Periodic casting out of changed data items from the structure can improve the likelihood that the system can reclaim storage resources for new requests. The number of data items that can be defined to the cache structure at any given time is finite. If you try to define a new data item to the cache structure or increase the size of an existing data entry, and there is insufficient unused cache structure storage available for the new data item, the system attempts to reclaim storage.

The system attempts to reclaim storage from unchanged data items stored in the cache structure. When you cast out data items and release the lock, the data items are considered unchanged and available for reclaim. If there are no unchanged data items, or insufficient storage is available from unchanged data items, the request fails.

How you cast out data items depends on the cast-out class you assign to each data item, and how your protocol uses these cast-out class assignments. For information on how to assign cast-out classes and on developing cast-out protocol, see Casting out Data Items and Reclaim Processing.

It is also important to cast out changed data from the cache structure because the changed data might be lost if the coupling facility or structure fails. The more often you cast out changed data, the fewer changed data items you lose if a failure occurs, and you thus minimize the amount of recovery processing you need to perform.