When automatic step restart is requested, the system saves the TTR and track balance of the end of the data set, for each data set with DISP=MOD that is on a direct access volume and used by the step. Saving occurs when each data set is first opened. If restart occurs, the saved TTRs indicate the ends of the data sets when the data sets open again. Thus, if the step writes data in such a data set during the original execution, the step writes over the data during the restart. The action described here does not occur if restart at a checkpoint occurs.

If a data set with DISP=MOD on tape is used in the restart step, the data set is not repositioned at the start of the restart execution. Data written into it during the restart execution follows the data written during the original execution. You may reposition the data set so the data written during the restart execution overlays the data written during the original execution.