As others have already pointed out, your disk performance here is
completely typical of a single pair of drives doing random read/write
activity. So the question you should be asking is how to reduce the
amount of reading and writing needed to run your application. The
suggestions at
http://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server address
that. Increases to shared_buffers and checkpoint_segments in particular
can dramatically reduce the amount of I/O needed to run an application.
On the last server I turned, random reads went from a constant stream of
1MB/s (with default value of shared_buffers at 32MB) to an average of
0.1MB/s just by adjusting those two parameters upwards via those guidelines.
If you haven't already made large increases to those values, I'd suggest
starting there before presuming you must get a different disk setup.
--
Greg Smith 2ndQuadrant US Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com www.2ndQuadrant.us