When allocating the structure, the coupling facility also determines the marginal structure size — the true minimum size at which the structure can be allocated. The marginal structure size is less than the minimum structure size and does not take into consideration the entry-to-element ratio or the percentage of storage used for event monitor controls. Thus, when allocating the storage in the structure, the percentage specified for event monitor controls is applied to the storage in the structure that is available beyond that designated as the marginal size. The entry-to-element ratio is applied to the storage that is available after the percentage for event monitor controls has been determined.

Should there not be enough space in a coupling facility to allocate the structure with the requested size, the system allocates the structure in the coupling facility in the preference list with the most available space, which satisfies the largest set of other allocation requirements. For a lock structure, the allocation fails if a large enough area in a coupling facility cannot be found to support the number of lock entries required.

The coupling facility ensures that the size of a structure is a multiple of the coupling facility storage increment (see Coupling Facility Storage Increment). If not, the coupling facility rounds up the size value to be a multiple of the increment. Ultimately, the actual size of the structure allocated in a coupling facility is based on storage allocation priorities with which the coupling facility control code complies and on storage constraints in the coupling facility itself. See Coupling Facility Considerations When Allocating a Structure.

A connected user of the structure can determine the structure's size by examining the connect answer area for both the maximum structure size and the actual structure size (fields CONAMAXSTRUCTURESIZE and CONASTRUCTURESIZE). An operator can display a structure's size by issuing the DISPLAY XCF,STRUCTURE command.