The line trace

A wrap-around line trace exists for each channel. This trace is kept in a 4 KB buffer for each channel in the channel initiator address space. Trace is produced for each channel, so it is ideal for problems where a channel appears to be hung, because information can be collected about the activity of this channel long after the normal trace has wrapped.

The line trace is always active; we cannot turn it off. It is available for both LU 6.2 and TCP channels and should reduce the number of times a communications trace is required.

We can view the trace as unformatted trace that is written to CSQSNAP. We can display the trace by following these steps:

1. Ensure that the CHIN procedure has a SNAP DD statement.

2. Start a CHIN trace, specifying IFCID 202 as follows:

   START TRACE(CHINIT) CLASS(4) IFCID(202)

3. Display the channel status for those channels for which the line trace is required:

   DISPLAY CHSTATUS(channel) SAVED

   This dumps the current line for the selected channels to CSQSNAP. See Snap dumps for further information.
   Notes:

   1. The addresses of the storage dump are incorrect because the CSQXFFST mechanism takes a copy of the storage before writing it to CSQSNAP.

   2. The dump to CSQSNAP is only produced the first time you run the DISPLAY CHSTATUS SAVED command. This is to prevent getting dumps each time you run the command.

      To obtain another dump of line trace data, stop and restart the current trace.

      1. We can use a selective STOP TRACE command to stop just the trace that was started to gather the line trace data. To do this, note the TRACE NUMBER assigned to the trace as shown in the example below:

         +ssid START TRACE(CHINIT) CLASS(4) IFCID(202)
              CSQW130I +ssid 'CHINIT' TRACE STARTED, ASSIGNED TRACE NUMBER 01
         

      2. To stop the trace, issue the following command:

         +ssid STOP TRACE(CHINIT) TNO(01)

      3. We can then enter another START TRACE command with a DISPLAY CHSTATUS SAVED command to gather more line trace data to CSQSNAP.

4. The line trace buffer is unformatted. Each entry starts with a clock, followed by a time stamp, and an indication of whether this is an OUTBOUND or INBOUND flow. Use the time stamp information to find the earliest entry.