Re: effective cache size on FreeBSD (WAS: Performance on SUSE w/

On Tue, 2005-10-11 at 16:54 +0200, Claus Guttesen wrote:
> > > I have a postgresql 7.4.8-server with 4 GB ram.
> > > #effective_cache_size = 1000 # typically 8KB each
> > >
> > > This is computed by sysctl -n vfs.hibufspace / 8192 (on FreeBSD). So I
> > > changed it to:
> > >
> > > effective_cache_size = 27462 # typically 8KB each
> >
> > Apparently this formula is no longer relevant on the FreeBSD systems as
> > it can cache up to almost all the available RAM. With 4GB of RAM, one
> > could specify most of the RAM as being available for caching, assuming
> > that nothing but PostgreSQL runs on the server -- certainly 1/2 the RAM
> > would be a reasonable value to tell the planner.
> >
> > (This was verified by using dd:
> > dd if=/dev/zero of=/usr/local/pgsql/iotest bs=128k count=16384 to create
> > a 2G file then
> > dd if=/usr/local/pgsql/iotest of=/dev/null
> >
> > If you run systat -vmstat 2 you will see 0% diskaccess during the read
> > of the 2G file indicating that it has, in fact, been cached)
>
> Thank you for your reply. Does this apply to FreeBSD 5.4 or 6.0 on
> amd64 (or both)?
>

Not sure about 6.0 (but I don't know why it would change) but definitely
on 5.4 amd64 (and I would imagine i386 as well).

Sven