Tracks and cylinders
The units used to request disk space on z/OS, taken from the layout of the 3390 disk.
Also written TRK, CYL, track, cylinder, 3390
Tracks and cylinders are the units in which you ask for disk space. In JCL you might write SPACE=(TRK,(10,5)) for ten tracks with five more each time it fills, or SPACE=(CYL,(1,1)) for the same in cylinders.
They come from the shape of an old IBM disk, the 3390. A track was one circle of data on one disk surface. A cylinder was the same track on every surface, 15 tracks stacked together. Modern storage is no longer built this way, but it still presents itself as 3390 volumes, so the units remain.
The sizes are worth knowing. A 3390 track holds up to 56,664 bytes and a cylinder holds 15 tracks, about 850,000 bytes. How much of a track your data actually uses depends on the block size. A block size of 27,998 fits two blocks on each track, which is why that number appears so often.
You can also ask for space in blocks, or in records with the AVGREC parameter, and let the system work out the tracks. In practice, small datasets are usually given tracks and large ones cylinders.
Where can I learn this hands-on?
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.
Start at ZedBankRelated
- Space allocationReserving disk space for a dataset up front, in tracks, cylinders or blocks, with room to grow.
- DASDDirect access storage device, the mainframe term for disk storage.
- Block sizeHow much data is moved between disk and memory in one go, written as BLKSIZE.
- VolumeA single unit of disk storage with a six-character label, on which datasets are placed.
- DD statementThe JCL statement that connects a name inside a program to a real dataset outside it.