why my systems cache only ~900 MB?

On a (32 bit) system with more than 1 GB memory it is impossible to cache more than about 900 MB. Why?

I have a system with 2 GB which I use as a SAN machine, on which I would like to use as much as possible for caching.

However, caching never goes beyond ~900 MB (i.e. when I read a mounted drive with dd):

# free
total used free shared buffers cached
Mem: 2076164 954444 1121720 0 860560 54076
-/+ buffers/cache: 39808 2036356
Swap: 2097144 0 2097144


Is it some kernel setting that I'm missing?

--
[wpkg.org]

Are you using a 2.4 kernel by any chance?

Any 2.6 kernel should be able to put the NFS cache in highmem.