Coupling facility structure sizing
The maximum number of sysplex variables for your structure size can be seen by issuing the .D V command in the BBI Journal or the following MVS command to display the structure:
D XCF,STR,STRNM=mvao_bmcab
For example, a 2 megabyte (2,000 KB) structure on a 2817 (z196) machine can contain approximately 1,100 sysplex variables. A 5,000 KB structure can contain approximately 4,600 sysplex variables. It is difficult to estimate the number of long variables that can be stored because long variables are internally implemented as invisible chains of short variables.
The amount of overhead storage consumed in a structure is related to the CFLEVEL. Higher CF Levels (versions) with increased features tend to have more overhead and will therefore support a fewer number of sysplex variables for the same size structure.
IBM typically provides notice to customers when new CF levels incur more overhead, alerting customers to evaluate if existing structures need to be made larger.
You might choose to allocate a smaller or larger structure to suit their needs. The optimum-sized structure is one that supports the maximum number of sysplex variables needed with a safety margin for unexpected variables or an unexpected increase in coupling facility overhead.
When planning your number of sysplex variables, you should note that sysplex variables are slower and consume more resources than SHARED variables. Variables that are referenced in only one MainView AutoOPERATOR subsystem, or those that have static values, are best stored as SHARED variables.
You may need to slightly increase the size of the coupling facility structure when migrating to a new machine or upgrading the CFLEVEL on the same machine, if the amount of overhead storage consumed by the coupling facility is more than before.