Josh Berkus <josh@agliodbs.com> writes:
> I'm not clear on where the memory needed by max_fsm_pages comes from.
> Is it taken out of the shared_buffers, or in addition to them?
In addition to. Basically max_fsm_pages and max_fsm_relations are used
to ratchet up the postmaster's initial shared memory request to the kernel.
> Further, Joe Conway gave me a guesstimate of 6k per max_fsm_pages
> which seems rather high ...
Quite ;-). The correct figure is six bytes per fsm_page slot, and
I think about forty bytes per fsm_relation slot (recent versions of
postgresql.conf mention the multipliers, although for some reason
the Admin Guide does not).
regards, tom lane