[Gllug] Constrained virtual machines

[Richard Jones]

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[1] 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 ...

Rich.

[1] But cannot prove - ask Dan Berrange, he's the local Xen guru :-)

-- 
Richard Jones, CTO Merjis Ltd.
Merjis - web marketing and technology - http://merjis.com
Internet Marketing and AdWords courses - http://merjis.com/courses - NEW!
Merjis blog - http://blog.merjis.com - NEW!
-------------- next part --------------
-- 
Gllug mailing list  -  Gllug at gllug.org.uk
http://lists.gllug.org.uk/mailman/listinfo/gllug