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.