Extent
One continuous piece of disk space belonging to a dataset; a dataset grows by adding extents.
Also written extents, primary extent, secondary extent, secondary allocation
An extent is a single continuous area of disk space that belongs to a dataset. When a dataset is created, it gets its primary allocation, often as one extent. When that fills up, z/OS adds a secondary allocation, which may be another extent, and so on.
There are limits on how many extents a dataset can have. An ordinary sequential dataset or PDS can have up to 16 extents on each volume, and a PDS must stay on one volume. VSAM and newer dataset types allow many more. When a dataset reaches its limit, or no more space can be found, the job ends with a space abend, such as B37, D37 or E37.
Each code has a different cause. In short, D37 usually means no secondary space was asked for, B37 that no more extents or volumes were available, and E37 that a PDS or volume ran out. All of them are fixed by giving the dataset more room: a larger primary allocation, a secondary allocation, or a compress for a PDS.
The VTOC records each dataset's extents. In ISPF 3.4, the I line command shows how many extents a dataset has.
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Right here, at ZedBank. You take a junior job at a simulated bank and learn each part of the work by doing it, with Zed beside you. There's nothing to install.
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- Space allocationReserving disk space for a dataset up front, in tracks, cylinders or blocks, with room to grow.
- Tracks and cylindersThe units used to request disk space on z/OS, taken from the layout of the 3390 disk.
- VolumeA single unit of disk storage with a six-character label, on which datasets are placed.
- VTOCThe volume table of contents: the list on each disk volume of which datasets it holds and exactly where they are.
- DASDDirect access storage device, the mainframe term for disk storage.
- AbendAn abnormal end: a program stopping in an uncontrolled way rather than finishing and reporting a result.