The log stream

A log stream is an application specific collection of data that is used as a log. The data is written to and read from the log stream by one or more instances of the application associated with the log stream. A log stream can be used for such purposes as a transaction log, a log for re-creating databases, a recovery log, or other logs needed by applications.

A system logger application can write log data into a log stream, which is simply a collection of data. Data in a log stream spans two kinds of storage:

Interim storage, where data can be accessed quickly without incurring DASD I/O.
DASD log data set storage, where data is hardened for longer term access. When the interim storage medium for a log stream reaches a user-defined threshold, the log data is offloaded to DASD log data sets.

There are two types of log streams:

Coupling facility log streams
DASD-only log streams.

The main difference between the two types of log streams is the storage medium system logger uses to hold interim log data:

In a coupling facility log stream, interim storage for log data is in coupling facility list structures. See Coupling facility log stream.
In a DASD-only log stream interim storage for log data is contained in local storage buffers on the system. Local storage buffers are data space areas associated with the system logger address space, IXGLOGR. See DASD-only log stream.

Your installation can use just coupling facility log streams, just DASD-only log streams, or a combination of both types of log streams. The requirements and preparation steps for the two types of log streams are somewhat different; see Setting up the system logger configuration.

Some key considerations for choosing either coupling facility log steams or DASD-only log streams are:

The location and concurrent activity of writers and readers to a log stream's log data
The volume of log data written to a log stream.

Coupling facility log streams are required when:

There needs to be more than one concurrent log writer and/or log reader to the log stream from more than one system in the sysplex.
There are high volumes of log data being written to the log stream.

DASD-only log streams can be used when:

There is not a need to have more than one concurrent log writer and/or log reader to the log stream from more than one system in the sysplex.
There are low volumes of log data being written to the log stream.

Note: Since DASD-only log streams always use staging data sets, high volume writers of log data may be throttled back by the I/O required to record each record sequentially to the log stream's staging data sets.

With z/OS® Release 3 and higher, you can also upgrade existing structure-based log streams to use a different coupling facility structure. See the topic "Upgrading an Existing Structure-based log stream" in z/OS MVS Programming: Assembler Services Guide.