Device Allocation

Enhanced Definition

Device allocation in z/OS is the process by which the operating system assigns specific hardware resources, such as disk drives (DASD), tape drives, or printers, to a running job or started task. It ensures that a program has exclusive or shared access to the necessary I/O devices required to process its data and perform its functions. This assignment is typically requested via Job Control Language (JCL) and managed by the Job Entry Subsystem (JES) and the z/OS supervisor.

Key Characteristics

- JCL-Driven: Device allocation requests are primarily specified through Data Definition (DD) statements within JCL, using parameters like UNIT, VOL, DSN, and DISP.
- Dynamic and Static: Devices can be allocated *statically* at job initiation (e.g., for specific tape drives) or *dynamically* during job execution (e.g., for temporary DASD space).
- Resource Serialization: The system ensures that devices requiring exclusive access (e.g., a tape drive for writing) are not simultaneously allocated to multiple jobs, preventing data corruption.
- Device Pooling: z/OS can allocate devices from a pool of generic devices (e.g., UNIT=SYSDA for any available direct access storage device) rather than requiring a specific device address.
- Deferred Allocation: For some devices, especially tape drives, allocation can be deferred until the device is actually opened by the program, improving resource utilization.
- Unallocation: Devices are typically unallocated automatically at job step termination or job completion, releasing them for use by other jobs.

Use Cases

- Accessing Datasets: A COBOL program might require a specific DASD dataset for input, allocated via a DD statement pointing to the dataset's name and volume.
- Tape Operations: Allocating a tape drive to mount a specific tape volume for backup, restore, or archival processing.
- Spooling Output: Directing job output (e.g., SYSOUT, reports) to the JES spool for later printing or viewing, which involves allocating a virtual printer device.
- Temporary Work Files: Creating and allocating temporary DASD space for intermediate files used during a job step, which are automatically deleted upon job step or job completion.
- System Utilities: Utilities like DFSORT or IDCAMS require device allocation for their input, output, and work files.

Related Concepts

Device allocation is fundamental to JCL and Dataset Management. JCL's DD statements are the primary mechanism for requesting device allocation, linking a program's logical file name to a physical device and dataset. JES (Job Entry Subsystem) plays a crucial role by interpreting these JCL requests, managing the job queue, and coordinating with the z/OS supervisor to acquire the necessary resources. It works closely with Resource Management components of the operating system to track available devices and prevent contention. Proper allocation is also critical for Data Integrity and System Performance, as incorrect or inefficient allocation can lead to job delays or failures.

Best Practices:

Use Generic Units: Whenever possible, specify generic device types (e.g., UNIT=SYSDA, UNIT=TAPE) rather than specific device addresses to allow the system more flexibility in allocating available resources.
Manage Disposition: Use appropriate DISP parameters (NEW, OLD, SHR, MOD) and their secondary actions (CATLG, KEEP, DELETE, PASS) to control dataset creation, retention, and deletion, ensuring resources are properly released or managed.
Optimize Temporary Datasets: For temporary work files, use SPACE=(CYL,(...)) or SPACE=(TRK,(...)) with UNIT=SYSDA and DISP=(NEW,DELETE) to ensure they are automatically deleted and their space is reclaimed.
Understand Exclusive vs. Shared: Be mindful of DISP=OLD (exclusive access) versus DISP=SHR (shared access) to prevent deadlocks or ensure data integrity when accessing existing datasets.
Monitor Device Contention: Regularly monitor system logs and performance metrics for device contention issues, especially for frequently used DASD volumes or tape drives, to identify bottlenecks and optimize job scheduling or resource configuration.