To ensure maximum protection of log data, the system logger allows duplexing of log write requests to both the coupling facility and to DASD. Duplexing provides failure-isolation of committed log data, even when the coupling facility and a connected MVS™ are in the same failure domain. For example, a failure domain is when the coupling facility and MVS are in the same central processor complex (CPC), or if the coupling facility storage is volatile. The records are written temporarily to a staging data set associated with the log stream. They are later offloaded asynchronously to permanent storage on the persistent log stream data sets. Offloading occurs when the HIGHOFFLOAD threshold of either the log stream or the staging data set is reached.

You can specify whether duplexing to staging data sets is either conditional or unconditional, depending on whether or not the coupling facility is in an independent failure domain. If conditional, the system logger chooses either the data space option or a staging data set; if unconditional, duplexing to a staging data set is forced.

MVS normally keeps a second copy of the data written to the coupling facility in a data space. The second copy is used to build a coupling-facility log in the event of an error. This is satisfactory as long as the coupling facility is failure-independent (in a separate central processor complex (CPC) and nonvolatile) from MVS.

The coupling facility is in the same CPC or uses volatile storage, the system logger supports staging data sets for copies of log stream data. That log stream data would otherwise be vulnerable to failures that impact both the coupling facility and the MVS images.