Whereas DUPLEX-mode checkpoint processing uses two checkpoint data sets in a primary-duplex scheme (as described previously), DUAL-mode checkpoint processing uses the same two data sets in a “flip-flop” scheme. DUAL mode is recommended if your installation is suffering from degraded system performance because of high checkpoint data set I/O. The use of the change log provides reduced I/O to the checkpoint data set during most, if not all, update accesses by a member of your multi-access spool configuration.

Note that if any checkpoint data set resides on a Coupling facility structure, you cannot use DUAL mode.