Re: FreeBSD config

On Thu, Feb 26, 2004 at 11:55:31AM -0700, scott.marlowe wrote:
> On Thu, 26 Feb 2004, Dror Matalon wrote:
>
> > Hi,
> >
> > We have postgres running on freebsd 4.9 with 2 Gigs of memory. As per
> > repeated advice on the mailing lists we configured effective_cache_size
> > = 25520 which you get by doing `sysctl -n vfs.hibufspace` / 8192
> >
> > Which results in using 200Megs for disk caching.
> >
> > Is there a reason not to increase the hibufspace beyond the 200 megs and
> > provide a bigger cache to postgres? I looked both on the postgres and
> > freebsd mailing lists and couldn't find a good answer to this.
>
> Actually, I think you're confusing effective_cache_size with
> shared_buffers.

No, I'm not.

>
> effective_cache_size changes no cache settings for postgresql, it simply
> acts as a hint to the planner on about how much of the dataset your OS /
> Kernel / Disk cache can hold.

I understand that. The question is why have the OS, in this case FreeBsd
use only 200 Megs for disk cache and not more. Why not double the
vfs.hibufspace to 418119680 and double the effective_cache_size to 51040.

>
> Making it bigger only tells the query planny it's more likely the data
> it's looking for will be in cache.
>
> shared_buffers, OTOH, sets the amount of cache that postgresql uses. It's
> generall considered that 256 Megs or 1/4 of memory, whichever is LESS, is
> a good setting for production database servers.
>

Actually last I looked, I thought that the recommended max shared
buffers was 10,000, 80MB, even on machines with large amounts of memory.

Regards,

Dror

--
Dror Matalon
Zapatec Inc
1700 MLK Way
Berkeley, CA 94709
http://www.fastbuzz.com
http://www.zapatec.com