Re: Constrained virtual machines
Richard Jones <rich <at> annexia.org>
2006-12-01 11:09:38 GMT
On Fri, Dec 01, 2006 at 10:44:02AM +0000, Richard Huxton wrote:
> Richard Jones wrote:
> >Be aware that disk performance under any VM I've seen sucks compared
> >to the real hardware ... So if you're testing a database, where disk
> >I/O performance really matters, it's not clear the test will be fair.
> Ah, but I don't care if disk access is 10 times slower than for-real. So
> long as I can then scale CPU and memory by the relevant amount to keep
> things balanced.
Actually I'm afraid that the effects might be more subtle than that.
Remember that disk I/O isn't just "XX megabytes/sec". It's a complex
mixture of types of operation, each having their own latency,
throughput and other peculiarities. I would expect that if issuing
commands directly over SATA to a directly connected disk you would
expect to see an entirely different set of results from issuing
commands through a hypervisor into a contended host domain and then
down to disk. The latencies I would expect to be considerably
greater, but effects like disk seek times would be lost in the noise,
and throughput would be reduced by only a constant factor. Be
interesting to see some real world tests through ...
 But cannot prove - ask Dan Berrange, he's the local Xen guru
Richard Jones, CTO Merjis Ltd.
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