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(a)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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