I/O configurations on the first IBM System/360 machines supported 16 channels connected to 4,096 peripheral I/O devices. Communication paths were physically dedicated. Connecting the control units directly to the CPU made the connections obvious. You took a device offline and online by unplugging and plugging the cable. Next, logical partitioning (LPARs) increased the number of shared devices and the four-digit device numbering increased I/O configurations.
Dynamic channel path management (DCM) replaced physically dedicated paths of communication with logical dynamic paths of communication that are managed inside ESCON (and later, FICON) directors, or switches, manipulated by firmware. Switch-to-switch connections, known as chaining and cascading, further complicated I/O configurations.
Today, a single z/OS machine supports four Logical Channel Subsystems (LCSSs), each of which can be configured with up to 15 LPARs. Combine this with the use of multi-imaging to share channels among LPARs and channel spanning between LPARs and LCSSs, can result in 1,024 possible channels. Add to this the fact that 336 of these channels can be configured to support FICON connectivity and there are potentially up to 83,328 switch ports to be configured and managed.
These I/O configurations, and the potential configurations of tomorrow, are far too large to be managed manually.
For example, to take a control unit offline for maintenance you must identify the control unit, tape devices, and ESCON director, which all have different numbers on each system. Next, you must identify each port in the ESCON director to which the control unit is attached and each CHPID from each system attached to the ESCON director.
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