The API trace facility issues programmer messages to report:

• Successful and unsuccessful completion of an ATBTRACE START, STOP, or LIST request.
• Allocation or other problems with the data set specified on START and STOP requests.
• Timing or sequence problems (for example, delays in processing requests, START requests issued before STOP processing completes, and so on).
• Suspension or termination of API tracing activity by APPC/MVS. These conditions might result in the loss of trace data.

When APPC/MVS terminates API tracing activity, APPC/MVS issues message ATB499I to the operator. You cannot restore tracing capability without bringing down APPC/MVS itself. Because this process is so disruptive to workload, consider it only as a last resort. If tracing activity is absolutely critical on this system, and you must restore the API trace facility by bringing down APPC/MVS and restarting it, make sure you follow these steps:

1. Use a SET APPC command to specify a parmlib member containing LUDEL statements for the APPC/MVS LUs
2. Allow the system some time to drain (or quiesce) APPC/MVS work
3. Only if necessary, use the methods in Using the VARY Command to Stop Work Immediately to accelerate the process.