The initial size allows you to enter RACF® data sharing with acceptable performance. Once you have done that, you might want to tune your system for optimal performance. Here are some things to consider as you tune your structure size.

It is often the case that a relatively small percentage of the profiles in a database account for a relatively high percentage of the database I/O which can occur when granting access to protected resources. This I/O can be reduced if space is provided in the coupling facility for these “high activity” profiles. (The reduction pertains to non-RACLISTed profiles only, because RACLISTed profiles do not require database I/O.) The reduction occurs because RACF looks in the coupling facility before going to the database. The situation is analogous to the well-known notion of working set, which has been used to describe the number of real storage page frames required to run a program without “thrashing”.

Providing coupling facility space beyond what is needed for the “high activity” profiles is less beneficial. For example, a data block containing a profile that is used only once requires I/O to bring it into the coupling facility. It is never referenced again, so nothing is gained.

One strategy for determining the optimum structure size is to monitor I/O rates to the RACF database and gradually increase the size of the structure until there is no significant reduction in the I/O rate.