When a batch job is started, CICS® VR obtains the name of the forward
recovery log stream defined for each VSAM data set that is updated by the
batch job and is defined with FRLOG REDO or ALL. However, CICS VR can write
log records reflecting updates made to one or more VSAM data sets to the same
forward recovery log stream.
When deciding how many forward recovery log streams to create and how to
define them to your VSAM data sets, you must consider such factors as transaction
performance, work load, and recovery performance.
The MVS™ logger
merges all the forward recovery log records from the various CICS VR instances
onto the shared forward recovery log. Some points to consider are:
- All data sets used by one batch job must use the same log stream, to reduce
the number of log streams written to at sync point.
- Share a forward recovery log stream between data sets that:
- Have similar security requirements
- Have similar backup frequency
- Are likely to need restoring in their entirety at the same time
- Log stream names must relate to the data sets. For example, PAYROLL data
sets could be mapped to a forward recovery log named PAYROLL.FWDRECOV.PAYLOG.
- Do not mix high update frequency data sets with low update frequency data
sets because this causes a disproportionate amount of unwanted log data to
be read during recovery of low frequency data sets.
- Do not put all high update frequency data sets on a single log stream
because you could exceed the throughput capacity of the log stream.
- If you define too many data sets to a single log stream, you could experience
frequent structure-full events when the log stream can not keep up with data
flow.
- Delete redundant data from log streams periodically so that the log streams
do not become excessively large. Typically, for a forward recovery log, deletion
of old data is related to the data backup frequency. For example, you might
keep the four most recent generations of backup so that when you delete a
redundant backup generation, you can also delete the corresponding redundant
forward recovery log records. These are the records older than the redundant
backup and they are no longer needed for forward recovery.