Re: Shared Buffers

I had the same quetions more or less. I have 256Mb ram and set it according to the 1/4 rule to ~64M
(shared_buffers=7500)after raising the /proc/sys/kernel/shmall and shmmax to 64M 
Today I decided to go for double the money, and switched to 128M shmem and restarted postgres with shared_buffers=15000
(8192*15000=~120M).I didn't notice any difference :) 
I assume that by "shared buffers" Pg means the area where common data is stored and by common data it means "index
blocks"(can't think what else can be shared between processes/queries). So unless you get many users hitting different
tables,you won't notice a difference.  

Is there a shred of truth in this or am I...?

cheers,
thalis


On Thu, 21 Jun 2001, Dennis wrote:

>
> When starting postmaster, you can add the -B option to indicate the amount
> of shared buffers you want postgres to use.  You can also specify the value
> in postgresql.conf.
>
> I read in the documentation that 1 shared buffer is 8192 bytes.   Somone
> posted a while back that it was most optimal to let postgres have about 1/4
> of the memory on a dedicated machine.
>
> So, I have 512 megs on a dedicated machine.  I divide that by 4 and then
> times the number of bytes in a meg and divide by 8192 and I get 16384.
> By the 1/4 standard, it seems I should set "-B 16384" at postgres startup.
>
> Postgres defaults to 64 Shared Buffers.  That is a big difference from
> 16384.  Is there any reason to my logic here or am I way off?
>
> Can anyone supply some more information or maybe suggest the most
> optimum way to use Shared Buffers.  Is the 1/4 standard good or should
> I use even more.. or much less?
>
> Thanks in advance.
> Dennis
>
> ---------------------------(end of broadcast)---------------------------
> TIP 3: if posting/reading through Usenet, please send an appropriate
> subscribe-nomail command to majordomo@postgresql.org so that your
> message can get through to the mailing list cleanly
>