Every time a system logger application writes a log block to a log stream, system logger automatically ensures there is a duplicate copy of the data as insurance against data loss due to coupling facility, system, or system logger failure. System logger keeps a duplex copy of data in log stream interim storage only. The duplicate copy is kept until the data is off-loaded from interim storage to DASD log data sets.

The way system logger duplexes log data depends on whether the log stream is a coupling facility or a DASD-only log stream.