Data set initialization

During initialization, the active subsystem can start but it is not XRF-capable if it does not find two MODSTAT data sets (unless global online change is enabled, in which case the MODSTAT data sets are not needed), two restart data sets (RDSs), and, if you used IMS Fast Path, four MSDBCPn data sets.

The alternate subsystem, however, cannot be started unless it finds these data sets on shared DASD.

Both the active subsystem and the alternate subsystem allocate the latest MODSTAT and RDS, but the alternate subsystem switches to the unused pair during startup. The active subsystem uses the last active MSDBCPn pair, and the alternate subsystem uses the other two.

In a shared-queues environment, the active IMS subsystem does not need the message-queue data sets, but the alternate subsystem IMS subsystem does use these data sets because the alternate subsystem does not participate in sharing the IMS message queues until it takes over the active IMS subsystem workload.