Device

Enhanced Definition

In the mainframe context, a device refers to a physical hardware unit that performs input or output operations, or both, for the system. These devices are managed by the z/OS operating system and communicate with the CPU via channels and control units to process data, serving as fundamental components for data storage, retrieval, and interaction.

Key Characteristics

- Addressability: Each device on a mainframe has a unique device address (e.g., 0A80), which is used by the operating system and channel programs to identify and communicate with it.
- Device Type: Devices are categorized by their function, such as DASD (Direct Access Storage Device) for disk storage, tape drives for sequential storage, printers for output, and terminals for interactive input/output.
- Channel-Attached: Mainframe devices are typically connected to the CPU via channels (I/O processors) and control units, which manage the flow of data and commands between the CPU and the device.
- Shared Access: Many devices, especially DASD volumes, can be shared across multiple LPARs (Logical Partitions) or sysplex members, requiring robust serialization and integrity mechanisms like GSR (Global Resource Serialization).
- Device Drivers/Support: z/OS includes specific device support modules and UCBs (Unit Control Blocks) to manage and interact with various types of hardware, abstracting the physical details from applications.
- Offline/Online Status: Devices can be brought online (available for use) or offline (unavailable) by system operators, impacting their accessibility to applications and requiring operator intervention.

Use Cases

- Data Storage: DASD devices (e.g., IBM DS8000 series) are used to store datasets, VSAM files, DB2 databases, IMS databases, and the z/OS operating system itself.
- Backup and Archival: Tape devices are extensively used for creating backups of critical data, archiving historical information, and transferring large volumes of data between systems.
- Printing Reports: Line printers and laser printers are used to generate reports, invoices, and other hardcopy output from batch jobs or online applications.
- Interactive Access: Terminals (e.g., 3270 terminals or emulators) serve as input/output devices for users to interact with TSO/ISPF, CICS transactions, or IMS transactions.
- Network Connectivity: Network adapters (e.g., OSA - Open Systems Adapter) function as I/O devices, enabling the mainframe to communicate with TCP/IP networks and other distributed systems.

Related Concepts

Devices are fundamental building blocks of the mainframe I/O subsystem. They are managed by channels and control units, which execute channel programs initiated by z/OS to perform I/O operations. Each device is represented in the operating system by a UCB (Unit Control Block) and is often allocated to a job via DD statements in JCL, which specify the device type, volume, and dataset information. The efficient management of devices is critical for overall system performance and data availability.

Best Practices:

Optimal Device Placement: Distribute frequently accessed datasets across multiple DASD volumes and control units to minimize I/O contention and improve throughput.
Regular Monitoring: Monitor device utilization, queue lengths, and error rates using tools like RMF (Resource Measurement Facility) to identify bottlenecks and potential issues.
Proper Allocation: Ensure JCL DD statements correctly specify device types and allocation parameters (UNIT, VOL=SER, DISP) to prevent allocation errors and ensure data integrity.
Path Redundancy: Implement multiple I/O paths (channels and control units) to critical devices (DASD, tape) to provide redundancy and high availability in case of a path failure.
Security Configuration: Secure access to devices and the data they contain using RACF or other security managers, controlling who can allocate, read from, or write to specific devices or volumes.