Device Support

Enhanced Definition

Device Support refers to the collection of software components within the z/OS operating system that enable it to recognize, control, and communicate with various I/O devices, such as DASD, tape drives, printers, and network adapters. It translates high-level I/O requests from applications and the operating system into device-specific commands, managing the intricate details of hardware interaction. In the z/OS environment, Device Support refers to the comprehensive set of software components and services that enable the operating system to recognize, control, and communicate with attached hardware devices. It translates high-level application requests into low-level device-specific commands, managing I/O operations and ensuring data integrity and system availability.

Key Characteristics

- Device Drivers: Specific software modules (often part of the operating system kernel or loadable modules) designed to interface with particular types or models of I/O devices, handling their unique command sets and protocols.
- I/O Configuration: Defined through tools like HCD (Hardware Configuration Definition), which creates an IOCDS (I/O Configuration Data Set) that describes the installed I/O devices, their channels, control units, and addresses.
- Channel Programs: Device support software constructs Channel Command Words (CCWs) and Channel Programs that are executed by the Channel Subsystem to perform actual I/O operations.
- Interrupt Handling: Manages and processes I/O interrupts generated by devices upon completion of an operation or in case of an error, allowing the CPU to efficiently multitask.
- Error Recovery: Includes mechanisms for detecting, reporting, and attempting recovery from I/O errors, enhancing system reliability and data integrity.
- Dynamic I/O: Provides the capability to add, remove, or modify I/O devices and their configurations without requiring a full IPL (Initial Program Load) of the operating system.

Use Cases

- Data Storage Access: Enabling applications (e.g., COBOL programs, DB2) to read from and write to DASD volumes (e.g., IBM DS8000) for data storage and retrieval.
- Backup and Archiving: Facilitating the use of tape drives for creating backups, archiving historical data, and migrating data between systems.
- Printing and Output Management: Controlling printers (local or network-attached) for generating reports, invoices, and other hardcopy output, often managed by JES (Job Entry Subsystem).
- Network Communication: Supporting OSA (Open Systems Adapter) cards to enable TCP/IP and SNA network connectivity for applications like CICS, IMS, and z/OS UNIX System Services.
- Console Operations: Managing console devices (e.g., HMC, 3270 terminals) for system operators to monitor system status, issue commands, and respond to messages.

Related Concepts

Device Support is fundamental to the I/O Subsystem of z/OS, acting as the crucial interface between the logical requests of the operating system and the physical Channel Subsystem and hardware. It relies on the HCD tool for defining the I/O configuration and works in conjunction with Access Methods (like VSAM, QSAM, BSAM) to provide a structured way for programs to perform I/O operations. Without robust device support, z/OS would be unable to interact with any external hardware, rendering the system inoperable.

Best Practices:

Maintain Current HCD Definitions: Regularly review and update HCD definitions to accurately reflect the physical I/O configuration, ensuring optimal performance and avoiding configuration mismatches.
Utilize Dynamic I/O: Leverage Dynamic I/O capabilities to implement configuration changes (e.g., adding new DASD, reconfiguring paths) without disruptive IPLs, minimizing downtime.
Apply PTFs and APARs: Ensure that relevant Program Temporary Fixes (PTFs) and Authorized Program Analysis Reports (APARs) related to device support are applied to address known issues and enhance stability or performance.
Monitor I/O Performance: Regularly monitor I/O metrics (e.g., response times, queue depths, error rates) using tools like RMF to identify bottlenecks or issues with device performance and configuration.
Plan for Redundancy: Design I/O paths with redundancy (e.g., multiple channels, control units, paths to devices) to ensure high availability and fault tolerance in case of hardware failures.