To record traces in an external file, specify MODE=EXT in the MODIFY TRACE command or as a TRACE start option.
Use external mode when you must collect large amounts of trace data, to lessen the chance of wraparound and lost data. However, unless you are using a tape, the trace data can still fill the external DASD or data file and begin overwriting itself if you do not ensure that the external trace file is large enough.
VTAM® allocates from common storage area (CSA) the number of 8-K VIT buffers specified on the command or start option. The default value is 2. VTAM now accumulates the VIT records in one of these buffers. When a process has insufficient room in the current buffer, the process writes the entire 8-K buffer to an external trace file using the generalized trace facility (GTF). After the buffer has been written, the process tries again to put its VIT records into the current buffer. If another process finds that there is no current buffer (because of another process writing it to GTF), the process tries to use one of the preallocated buffers. If no other buffers are available, either because they are all in the process of being written to GTF or because BFRNUM=0 was specified, VTAM writes the individual records to the external trace file.
Avoid using BFRNUM=0 because writing to the external trace file involves large system overhead. Also, allot enough 8K buffers to prevent individual records from being written to the external trace file. Depending on the situation, VTAM might write the 8K buffers to the external trace file out of order.
The external trace file contains the same information as the internal trace table, except it does not contain the first-line header record and the BUFF VIT record. The BUFF VIT record is the first VIT record in the 8K buffer. This record contains the buffer sequence number, which you can use to determine whether the 8K buffers were all written in the correct sequence.
The external trace file is produced by GTF, and the default file name is SYS1.TRACE. You can print the internal trace data with IPCS or TAP. If you use IPCS to print the data, specify the GTFTRACE option, and set USR(FE1). See "Formatting and printing trace records" in z/OS Communications Server: SNA Diagnosis Vol 1, Techniques and Procedures for information about using TAP and IPCS to print output.