For availability, a minimum of two physical paths must be provided between any two systems. This can be accomplished with two physical CTCs, structures in each of two different CFs, or a combination of CTCs and coupling facility structures.

Most environments will find the rate of XCF traffic can be handled by the two paths which were configured for availability. Only for environments with very high rates of XCF traffic would additional paths be required.

The XCF message rate capacity of a path is affected by many factors:

1. The size of the message
2. How the paths are defined
3. If the path is also used for other (non-XCF) functions

Based on these factors, message rates (XCF IN+OUT), have been observed from 1000/sec to 5000/sec on a CTC, up to 9000/sec via an ICB and up to 4000/sec per HiPerLink. The adage "Your mileage may vary" is certainly true here.

When using coupling facility structures for XCF messaging, there is also a cost in coupling facility CPU utilization to plan for. As an example, running 1000 XCF messages/sec through an R06 coupling facility would utilize approximately 10% of one coupling facility processor. Additionally, if you use coupling facility structures as XCF paths, make sure the structure size is adequate. You can use the CF Sizer tool available at http://www.ibm.com/systems/support/z/cfsizer/ to obtain an initial estimate for the structure size. If the structure is too small, you will see an increase in the number of REQ REJECT and AVG Q LNGTH, and these events will definitely affect response time.