Requesting structure size

The size of a coupling facility structure includes the total amount of storage required by both the application and by the coupling facility itself for structure control information, and is specified in size units of kilobytes, megabytes, gigabytes or terabytes. Determining the correct size is based on the application's use of the structure, characteristics of the workload environment in which the structure is to be used, and the CFLEVEL of the coupling facility in which the structure is allocated. The application provides guidance so that you can estimate an appropriate size for a structure. The IBM Z® Coupling Facility Structure Sizer is also available to help estimate the amount of storage required for a structure. (See Coupling Facility sizer for more details.) You should use RMF reports to monitor the structure's utilization. Also, you can use structure full monitoring to monitor the structure utilization. The combined functions of structure full monitoring and automatic structure altering allow the system to tune the size of the structure when it reaches a full threshold.

The size of a structure can be specified by the application in its code or by the installation in its CFRM policy. The size that you specify in the CFRM policy with the SIZE specification is the requested maximum structure size. (The system might override this maximum size during rebuild processes. See How MVS initially allocates the structure.) You can change the structure size with the ALTER function if the application has specified that ALTER is allowed. ALTER allows you to vary the structure size between an initial size (specified by INITSIZE in the CRFM policy) and the maximum size. The coupling facility determines the minimum structure size, the minimum amount of coupling facility space required to allocate the structure with the specified entry-to-element ratio. It is possible to have the system automatically alter the size of a structure when the system determines that the structure has surpassed a “structure full” threshold. In addition to the application specfiying that the structure is able to be altered, the installation must indicate in the CFRM policy that the system is to monitor the structure and automatically alter its size when required. The MINSIZE specification in the CFRM policy indicates the smallest size to which the structure can be automatically altered.