Coupling facility: At least two coupling facilities at CFLEVEL=11 or higher are required. IBM® recommends a minimum of three coupling facilities at CFLEVEL=11 or higher so that if one coupling facility is removed for maintenance, there will still exist two others to contain the duplexed pair of structures. The coupling facilities must all be defined in the preference list for the structure to be duplexed. You may need to add additional coupling facility storage resources and processor capacity, as well as additional z/OS® to CF link capacity, because two structure instances will be allocated for an extended period of time and for each duplexed coupling facility operation, two operations will be driven, one for each structure.

See Understanding the coupling facility level (CFLEVEL) for sizing and other considerations before migrating to any new CFLEVEL.

• CFLEVEL=16 to CFLEVEL=16 duplexing: You can improve duplexing performance by duplexing between two coupling facilities at CFLEVEL=16 or higher. See the web page at http://www.ibm.com/systems/z/advantages/pso/cftable.html for information on what server models support CFLEVEL 16. To take advantage of this performance optimization, you must enable the optional DUPLEXCF16 function using the COUPLExx FUNCTIONS statement (see z/OS MVS Initialization and Tuning Reference for more information) or the SETXCF FUNCTIONS command (see z/OS MVS System Commands for more information).

CF-to-CF Links: CF-to-CF links provide connectivity between a pair of coupling facilities in which a duplexed pair of structure instances are to be allocated. The link connectivity between the coupling facilities must be bidirectional, and there should be more than one link providing CF-to-CF connectivity in each direction for highest availability.