Deallocation

Enhanced Definition

Deallocation in z/OS is the process of releasing system resources that were previously allocated to a job, step, or program. This action makes the resources available for use by other tasks or systems, preventing resource exhaustion and ensuring efficient system operation. Deallocation in the mainframe/z/OS context refers to the process of releasing system resources that were previously allocated to a job, job step, program, or transaction. Its primary purpose is to make these resources available for other tasks, improve system efficiency, and ensure proper cleanup after a process completes.

Key Characteristics

- Resource Types: Applies to a wide range of resources including datasets (DASD, tape), memory (storage), devices (printers, communication lines), control blocks, and system services (e.g., enqueues).
- Automatic vs. Explicit: Deallocation can occur automatically at job or step termination (e.g., temporary datasets) or explicitly through program code (e.g., CLOSE statements in COBOL) or JCL parameters.
- Timing: Resources are typically deallocated at the end of a job step, the end of a job, or when a program explicitly releases them, depending on the resource type and how it was allocated.
- Resource Availability: Upon deallocation, the released resource becomes free and accessible to other jobs or tasks, improving overall system throughput and preventing resource contention.
- Operating System Role: z/OS manages the deallocation process, ensuring that resources are properly released, their status is updated in system control blocks, and any associated locks are removed.

Use Cases

- JCL Dataset Disposition: Using DISP=(OLD,DELETE) or DISP=(NEW,DELETE) in JCL to explicitly delete a dataset after its use, thereby deallocating its space on DASD.
- Temporary Dataset Management: z/OS automatically deallocates and deletes temporary datasets (e.g., DSN=&&TEMP) at the end of the job that created them, freeing up space.
- COBOL File Closing: A CLOSE statement in a COBOL program deallocates the file and its associated buffers, making the file available for other programs or jobs.
- CICS Storage Release: A CICS application using EXEC CICS FREEMAIN to release dynamically acquired storage (e.g., GETMAIN areas) back to the CICS DSA (Dynamic Storage Area).
- DB2 Resource Release: When a DB2 transaction completes (via COMMIT or ROLLBACK), DB2 automatically deallocates locks, cursors, and other resources held by that transaction.

Related Concepts

Deallocation is the inverse operation of allocation, where resources are acquired for use. It is a fundamental aspect of resource management within z/OS, ensuring that system resources are efficiently shared among multiple concurrent workloads. It works closely with Job Entry Subsystem (JES2/JES3) which manages job execution and resource cleanup, and the z/OS operating system itself, which provides services for tracking and releasing resources. Proper deallocation is crucial for preventing resource contention, deadlocks, and storage leaks.

Best Practices:

Explicit Deallocation: Whenever possible, explicitly deallocate resources when they are no longer needed within application programs (e.g., CLOSE files, FREEMAIN storage) rather than relying solely on automatic system cleanup.
Error Handling: Implement robust error handling around deallocation routines to ensure resources are released even if unexpected conditions occur during program execution.
Resource Monitoring: Regularly monitor system resource usage (e.g., using RMF, SMF, or OMEGAMON) to identify potential resource leaks or inefficient deallocation practices.
Understand JCL FREE Parameter: Utilize the FREE=CLOSE or FREE=END parameter on DD statements in JCL to control precisely when datasets are deallocated, optimizing resource availability for subsequent steps or jobs.
CICS FREEMAIN Discipline: In CICS, always pair EXEC CICS GETMAIN with a corresponding EXEC CICS FREEMAIN to