Volume
A single unit of disk storage with a six-character label, on which datasets are placed.
Also written VOLSER, volume serial, volser
A volume is one storage unit as z/OS sees it, identified by a six-character label called the volume serial, or VOLSER. Datasets are allocated space on volumes, and the catalog records which volume each dataset is on.
In the past this mapped to a physical disk pack. Today it is a logical unit carved out of a storage array, but the model persists because so much software depends on it.
Sites group volumes into storage pools with different characteristics: fast storage for production data, cheaper storage for archives, separate pools for temporary work. Automated storage management then chooses a suitable volume when a dataset is created, based on rules written against its name. This is why you can usually create a dataset without ever naming a volume.
The term becomes relevant when something goes wrong: a volume filling up, a dataset needing to be found on a specific volume because it is not catalogued, or a job explicitly requesting one. In routine work the system handles placement for you.
Related terms
- DASDDirect access storage device, the mainframe term for disk storage.
- CatalogThe system's index of where datasets live, so you can refer to a dataset by name without knowing which disk it is on.
- DatasetWhat a mainframe calls a file: a named collection of records with a defined structure, rather than a loose stream of bytes.
- Space allocationReserving disk space for a dataset up front, in tracks, cylinders or blocks, with room to grow.