Linux mis-reporting memory

Sorry, I know this is probably more a linux question, but I'm guessing
that others have run into this...

I'm finding this rather interesting report from top on a Debian box...

Mem: 32945280k total, 32871832k used, 73448k free, 247432k buffers
Swap: 1951888k total, 42308k used, 1909580k free, 30294300k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
12492 postgres 15 0 8469m 8.0g 8.0g S 0 25.6 3:52.03 postmaster
7820 postgres 16 0 8474m 4.7g 4.7g S 0 15.1 1:23.72 postmaster
21863 postgres 15 0 8472m 3.9g 3.9g S 0 12.4 0:30.61 postmaster
19893 postgres 15 0 8471m 2.4g 2.4g S 0 7.6 0:07.54 postmaster
20423 postgres 17 0 8472m 1.4g 1.4g S 0 4.4 0:04.61 postmaster
26395 postgres 15 0 8474m 1.1g 1.0g S 1 3.4 0:02.12 postmaster
12985 postgres 15 0 8472m 937m 930m S 0 2.9 0:05.50 postmaster
26806 postgres 15 0 8474m 787m 779m D 4 2.4 0:01.56 postmaster

This is a machine that's been up some time and the database is 400G, so
I'm pretty confident that shared_buffers (set to 8G) should be
completely full, and that's what that top process is indicating.

So how is it that linux thinks that 30G is cached?
--
Decibel!, aka Jim C. Nasby, Database Architect decibel(at)decibel(dot)org
Give your computer some brain candy! www.distributed.net Team #1828