DASD-only log stream

Figure 1 shows a DASD-only log stream spanning two levels of storage; local storage buffers for interim storage, which is then offloaded 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 a 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 offloads 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 a DASD-only log stream and a coupling facility log stream can have data in multiple DASD log data sets; as a log stream fills log data sets on DASD, system logger automatically allocates new ones for the log stream.