Re: PostgreSQL block size vs. LVM2 stripe width

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?

>>> 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.

Servus
Manfred