Selecting the Type of Message Log

If you choose to do anything other than bypass the message log, your choice of value for the DATASET_STATUS parameter determines the type of message log:

Value: Type of TP Message Log
NEW: A new log for each TP instance
OLD: A log for only the most recent TP instance
MOD: A cumulative log

Depending on the type of message log you want to use, you either allow APPC/MVS to allocate the data set, or allocate it yourself. If you allocate it yourself, use a record length of 133. (Each message written to the data set is one 133-byte record; messages are either padded with blanks or truncated to fit.)

For a new log for each TP instance (DATASET_STATUS is NEW):
In this case, APPC/MVS allocates a data set for you, specifying only primary space. The MSGLIMIT value for the class determines how many messages are logged for each TP instance, and APPC/MVS allocates a data set large enough to contain all those messages.

When you specify NEW, the data set is cataloged and kept once it is created. If you also specified ALWAYS for KEEP_MESSAGE_LOG, you might accumulate an inordinate number of data sets, because a new one is created for each instance of a TP.
For a log of only the most recent TP instance (DATASET_STATUS is OLD):
If you specify a DATASET_STATUS of OLD, you must pre-allocate the data set. Estimate the amount of space required based on your knowledge of the TP's processing. You may specify both primary and secondary space. If you specify secondary space, the system allocates it, up to the extent limit for the type of data set, when the specified primary space is insufficient for the number of messages logged. The MSGLIMIT value for the class determines how many messages are collected in storage for the TP instance; the size of the data set, and any additional space the system can allow, determine how many of those messages are written to the data set.

To make sure you obtain all pertinent diagnostic information, you might have to adjust the specified amounts of primary and secondary space. If you don't have enough space allocated, a X'x37' abend results when the system attempts to write messages to the data set.
For a cumulative log (DATASET_STATUS is MOD):
If you specify a DATASET_STATUS of MOD, IBM® recommends that you pre-allocate the data set. If you do not, various installation or system defaults might result in the allocation of a data set that is not large enough for the amount of messages you want to collect.
Estimate the primary and secondary space required for a cumulative log by considering the following factors:
- How many messages might be generated by one instance of the TP.
- How many instances of the TP might run before you need to view the message log.
- The MSGLIMIT value specified for the class in which the TPs run. Remember that this value applies for each instance of the TP. For example, if the MSGLIMIT for the class is 50 messages, a cumulative log contains one set of no more than 50 messages for each TP instance.
Just as you may for a data set with a value of OLD, you may specify both primary and secondary space. If you specify secondary space, the system allocates it, up to the extent limit for the type of data set, when the specified primary space is insufficient for the number of messages logged.

To make sure you obtain all pertinent diagnostic information, you might have to adjust the specified amounts of primary and secondary space. If you don't have enough space allocated, a X'x37' abend results when the system attempts to write messages to the data set.

For multi-trans TPs only, APPC/MVS allows messages to wrap when the MSGLIMIT value is exceeded, so you can obtain more recent messages for each TP instance, at the cost of overwriting older messages. For more information about the MSGLIMIT value, and how it applies for standard versus multi-trans TPs, see Choosing a Value for the MSGLIMIT Parameter.