Erasing records from a KSDS may result in space for empty CAs that is not reclaimed. Over time, this can produce DASD space that is fragmented and performance that is degraded due to an index structure that is unnecessarily complicated by the empty CAs. To reclaim the empty CA space and streamline the index structure, you can reorganize the KSDS. To reduce the need for reorganizing a KSDS, you can cause empty CA space on DASD to be reclaimed automatically, so that it may be reused at a later time when a CA split is required. The CAs that have been reclaimed are available to be used for new records without any processing to obtain new space. A CA split that reuses a reclaimed CA will not change the high-used RBA.

CA reclaim provides for improved DASD space usage, but it requires additional I/O to keep track of the reclaimed CAs so that they can be reused. The cost of this I/O may not be justified if there are no or very few CA splits to reuse empty CAs.

To determine if CA reclaim is desirable for a data set, use the EXAMINE DATASET command, which shows the number of empty CAs in a KSDS with message IDC01728I. For more information about the EXAMINE DATASET command, see EXAMINE Command. For a description of message IDC01728I, see z/OS MVS System Messages, Vol 6 (GOS-IEA).