which is very fast. Since your host likely has much more free RAM
> for I/O cache than the guest, dd in the host is able todo much more
> I/O to fast RAM before the OS has to start actually writing to slow
> disk. The guest which is likely writing to slow disk much sooner giving
> the lower results.
>
> At the very least you need to request use of O_DIRECT with dd
> so it is writing straight to disk. Or try a formal benchmark tool
> like iozone
>
> Daniel
> --
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