Re: [HACKERS] PostgreSQL block size vs. LVM2 stripe width

[markw@osdl.org]

On 30 Mar, Manfred Koizar wrote:
> On Mon, 29 Mar 2004 08:50:42 -0800 (PST), markw@osdl.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