Tape
Storage that holds data in sequence on cartridges, used on the mainframe for backups, archives and large files read from start to end.
Also written tape volume, virtual tape, tape cartridge, tape management
Tape stores data in one long sequence. To reach a record near the end, you have to pass everything before it. That makes it poor for looking things up but very good, and cheap, for large amounts of data that are written once and read in order.
On the mainframe, tape is used for backups, long-term archives, copies kept at another site for disaster recovery, and migration of datasets that are rarely used. Tape datasets are always sequential. A tape volume has a six-character serial number, like a disk volume.
Most mainframe tape today is virtual. A virtual tape system uses disk storage, and sometimes real tape behind it, but presents itself to z/OS as tape drives. Jobs that were written for real tape keep working, and they no longer wait for a person to mount a cartridge.
In JCL, a tape dataset is requested with a unit name the site defines, and it often belongs to a generation data group. A tape management system keeps track of which tapes hold what, and when each one can be reused.
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- DASDDirect access storage device, the mainframe term for disk storage.
- VolumeA single unit of disk storage with a six-character label, on which datasets are placed.
- DatasetWhat a mainframe calls a file: a named collection of records with a defined structure, rather than a loose stream of bytes.
- Generation data groupA set of datasets holding successive versions of the same file, so today's copy, yesterday's and last week's all exist under one name.
- Sequential datasetThe simplest kind of dataset: records stored one after another and normally read from start to finish.
- Disaster recoveryThe plans, copies of data and second systems that let an organisation keep running if a whole data centre is lost.