Re: PostgreSQL block size vs. LVM2 stripe width

On 30 Mar, Manfred Koizar wrote:
> On Mon, 29 Mar 2004 08:50:42 -0800 (PST), markw(at)osdl(dot)org wrote:
>>In this case, I've only done 1 per each combination. I've found the
>>results for this test to be reproduceable.
>
> Pardon?

I haven't repeated any runs for each combination, e.g. 1 test with 16kb
lvm stripe width and 2kb BLCKSZ, 1 test with 16kb lvm stripe width and
4kb BLCKSZ...

>>>> Linux-2.6.3, LVM2 Stripe Width
>>>>BLCKSZ
>>>>(going down) 16 KB 32 KB 64 KB 128 KB 256 KB 512 KB
>>>>2 KB 2617 2656 2652 2664 2667 2642
>>>>4 KB 4393 4486 4577 4557 4511 4448
>>>>8 KB 4337 4423 4471 4576 4111 3642
>>>>16 KB 4412 4495 4532 4536 2985 2312
>>>>32 KB 3705 3784 3886 3925 2936 2362
>
>>> Does this mean that you first ran all test with 8 KB, then with 4, 2, 16
>>> and 32 KB BLCKSZ? If so, I suspect that you are measuring the effects
>>> of something different.
>>
>>Yes, that's correct, but why do you suspect that?
>
> Gut feelings, hard to put into words. Let me try:
>
> Nobody really knows what the "optimal" BLCKSZ is. Most probably it
> depends on the application, OS, hardware, and other factors. 8 KB is
> believed to be a good general purpose BLCKSZ.
>
> I wouldn't be surprised if 8 KB turns out to be suboptimal in one or the
> other case (or even in most cases). But if so, I would expect it to be
> either too small or too large.
>
> In your tests, however, there are three configurations where 8 KB is
> slower than both 4 KB and 16 KB. Absent any explanation for this
> interesting effect, it is easier to mistrust your numbers.
>
> If you run your tests in the opposite order, on the same hardware, in
> the same freshly formatted partitions, and you get the same results,
> that would be an argument in favour of their accurancy.
>
> Maybe we find out that those 1.5% are just noise.

I did reformat each partition between tests. :) When I have tested for
repeatability in the past I have found results to fluxuate up to 5%, so
I would claim the 1.5% to be noise.

Mark