CICS® system programmers need to consult with their MVS™ system programmers
to plan for the storage that is required by the log streams needed by the
many CICS log managers operating in the sysplex.
Each log stream is a sequence of blocks of data, which the MVS system logger internally
partitions over three different types of storage:
- Primary storage, which holds the most recent records that were written
to the log stream. Primary storage can consist of either:
- A structure within a coupling facility. Log data written to the coupling
facility is also copied to either a data space or a staging data set.
- A data space in the same MVS image as the system logger. Log data written
to the data space is also copied to a staging data set.
- Secondary storage--when the primary storage for a log stream becomes
full, the older records automatically spill into secondary storage, which
consists of data sets managed by the storage management subsystem (SMS). Each
log stream, identified by its log stream name (LSN), is written to its own
log data sets.
- Tertiary storage--a form of archive storage that is used as specified
in your hierarchical storage manager (HSM) policy. Optionally, older records
can be migrated to tertiary storage, which can be either DASD data sets or
tape volumes.
See the different levels of log stream storage in Figure 10 and Figure 11.
Figure 10. The types of storage used by the MVS system logger. This diagram shows
a log stream that uses a coupling facility. Primary storage consists of space
in a structure within the coupling facility, and either space in a staging
data set or a data space in the same MVS image as the system logger. Secondary storage
consists of a series of data sets on disk storage to which the elements of
the log structure in the coupling facility are mapped. Tertiary storage is
the DFHSM storage holding older levels of the secondary storage data sets.
Figure 11. The types of storage used by the MVS system logger. This diagram shows a log
stream that uses DASD-only logging. Primary storage consists of a data space
in the same MVS image as the system logger, and a single staging data set. Secondary
storage consists of a series of data sets on disk storage to which hold successive
copies of the single staging dat set. Tertiary storage is the DFHSM storage
holding older levels of the secondary storage data sets.
The CICS log manager supports the DASD-only option
of the MVS system logger. This means that individual CICS log streams can use either coupling facility
log structures or DASD-only logging.
Take the following points into account when deciding which log streams should
be defined to use the coupling facility and which to use DASD-only:
- A coupling facility log stream must be used if you want to allow simultaneous
access from CICS regions running in different MVS images. (Simultaneous access to a DASD-only
log stream is limited to CICS regions in the same MVS image.)
For example, assume that you
are using RLS and have several CICS application-owning regions (AORs) running
on different MVS images. Because the forward recovery log must be accessible from
all the AORs, it must be defined as a coupling facility log stream. A CICS system log, on the other hand, is only ever accessed by a single CICS region, and can therefore always be defined as a DASD-only log stream.
Without a coupling facility, you cannot share general log streams across MVS images.
- Defining all your CICS log streams to use structures within a single coupling facility is not recommended--see Coupling facility log streams.
- DASD-only log streams are easier to define and administer than coupling
facility log streams.
- The CPU cost of a log write to a DASD-only log stream is greater than
that of a write to a coupling facility log stream. For more information, see
the CICS Performance Guide .
- If the amount of available coupling facility space is limited, you may
want to define some DASD-only log streams in order to minimize the amount
of space allocated to log structures.
Notes:
- Define a single-system sysplex (which must use a sysplex
couple data set) with PLEXCFG=MONOPLEX. This is required for stand-alone MVS systems
that use MVS system logger facilities.
- Define sysplexes that have two or more MVS images with PLEXCFG=MULTISYSTEM.
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