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.

Related topic

Coupling-facility-usage-and-configuration

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 BMC AMI Ops Automation 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.

Warning

Do not create overly large structures or structures that are less than 60% utilized.

For example, do not create a 20,000 KB structure or allocate a structure for 20,000 sysplex variables when you will need only a maximum of 200 sysplex variables.

Such large or under utilized-structures will consume significant amounts of CPU and storage. Based on the size of the structure, the internal code in the coupling facility will allocate an equal number of control blocks, and also search or scan that same number of blocks even though very few sysplex variables have been created.

 

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