Re: shared_buffers performance

Gaetano Mendola wrote:
> Hi all,
> I started to do some performance tests (using pgbench) in order to
> estimate the DRBD impact on our servers, my plan was to perform some
> benchmarks without DRBD in order to compare the same benchmark with
> DRBD.
> I didn't perform yet the benchmark with DRBD and I'm already facing
> something I can not explain (I performed at the moment only reads test).
>
> I'm using postgres 8.2.3 on Red Hat compiled with GCC 3.4.6.
>
> I'm using pgbench with scaling factor with a range [1:500], my server
> has 4 cores so I'm trying with 16 client and 4000 transaction per
> client: pgbench -t 4000 -c 16 -S db_perf. I did 3 session using 3 different
> values of shared_buffers: 64MB, 256MB, 512MB and my server has 2GB.
>
> The following graph reports the results:
>
> http://img84.imageshack.us/my.php?image=totalid7.png
>
> as you can see using 64MB as value for shared_buffers I'm obtaining better
> results. Is this something expected or I'm looking in the wrong direction?
> I'm going to perform same tests without using the -S option in pgbench but
> being a time expensive operation I would like to ear your opinion first.

I have complete today the other benchmarks using pgbench in write mode as well,
and the following graph resumes the results:

http://img440.imageshack.us/my.php?image=totalwbn0.png

what I can say here the trend is the opposite seen on the read only mode as
increasing the shared_buffers increases the TPS.

I still didn't upgrade to 8.2.7 as suggested by Greg Smith because I would like
to compare the results obtained till now with the new one (simulations running
while I write) using postgres on a "DRBD partition"; sure as soon the current
tests terminate I will upgrade postgres.

If you have any suggestions on what you would like to see/know, just let me know.

Regards
Gaetano Mendola