The structure alter function can expand or contract a structure within the range of its maximum and minimum size. The maximum size (MSS) of a structure is set at the time of structure allocation and remains constant as long as this instance is allocated. The structure allocation algorithm determines the MSS based on whether the structure is allocated by counts or by size/ratios. The actual structure size may be less than or equal to the MSS value. The minimum size is set by the coupling facility, and is determined by calculating the minimum amount of space required for the coupling facility to allocate the structure with the specified entry-to-element ratio. See Specifying the Structure Size for a discussion about the maximum and minimum structure size.

Structure alter cannot change the maximum size of a structure. However, the minimum structure size could change if you reapportion the structure with an entry-to-element ratio that is different from the previous ratio or with an EMC storage percentage that is different from the previous percentage.

For example, when allocating the structure, the coupling facility 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. The system will process a request to alter a structure to the marginal structure size value only if a change to the structure's entry-to-element ratio is permitted. When such a structure alter occurs, the minimum structure size value also changes to the altered structure size.