Whenever you write a data item to the cache structure, the system first attempts to allocate cache structure resources (a data entry, a directory entry, or both) that are unallocated or not in use. When there are insufficient resources available, the system attempts to reclaim currently allocated resources in the cache structure to satisfy the request.

Data items that are marked changed or that are locked for cast-out cannot be reclaimed. If the system cannot reclaim sufficient resources to satisfy the request, the request fails.