Putting tables or indexes in SSD or RAM: avoiding double caching?

Hi,

I have sen many posts on using SSDs, and iodrive
<http://www.fusionio.com>in particular, to accelerate the performance
of Postgresql (or other DBMS)
-- e.g. this discussion<http://groups.google.co.il/group/pgsql.performance/browse_thread/thread/1d6d7434246afd97?pli=1>.
I have also seen the suggestion to use RAM for the same purpose by creating
a tablespace on a RAM mount
point.<http://magazine.redhat.com/2007/12/12/tip-from-an-rhce-memory-storage-on-postgresql/>Granted
these make most sense when the whole database cannot fit into main
memory, or if we want to avoid cold DB response times (i.e waiting for the
DB to "warm up" as stuff gets cached in memory).

My question is this: if we use either SSD or RAM tablespaces, I would
imagine postgresql will be oblevient to this and would still cache the
tablespace elemenst that are on SSD or RAM into memory - right? Is there a
way to avoid that, i.e. to tell postgress NOT to cache tablespaces, or some
other granularity of the DB?

Thanks,

-- Shaul

*Dr. Shaul Dar*
Email: info(at)shauldar(dot)com
Web: www.shauldar.com