VSAM KSDS CI reclaim for full-function databases

In data-sharing or XRF environments, IMS can reclaim the storage used for empty VSAM KSDS control intervals (CIs) in full-function databases by using a process called CI reclaim.

CI reclaim enhances the performance of database GN, GU, and ISRT calls by reducing the number of empty CIs that VSAM reads on subsequent DL/I calls.

After IMS commits the deletion of the last record in a CI, IMS flags the CI for reclamation. IMS reclaims the CI the next time the CI is read during a DL/I call that has update access. On a single DL/I call, IMS reclaims every empty CI flagged for reclamation that it reads until the DL/I call has been satisfied.

CI reclaim is not a replacement for the routine reorganization of KSDS data sets.

CI reclaim works only under the following circumstances:

If you are using VSAM subpools, you have specified ERASE=YES, or accepted it as the default, on the DBD statement of the DFSVSMxx PROCLIB member.
Your KSDS uses unique keys.
The application program issuing the GN, GU, or ISRT call has update access to the database. Using a processing option PROCOPT of "I", "R", "D" or "A" guarantees the data set is open for update.

Restriction: SHISAM databases do not support CI reclaim. When a large number of records in a SHISAM database are deleted, particularly a large number of consecutive records, serious performance degradation can occur. Eliminate empty CIs and resolve the problem by using VSAM REPRO.

Related Reading: For information about the VSAM REPRO command, see z/OS® DFSMS Access Method Services for Catalogs.