Compression of large objects is also a good idea, but I'm not sure how
it would be implemented, or how it would affect reads/writes (you
can't really seek with zlib, which is what I would use).
Look at lzo realtime compression utility at:
http://wildsau.idv.uni-linz.ac.at/mfx/lzop.html
It is extremly fast. I have to support first class Austrian technology ;-)
Andreas