David Sciaraffa explains that DB2 9 uses autonomic algorithm to determine the number of prefetchers (IO_SERVERS) and page-cleaners (IO_CLEANERS) to determine the optimum values and they are described here:

Prefetchers: IO_SERVERS

Page Cleaners: IO_CLEANERS

Page-cleaners aren’t affected by the SAN configuration, but prefetchers are. The DB2_PARALLEL_IO registry variable will need to be set manually to accommodate this, since the autonomic algorithm doesn’t. Basically the DB2_PARALLEL_IO registry variable should be set to reflect the number of physical spindles in extentsize underneath a single container. For example, if a table space has 2 identical containers each with 3 spindles with stripe size = extentsize, then,

db2set DB2_PARALLEL_IO=:3

For more info regarding this registry variable, please see this link.