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.
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.
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.