Device Class

Enhanced Definition

In z/OS, a **Device Class** is a logical grouping of I/O devices that share similar characteristics, capabilities, and operational attributes. It provides a generalized way for the operating system and applications to request and manage I/O resources without needing to specify a particular physical device. A Device Class, in the context of IBM z/OS, is a logical grouping of I/O devices that share similar characteristics and capabilities, such as direct access storage devices (DASD), magnetic tape drives, or printers. It allows the operating system and applications to refer to a category of devices rather than a specific physical device, simplifying resource allocation and JCL.

Key Characteristics

- Logical Grouping: Organizes physical I/O devices (e.g., disk drives, tape drives, printers) into categories based on their fundamental type and function.
- Standard Categories: Common device classes include DASD (Direct Access Storage Devices), TAPE (Magnetic Tape Devices), UNITRECORD (printers, card readers/punches), and CONSOLE (system consoles).
- Allocation Abstraction: Allows JCL and programs to request a device from a class (e.g., UNIT=DASD) rather than a specific device address or model, providing flexibility and device independence.
- System Definition: Device classes are defined during system generation (SYSGEN) or dynamically via I/O configuration, associating specific device types and their attributes with a class.
- Hierarchy: Device classes sit above generic device types (e.g., 3390 for DASD, 3490 for tape) and specific device types (e.g., 3390-3, 3490E). A class can encompass multiple generic types.
- Resource Management: Facilitates resource management by allowing the system to select an available device from the specified class, optimizing I/O operations and resource utilization.

Use Cases

- JCL Device Allocation: Specifying UNIT=DASD or UNIT=TAPE in a JCL DD statement to request any available disk or tape drive, respectively, for a dataset.
- System Generation (SYSGEN): Defining the available I/O configuration to z/OS, where device classes are fundamental in categorizing and managing the physical hardware.
- Dynamic I/O Configuration: Adding or removing I/O devices and their associated classes to a running z/OS system without requiring a system IPL.
- Workload Management (WLM): Indirectly influencing resource allocation by ensuring that jobs requesting devices from a specific class can be dispatched to systems with those resources.
- Security Definitions (RACF): Controlling access to specific device classes or generic device types within a class, for example, restricting who can allocate tape drives.

Related Concepts

Device classes are a foundational concept for I/O management in z/OS. They work in conjunction with the UNIT parameter in JCL, where UNIT can specify a device class, a generic device type (e.g., 3390), or a specific device address. Device classes are defined within the I/O Configuration Program (IOCP) or dynamically, which maps physical devices to their logical categories. They are also related to Storage Management Subsystem (SMS), which can manage dataset placement on DASD volumes, often leveraging the underlying device class definitions.

Best Practices:

Use Generic UNIT Specifications: Whenever possible, use device class names (e.g., UNIT=DASD) or generic device types (e.g., UNIT=3390) in JCL DD statements to provide maximum flexibility and device independence.
Consistent Naming: Maintain consistent and meaningful naming conventions for device classes across your z/OS environment to improve clarity and manageability.