Figure 1 shows a DASD-only log stream spanning two levels of storage: local storage buffers for interim storage, which is then off-loaded to DASD log data sets for more permanent storage.

A DASD-only log stream has a single-system scope; only one system at a time can connect to DASD-only log stream. Multiple applications from the same system can, however, simultaneously connect to a DASD-only log stream.

Figure 1. Log Stream Data in Local Storage Buffers and DASD Log Data Sets

When a system logger application writes a log block to a DASD-only log stream, system logger writes it first to the local storage buffers for the system and duplexes it to a DASD staging data set associated with the log stream. When the staging data set space allocated for the log stream reaches the installation-defined threshold, system logger off-loads the log blocks from local storage buffers to VSAM linear DASD data sets. From a user's point of view, the actual location of the log data in the log stream is transparent.

Both DASD-only log streams and coupling facility log streams can have data in multiple offload data sets. System Logger automatically allocates new offload data sets for log streams.