The structure of VSAM prime indexes for a KSDS and a VRRDS is built to create a single index record at the lowest level of the index, the sequence set, to provide pointers to each CI within a single CA. Each entry contains a compressed value representing the highest key that can be contained within that control interval. The value stored for the control interval containing records with the highest key in that control area represents the highest record-key value that can be contained in that control area. Once all the records are deleted from any single control interval, the current high-key value is no longer associated with that control interval's entry in the sequence set record. It becomes a “free” control interval in which records containing any key within the range of keys for that control area can be inserted. This is called a CI reclaim.

The last empty control interval within the control area is not CI reclaimed. The high-key value for that control interval is maintained and it becomes the highest key for any record that can be inserted into that control area.When CA reclaim is not enabled, a KSDS may have many empty control areas and may continue to grow in size. This will result when applications continually add records with keys that are in ascending sequence, followed by another or the same application that deletes old records. During the deletion processing, the high-key value that was associated with that CA will be maintained, requiring that only records falling within that high-key range are eligible for insertion into that control area. If the record keys are always getting higher, no new records will qualify for insertion into those empty control areas. The result is a data set in which a majority of the space is occupied by empty control intervals.

One option a user has to reclaim this space is to reorganize the data set. This will require a logical copy of the data set, followed by a deletion of the old data set and a reload operation from the logical copy. However, such data set reorganization is time consuming. To minimize the need for reorganizing data sets, use the CA reclaim function. With CA reclaim, empty CA space is reclaimed automatically and CA split processing uses control areas that have been reclaimed instead of using space at the end of the data and index. For more information, refer to Reclaiming CA Space for a KSDS.