Restarting from a checkpoint

When a job step is restarted from a checkpoint, the restart routine repositions tape volumes containing data sets that were open when the checkpoint was taken. Specifically, the restart routine does the following:

Restores applicable control blocks to the conditions that existed when the checkpoint was taken.
Ensures that the first existing record on the tape is a standard volume label (VOL1), and verifies the volume serial number shown in the label. If the volume is a reel:
- Uses the data set sequence number shown in the JFCB to position the tape at the interrecord gap preceding the first record of the required data set. The method of positioning is the same as explained in Positioning the volume to the data set. The data set labels are not reprocessed.
- Uses the block count shown in the DCB to reposition the tape to the proper record within the data set. This positioning is always performed in a forward direction. If the block count is 0 or a negative number, the tape remains positioned at the interrecord gap preceding the first record.
If the volume is a cartridge, then the block identifier saved in the checkpoint is used to position the tape to the proper record.

If a SYSOUT data set was open when the checkpoint was taken, the data set written into during restart differs from the data set used originally. The system writes job separators at the beginning of the SYSOUT data set used during restart.