If a data set that is not a striped data set resides on multiple volumes, you can code a volume sequence number to specify the volume on which reading or writing is to begin. If you do not code a volume sequence number and the data set is not striped, the system must identify the volume that contains the logical end of the data. Data might not have been written on all the volumes. After the system identifies the last volume, it positions the read/write mechanism on that volume.

In DASD and tape data set labels there is an indicator on the last volume containing user data. When you do not specify a volume sequence number, the system looks in the data set label for the indicator that identifies the last volume, and then selects the volume on which to begin writing as follows:

In the DD: DISP section “How the System Determines the Last Volume”, SMS-managed DASD and Non-SMS-managed DASD processing are described separately. The Non-SMS-managed DASD description can be deleted. The SMS-managed DASD subtitle can be changed to be merely DASD.

DASD: The system tests the data set label on the first volume in the list. If the label indicates it contains the end of the data set, the system selects that volume. Otherwise, it checks each subsequent volume until it finds one that has a last-volume indicator. (To begin writing, the system will not select later volumes that might also have the last-volume indicator by virtue of having previously contained parts of the data set.)

Tape: See the information on Minimizing Tape Mounts.