DB2 Universal Database™ for iSeries™ automatically spreads the data across the disk devices available in the auxiliary storage pool (ASP) where the data is allocated. This ensures that the data is spread without user intervention.
The spreading allows the database manager to easily process the blocks of rows on different disk devices in parallel. Even though DB2 Universal Database for iSeries spreads data across disk devices within an ASP, sometimes the allocation of the data extents (contiguous sets of data) might not be spread evenly. This occurs when there is uneven allocation of space on the devices, or when a new device is added to the ASP. The allocation of the table data space may be spread again by saving, deleting, and then restoring the table.
Maintaining an even distribution of data across all of the disk devices can lead to better throughput on query processing. The number of disk devices used and how the data is spread across these devices is taken into account by the optimizer while costing the different plan permutations.