Channel
The connection between the mainframe and its devices, such as disk and tape, managed by its own processors so the main processors do not have to.
Also written FICON, CHPID, channel path, channel subsystem, ESCON
A channel is the path between the mainframe and its input and output devices, such as disk storage, tape and some network equipment. Mainframes are known for very high I/O capacity, and channels are a big part of why.
The channel subsystem has its own processors. When a program reads a record, z/OS starts the I/O and the channel subsystem carries it out, while the main processors continue with other work. Many thousands of I/Os can be in progress at once.
Modern channels use FICON, which runs over fibre optic cables. The older ESCON is still mentioned in documentation. Each channel path has an identifier, a CHPID, and each device is usually reachable through several paths, so the failure of one cable or port does not cut it off.
Channels are defined in the IODF. You will see them in messages about a path going offline, and in performance reports that show how busy each path is. Developers rarely deal with them directly.
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- DASDDirect access storage device, the mainframe term for disk storage.
- IODFThe I/O definition file: the dataset that describes all the hardware connected to the mainframe, such as disks, tape drives and network devices.
- TapeStorage that holds data in sequence on cartridges, used on the mainframe for backups, archives and large files read from start to end.
- IBM ZIBM's current family of mainframe hardware, and the brand name you will see on most modern mainframe job adverts.
- VolumeA single unit of disk storage with a six-character label, on which datasets are placed.
- HMCThe Hardware Management Console: the workstation used to control the mainframe hardware, its LPARs and IPLs.