Hardware Configuration Definition

HCD

Enhanced Definition

Hardware Configuration Definition (HCD) is an interactive, menu-driven component of z/OS that defines and manages the hardware configuration of an IBM mainframe system. It allows system programmers to specify the logical and physical connections between processors, channels, control units, and I/O devices, translating these definitions into an `Input/Output Definition File (IODF)`. This `IODF` is crucial for the operating system to recognize and utilize the installed hardware.

Key Characteristics

- Centralized Management: Provides a single point of control for defining and managing the entire I/O configuration, including processors, channels, control units, and devices, through an ISPF-based interface.
- IODF Generation: Its primary output is an Input/Output Definition File (IODF), which is a binary data set containing the complete hardware configuration information required by z/OS.
- Dynamic I/O Configuration: Supports dynamic activation of I/O changes without requiring a system IPL, allowing for non-disruptive hardware modifications and additions.
- Extensive Validation: Performs comprehensive validation checks during the definition process and IODF generation to ensure the configuration is syntactically correct and adheres to hardware rules and capabilities.
- Processor and LPAR Awareness: Allows definition of configurations for multiple processors and logical partitions (LPARs) within a single IODF, managing shared and dedicated resources.
- Migration and Coexistence: Facilitates migration between different hardware generations and z/OS releases, and supports coexistence with older configuration methods like IOCP during transitions.

Use Cases

- New Hardware Installation: Defining and integrating new processors, channels, control units, or I/O devices (e.g., DASD, tape drives, network adapters) into an existing z/OS environment.
- Hardware Reconfiguration: Modifying existing I/O paths, changing device addresses, or reassigning devices to different control units or channels to optimize performance or accommodate workload changes.
- Dynamic I/O Changes: Activating minor I/O configuration changes (e.g., varying a device online/offline, adding a path) dynamically without an IPL using the VARY command or HCD's dynamic activation features.
- System Cloning/Replication: Creating a new IODF based on an existing one for cloning a system or setting up a disaster recovery environment with similar hardware.
- Problem Determination: Reviewing the active IODF to understand the current hardware configuration when diagnosing I/O-related issues or verifying device availability.

Related Concepts

HCD is a cornerstone of z/OS system management, closely integrated with several other components. It effectively replaces the older IOCP (I/O Configuration Program) for defining I/O configurations. The IODF generated by HCD is loaded during IPL to initialize the operating system's understanding of the hardware. Dynamic I/O changes, managed through HCD, interact with IOS (I/O Supervisor) to activate changes without an IPL. HCD also works in conjunction with PARMLIB members (e.g., IEASYSxx) that specify which IODF to use and define other system parameters related to I/O, forming the complete system definition.

Best Practices:

Version Control for IODFs: Maintain strict version control for IODF source definitions and generated IODF files to track changes and facilitate rollback if necessary.
Test Changes Thoroughly: Always test new or modified IODFs in a non-production environment before activating them in production, especially for significant configuration changes.
Utilize Dynamic I/O: Leverage HCD's dynamic I/O capabilities to implement