System Reconfiguration

Reconfiguration is the process of adding hardware units to, or removing hardware units from, a configuration. Units can be either:

Online: Units in use by a system are called online. When both physically and logically online, a unit is available to be used by the system.
Offline: Units not in use by a system are called offline. When either physically or logically offline, a unit is not available to be used by the system.

An installation can use reconfiguration to:

Adapt a system to changing work loads by configuring units online or offline as required.
Perform maintenance on a part of a complex while the other part continues normal operation.
Possibly recover system operation by configuring failing units offline.

Hardware unit or units may be put offline before initialization of the system or systems. To do this, an operator can deselect through the system console such units as central processors (CPU), storage elements, or channel paths. Note that an operator should never deselect a unit during system operation, because the operating system is not notified of the removal.

During system operation, the operating system configures failing units offline with or without any intervention. When intervention is needed, the operator:

Can enter a CONFIG command that identifies the CONFIGxx member of Parmlib that specifies the reconfiguration. (See z/OS MVS Initialization and Tuning Reference.)
Can enter a CONFIG or VARY command that directly configures units online or offline. (See MVS system commands reference.)

For maintenance of hardware, prepare reconfiguration actions to configure hardware units offline for repair and back online so the system can use them. Also, hardware units and channel paths are reconfigured offline then online during reconfiguration of partitionable systems.