What an I/O configuration is

An I/O configuration is the set of hardware resources that are available to the operating system, and/or coupling facility, and the connections between these resources. Hardware resources typically include:
  • Channels (all channel types including coupling links that connect between z/OS and a coupling facility)
  • FICON directors (switches)
  • Control units (currently known as logical control unit headers)
  • I/O devices
  • PCIe functions

When you define an I/O configuration, you provide both physical and logical information about these resources. For example, in defining a device you provide physical information, such as its type and model, and logical information, such as the identifier you will assign to the device. Planning your I/O configuration definition describes the information you need to provide for each resource.

With Hardware Configuration Definition (HCD), you perform the hardware and software I/O configuration processes through a single, interactive end-user interface. As you enter data, HCD performs validation checking to help avoid data entry errors before you attempt to use the I/O configuration. Additionally, HCD provides a discovery and I/O auto-configuration function for FICON attached disk and tape storage devices to add definitions about already existing I/O paths.

The output of HCD is an I/O definition file (IODF), which contains I/O configuration data. An IODF can define multiple hardware and software configurations to the z/OS® operating system. When you activate an IODF, HCD defines the I/O configuration to the channel subsystem and the operating system.

Through the HCD activate function or the MVS™ ACTIVATE operator command, you can make changes to the current configuration without having to initial program load (IPL) the software or power-on reset (POR) the hardware. Making changes while the system is running is known as dynamic configuration or dynamic reconfiguration.

You select the I/O configuration when you:
  • POR
  • IPL
  • Activate a dynamic configuration change.

IPL and activation require that you identify the IODF that contains the definition of your configuration. A data set called an I/O configuration data set (IOCDS) is used at POR. An IOCDS can be created from a configuration definition in an IODF. The IOCDS contains the configuration for a specific processor, while the IODF contains configuration data for multiple processors.