buffer cache usage

[Richard Kearsley]

Hi
I'm using FreeBSD and nginx as a web server serving static files. I 
always like the "buffer cache" which lets frequently used files stay in 
memory rather than read from disk again.
I have a new server with 32GB of ram and I notice a lot of memory is 
listed as "Inact":

Mem: 688M Active, 26G Inact, 3758M Wired, 823M Cache, 3281M Buf, 173M Free

What's preventing this ram from being used as buffer cache for files on 
the hard disks? Is there a limit or a tunable stopping it? I never had a 
server with this amount of ram before.
It's a very busy server, I'm sure a lot of files are accessed regularly 
as the disks are always busy.

Using 9.1 amd64:
FreeBSD richard 9.1-RELEASE FreeBSD 9.1-RELEASE #0 r243825: Tue Dec 4 
09:23:10 UTC 2012 
root at farrell.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC amd64

Many thanks