postgresql, max_fsm_pages on FreeBSD 5.3

[Claus Guttesen]

Hi.

I'm running a postgresql 7.4.6 db, where the largest
table having 13 mill. records. The db-server is a
quad-opteron with 4 GB RAM.

I've configured shared_buffers to 8192, sort_mem to
4096 and effective_cache_size to 4096.

max_fsm_relations and max_fsm_pages was easy to
configure when I found
http://www.desknow.com/kb/idx/12/061/article/
(max_fsm_pages and max_fsm_relations). The odd thing
is that according to a vacuum verbose where total
pages needed always seems to be approx. 15.000 pages
larger than what max_fsm_pages is configured to.

So I keept increasing max_fsm_pages, from 60000, 80000
and now 100000. This morning my beloved crontab send
me a verbose vacuum telling me "114848 total pages
needed".

max_fsm_relations remains steady at 300 (245 relations
according to the output).

Not that I mind increasing this value, but is there
any limit to this value, or will I reach some
thresshold where this setting will become
counterproductive?

The docs says that max_fsm_pages "Sets the maximum
number of disk pages for which free space will be
tracked in the shared free-space map." But what does
that actually mean?

Postgresql does seem to perform better when I tweak
max_fsm_pages.

The above mentioned shared buffers, sort mem and
effective cache size did become counterproductive when
I increased them too much.

regards
Claus