Re: [PATCH v2] kvm tools: Add QCOW level2 caching support

[Ingo Molnar <mingo@elte.hu>]

* Ingo Molnar <mingo@elte.hu> wrote:

> This introduces a ~1msec delay and thus simulates IO, but the 
> delays are *constant* [make sure you use a high-res timers kernel], 
> so they do not result in nearly as much measurement noise as real 
> block IO does.
> 
> The IO delays will still be there, so any caching advantages (and 
> CPU overhead reductions) will be measurable very clearly.
> 
> This way you are basically 'emulating' a real disk drive but you 
> will emulate uniform latencies, which makes measurements a lot more 
> reliable - while still relevant to the end result.
> 
> So if under such a measurement model you can prove an improvement 
> with a patch, that improvement will be there with real disks as 
> well - just harder to prove.

Another risk that the current situation carries in itself, beyond 
making it more difficult to measure improvements, is that based on a 
"bad" Bonnie outlier or artifact you might throw away a perfectly 
good change accidentally!

So whenever you think you are fighting noise you need to improve your 
measurements, as entropy is a pretty tough opponent to beat.

Thanks,

	Ingo