When a table space is created, PREFETCHSIZE n can be optionally specified, where n is the number of pages the database manager will read if prefetching is being performed. If you do not specify the prefetch size on the CREATE TABLESPACE statement, the database manager uses the value given by this parameter.
Recommendation: Using system monitoring tools, you can determine if your CPU is idle while the system is waiting for I/O. Increasing the value of this parameter may help if the table spaces being used do not have a prefetch size defined for them.
This parameter provides the default for the entire database, and it may not be suitable for all table spaces within the database. For example, a value of 32 may be suitable for a table space with an extent size of 32 pages, but not suitable for a table space with an extent size of 25 pages. Ideally, you should explicitly set the prefetch size for each table space.
To help minimize I/O for table spaces defined with the default extent size (dft_extent_sz), you should set this parameter as a factor or whole multiple of the value of the dft_extent_sz parameter. For example, if the dft_extent_sz parameter is 32, you could set dft_prefetch_sz to 16 (a factor of 32) or to 64 (a whole multiple of 32). If the prefetch size is a multiple of the extent size, the database manager may perform I/O in parallel, if the following conditions are true: