The Generalized Trace Facility (GTF)

The generalized trace facility (GTF) is a service aid you can use to record and diagnose system and program problems. GTF is part of the MVS™ system product, and you must explicitly activate it by entering a START GTF command.

Use GTF to record a variety of system events and program events on all of the processors in your installation. If you use the IBM-supplied defaults, GTF lists many of the events that system trace lists, showing minimal data about them. However, because GTF uses more resources and processor time than system trace, IBM® recommends that you use GTF when you experience a problem, selecting one or two events that you think might point to the source of your problem. This will give you detailed information that can help you diagnose the problem. You can trace combinations of events, specific incidences of one type of event, or user-defined program events that the GTRACE macro generates. For example, you can trace:

Channel programs and associated data for start and resume subchannel operations, in combination with I/O interruptions
I/O interruptions on one particular device
System recovery routine operations

The events that GTF traces are specified as options in a parmlib member. You can use the IBM supplied parmlib member or provide your own. Details of GTF operation, which include storage that is needed, where output goes, and recovery for GTF are defined in a cataloged procedure in SYS1.PROCLIB.

GTF can trace system and program events both above and below 16 megabytes. For each event it traces, GTF produces trace records as its output. You can have GTF direct this output to one of the following places:

A trace table in virtual storage.
A data set on a tape or direct access storage device (DASD).

Choose a trace table for your GTF output when maintaining good system performance is very important to your installation. The trace table cannot contain as much GTF trace data as a data set, but will not impact performance as much as a data set because there is no I/O overhead.

Choose a data set or sets when you want to collect more data than will fit in a trace table. Writing trace data to a data set does involve I/O overhead, so choosing this option will impact system performance more than a trace table.

GTF can use only one table in virtual storage, but can use up to 16 data sets. If you specify more than one data set, all of them must reside on devices of the same class, tape, or DASD.

Other components, such as OPEN/CLOSE/EOV and VSAM have special GTF support. See z/OS DFSMSdfp Diagnosis for complete details.