The JES2 checkpointing mechanism preserves processing information during a program's operation to allow that processing to be restarted or duplicated. Your checkpoint configuration can be set up in either of two modes: DUPLEX or DUAL. DUPLEX mode processing allows MAS configuration members to maintain a primary checkpoint data set (CKPT1) and, optionally, a backup checkpoint data set (CKPT2). DUAL mode processing uses the same two checkpoint data sets in a “flip-flop” scheme designed to aid system performance. DUAL mode processing is recommended for installations with high checkpoint data set I/O.

If one or both checkpoint data sets at your installation reside on a coupling facility structure instead of on DASD, you cannot use DUAL mode; JES2 processing forces the mode to DUPLEX. For a more detailed discussion of both DUPLEX and DUAL mode processing, refer to z/OS JES2 Initialization and Tuning Guide, SA32-0991.