Finding target segments
When IMS accesses the target segment from the secondary index entry or logical child segment, it must first determine the partition in which the target resides.
IMS uses the key in the secondary index or logical child to determine the partition. Next it must determine the location in the target partition database data set. It compares the partition ID and reorganization number of the target partition with the partition ID and reorganization number stored in the EPS. If they match, IMS uses the RBA in the EPS to locate the target segment. If they do not match, the RBA in the EPS cannot be used.
When the RBA in the EPS cannot be used, IMS uses the information in the ILE to locate the target segment. The ILE key is found by using the ILK from the EPS and the target's segment code. The ILE is read from the ILDS of the partition determined from the target's key.
The following figure illustrates a situation in which the RBA in the EPS cannot be used. In the figure, the target partition has been reorganized three times since the EPS was accurate. This has moved the target segment and updated the reorganization number in the partition data set. The EPS still contains a reorganization number of 5, but the reorganization number in the partition data set is now 8. The information in the ILE has been updated by the HD Reorganization Reload utility. IMS uses the ILK from the EPS to find the ILE and uses the RBA in the ILE to find the target segment.
Even though the retrieval is indirect, often the CI containing the ILE will already be in an IMS buffer pool.
Recommendation: If possible, avoid the indirect process of locating target segments. Instead, get the target segment location from the EPS without reading the ILE. The self-healing process allows IMS to limit the use of ILEs.