For databases that are directly accessed by the user (rather than behind the 
scenes by a background server) you could add an initialization script in front 
of the access session, where the user would be aware that the database content 
would have to be staged to disk before being usable.  However, there isn't a 
real net gain in such a scheme in that you would have to maintain an HSM file 
system "window" large enough to accommodate the full size of the database for 
it to be recallable, meaning that you're always committing that much space 
anyway.

   Richard Sims