Each CICS® region uses a system log, which can be defined as DUMMY if recovery action is not required for the region. A DUMMY system log prevents warm restarts. If CICS uses only nonrecoverable data sets, this system log is optional. However, if CICS uses recoverable data sets, it is mandatory, because the system log is used for dynamic transaction backout.

The CICS system log is a single logical stream made up of two separately defined log streams: the primary system log and the secondary system log. If CICS uses recoverable data sets, one primary system log and one secondary system log are needed for each CICS instance.

CICS and DFSMStvs should share the log of logs so that forward recovery can be done correctly.