Operating RACF in data sharing mode (available only when RACF is enabled for sysplex communication) requires use of a coupling facility.

RACF sysplex data sharing is designed to address problems that can occur when many systems share a RACF database. RACF uses the coupling facility as a large sysplex-wide store-through cache for the RACF database that reduces contention and I/O to DASD. Serialization is done using global resource serialization instead of RESERVE/RELEASE while in data sharing mode. RACF sysplex data sharing also provides improved efficiency of invalidating resident data blocks. For more information on sysplex data sharing, see Sysplex considerations.

If you have a non-shared database, you can still take advantage of the possible performance improvement that the coupling facility offers. See Using the coupling facility with a single MVS image for details.