Figure 1 shows how a coupling facility log stream
spans two levels of storage; the coupling facility for interim storage
and DASD log data sets for more permanent storage. When the coupling facility space
for the log stream fills, the data is offloaded to DASD log data sets.
A coupling facility log stream can contain data from multiple systems,
allowing a system logger application to merge data from systems across
the sysplex.
Figure 1. Log Stream Data
on the Coupling Facility and DASD
When a system logger application writes a log block to a coupling facility log stream, system logger writes it first to a coupling facility list structure. System logger requires that a coupling facility list structure be associated with each log stream. When the coupling facility structure space allocated for the log stream reaches the installation-defined threshold, system logger moves (offloads) the log blocks from the coupling facility structure to VSAM linear DASD data sets, so that the coupling facility space for the log stream can be used to hold new log blocks. From a user's point of view, the actual location of the log data in the log stream is transparent.