Purpose of Database Reorganization

Database reorganization is the process of changing how the data in the database is organized to improve performance. There are two types of database reorganization: physical reorganization (to optimize the physical storage of the database) and restructuring (to alter the database structure).

The most common reasons to reorganize a database are:

• To reclaim and consolidate free space that has become fragmented due to repeated insertion and deletion of segments.

• To optimize the physical storage of the database segments for maximum performance (get dependent segments that are in distant blocks, which increases physical I/O, back in the same block as the parent or root), a situation that is normally the result of high update activity on the database.

• To alter the structure of the database, change the size of the database data sets, alter the HDAM root addressable area, and add or delete segment types.

The solution to the first two reasons is reorganization. The solution to the last reason is restructuring.

The need for reorganization is always due to change (for example, setting up a new database, amending the structure of the database as application requirements change, or as a result of update activity against the database). If you do not update a database, then after you have it in an optimum state for performance, there is no further need to reorganize it.

Reorganizing and restructuring the databases is only part of the process of tuning and monitoring access to IMS databases. You can also tune IMS DB itself and the applications that access the databases. These topics and more are covered in more detail in:

• IMS Version 9: Administration Guide: Database Manager.

• The IBM Redbook IMS Version 7 Performance Monitoring and Tuning Update.