Best disk caching method (and PGSQL performance)

[Jim C. Nasby]

After reading the thread at
http://www.freebsd.org/cgi/getmsg.cgi?fetch=25060+34014+/usr/local/www/db/text/2003/freebsd-performance/20030727.freebsd-performance
and the post at
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=utf-8&selm=199907290910.CAA06665%40implode.root.com
I'm a bit confused about how best to get FBSD to cache disk I/O on a
PostgreSQL server. Reading the VM article at
http://www.daemonnews.org/200001/freebsd_vm.html has certainly made
things a bit clearer, but I want to make sure I'm understanding things
before I go tweaking stuff.

The inactive queue is only comprised of pages that have been read in as
mapped memory, correct? I believe PGSQL does not make use of
memory-mapped I/O, so does that mean that the only place data reads are
being cached is in the buffer pool? (Note that this contradicts the
google link).

If that's indeed the case, then it seems like the only way to get a
decent amount of data caching is by increasing the buffer size (which
apparently means increasing kern.nbuf, which also means increasing
KVA_PAGES (though I'm not at all sure about this).

If this isn't the case, why have the buffer pool at all? Why not just
leave buffered disk pages in the inactive or cache queues?

On another note, does anyone know for certain that PGSQL on FBSD is
fsyncing only the WAL? I'm seeing a pretty large amount of activity on
my primary partition and I'm wondering if it's fsyncing everything.
Also, has anyone played with the other fsync options?
-- 
Jim C. Nasby, Database Consultant                  jim at nasby.net
Member: Triangle Fraternity, Sports Car Club of America
Give your computer some brain candy! www.distributed.net Team #1828

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