DEVTYPE - Device Type

Enhanced Definition

DEVTYPE, or Device Type, is an attribute or parameter used in IBM z/OS to specify the *class* or *category* of an I/O device required for a dataset or file. It allows the operating system to allocate an appropriate physical device from a pool of available devices that match the specified type, promoting device independence. In z/OS, `DEVTYPE` (Device Type) refers to the specific model or class of an I/O device, such as a disk drive, tape drive, or printer. It defines the device's hardware characteristics, capabilities, and the software interface required for the operating system to interact with it, enabling z/OS to manage I/O operations effectively.

Key Characteristics

- Abstract Specification: DEVTYPE specifies a *type* of device (e.g., 3390 for DASD, TAPE for tape drives) rather than a specific physical device address.
- JCL Parameter: Primarily used within the UNIT parameter of a JCL DD statement (e.g., UNIT=3390, UNIT=TAPE).
- Device Independence: Enables applications to be written without hardcoding specific hardware addresses, allowing them to run on different physical devices of the same type.
- System-Defined Values: The valid DEVTYPE values are predefined by z/OS and the installed hardware configuration, often representing standard IBM device models or generic categories.
- Resource Allocation: The z/OS Allocation component uses the DEVTYPE to select an available device from a pool that satisfies the request.
- Can be Grouped: System programmers can define generic UNIT group names (e.g., SYSDA, WORK) that map to one or more DEVTYPEs.

Use Cases

- Allocating DASD Datasets: Specifying UNIT=3390 for new datasets to be created on IBM 3390-compatible Direct Access Storage Devices.
- Allocating Tape Datasets: Using UNIT=TAPE for generic tape allocation, or UNIT=3490 for a specific tape drive model, for sequential datasets.
- Temporary Datasets: Frequently using UNIT=SYSDA or UNIT=VIO (Virtual I/O) for temporary work files that do not require specific device characteristics.
- Printer Output: Directing SYSOUT to a specific printer class or type using UNIT=PRINTER or a system-defined printer group.
- Utility Program Input/Output: Many z/OS utilities (e.g., IDCAMS, IEBGENER) require DEVTYPE specifications for their input and output datasets.

Related Concepts

DEVTYPE is a fundamental component of the JCL DD statement's UNIT parameter, which is crucial for dataset allocation. It works in conjunction with the Storage Management Subsystem (SMS), where SMS constructs like Storage Class and Data Class can either override or complement DEVTYPE specifications, providing more granular control over storage allocation. DEVTYPE is a cornerstone of device independence in z/OS, allowing programs to function without needing to know the underlying physical device addresses, which are instead handled by the operating system based on the specified type.

Best Practices:

Use Generic Device Types: Whenever possible, use generic DEVTYPE values like SYSDA for temporary DASD or TAPE for tape to maximize flexibility and allow the system to choose the best available resource.
Leverage SMS for Managed Datasets: For datasets managed by SMS, rely on Storage Class and Data Class to define storage characteristics, often allowing you to omit the UNIT parameter or use a very generic UNIT value.
Avoid Specific Device Addresses: Unless absolutely critical for a unique hardware configuration or testing, avoid specifying explicit device addresses (e.g., UNIT=001A) to maintain system flexibility and portability.
Understand UNIT Group Names: Be aware that UNIT can specify a DEVTYPE, a specific device address, or a *group name* (e.g., WORK, PROD) defined by system programmers, which maps to one or more DEVTYPEs.
Consult System Programmers: For complex or non-standard DEVTYPE requirements, or when defining new UNIT group names, always consult with system programmers to ensure proper configuration and resource availability.