DFSMShsm monitors the space used by the three control data sets and the journal data set and informs the system operator with a warning message when the space used exceeds a specified threshold. This threshold can be different for each control data set and different for the journal data set. You can express the threshold as a percentage with the SETSYS MONITOR command, or you can accept the DFSMShsm defaults (threshold of 80%). When the space that is used by the data set exceeds this percentage of total space, DFSMShsm issues warning message ARC0909E or ARC0911E to the operator. If no records are written because the MCDS, BCDS, or OCDS is full, DFSMShsm holds the requested function and issues warning message ARC0910E to the operator. Depending on the message, the operator can schedule either an increase in journal size or a reorganization of the control data sets.

Space information is obtained for each control data set and for the journal data set. This information includes the total space allocated for each data set and the high used relative byte address (RBA).