In a CXRC environment, the SDMs communicate with each other using the master data set. At regular intervals, the SDM writes data to the master data set. This data includes the last consistency group written to the secondary volumes and the last consistency group written to the journal data sets. In the same I/O operation, the SDM reads the status of the last consistency group written to the secondary volumes and the last consistency group written to the journal data set by all other SDMs in the coupled sessions. Figure 1 illustrates the master data set concept.
There is a value in the tuning table that controls the frequency
of I/Os to the master data set. If the value is set too high, delays
will result that slowdown coupled SDM processing.
Figure 1. Master data set