Multi-volume dataset
A dataset that is stored on more than one disk or tape volume, continuing onto the next when one is full.
Also written multivolume dataset, volume count, VOL=(,,,n), spanning volumes
A dataset normally lives on one volume. A multi-volume dataset is spread across several: when its space on the first volume is used up, it continues on the next.
For disk datasets, the number of volumes a dataset may use is set by a volume count. In JCL it is the fourth value of the VOL parameter, as in VOL=(,,,5) for up to five volumes. At SMS sites, the data class usually supplies it. A sequential dataset can span up to 59 volumes. Some dataset types, such as an ordinary PDS, cannot span volumes at all.
Tape datasets often span volumes too, because a single large file can fill more than one tape.
The catalog records every volume a dataset is on, so programs read it as one dataset without knowing where the pieces are. Multi-volume datasets matter mostly when they run out of space: a job may fail because no more volumes are allowed, even though plenty of space is free elsewhere. Raising the volume count, or allocating more space on each volume, solves it.
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- VolumeA single unit of disk storage with a six-character label, on which datasets are placed.
- ExtentOne continuous piece of disk space belonging to a dataset; a dataset grows by adding extents.
- 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.
- Space allocationReserving disk space for a dataset up front, in tracks, cylinders or blocks, with room to grow.
- TapeStorage that holds data in sequence on cartridges, used on the mainframe for backups, archives and large files read from start to end.