UNIT parameter
A DD statement parameter that says what kind of device a dataset should be on, such as disk or tape.
Also written UNIT=, UNIT=SYSDA, SYSALLDA, esoteric unit name
UNIT on a DD statement says what kind of device a new dataset goes on. UNIT=SYSDA asks for a disk. Names such as SYSDA, SYSALLDA and TAPE are not built into z/OS. They are group names, called esoteric names, that each site defines for its own devices, although SYSDA and SYSALLDA are almost universal.
A device type can also be given directly, such as UNIT=3390. Together with VOL=SER, UNIT can place a dataset on one particular volume.
At sites using SMS for disk, UNIT is often not needed. SMS chooses the storage group and volume, and may ignore the UNIT value. Many older jobs still carry it, which does no harm.
UNIT matters more for tape, where it selects tape drives, and for datasets on volumes that are not managed by SMS. If a job fails because it cannot find a suitable device, check whether the UNIT name exists at this site.
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- DD statementThe JCL statement that connects a name inside a program to a real dataset outside it.
- VolumeA single unit of disk storage with a six-character label, on which datasets are placed.
- DASDDirect access storage device, the mainframe term for disk storage.
- TapeStorage that holds data in sequence on cartridges, used on the mainframe for backups, archives and large files read from start to end.
- SMSThe Storage Management Subsystem: the z/OS feature that decides where new datasets go and how they are backed up, kept and deleted, using rules the site writes.
- Multi-volume datasetA dataset that is stored on more than one disk or tape volume, continuing onto the next when one is full.