Preventing segment splits
Compressed segments are stored in the database as variable-length segments to save space. Most database access methods store other segments physically adjacent to compressed segments. A compressed segment, however, may increase in length if it is modified, then replaced in the database. For full-function databases, IMS splits the segment data and its prefix. For data entry databases (DEDBs), IMS performs a delete/insert and relocates the segment. Both situations can cause database performance to degrade.
DATA PACKER/IMS provides the following two features to help prevent segment splits when compressed segments are replaced in the database:
- Minimum Segment Length (MSL)
- Fixed Pad Size (FPS)
Specifying MSL or FPS for a segment with high replacement activity can prevent frequent reorganizations. You can specify MSL or FPS for segment entries using the online or batch registration procedures. The value can be changed at any time. The change takes effect at database open time. The best time to change the MSL or FPS is when you perform a database reorganization with the unload in expanded format; in this way, you avoid adversely affecting database performance.