Deferred Mount
Deferred mount is a z/OS allocation technique used primarily for tape devices, where the actual mounting of a tape volume by an operator is delayed until the dataset residing on that volume is explicitly opened by the executing program. This contrasts with a *premature mount*, where a tape is requested and mounted at the beginning of a job step, even if it might not be used immediately or at all.
Key Characteristics
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- Resource Optimization: It conserves tape drive resources and operator time by only requesting a tape mount when it's genuinely needed by the application.
- JCL Control: Typically controlled via the
UNITparameter in JCL, often by specifyingUNIT=DEFERor implicitly through certain allocation scenarios (e.g.,UNIT=AFFfor a tape dataset that is not the first one allocated to that unit name). - Dynamic Allocation: Often associated with dynamic allocation, where device resources are acquired at the point of use rather than at job step initiation.
- Operator Intervention: Reduces unnecessary operator intervention for tape mounts that might not be required, improving overall system efficiency.
- Potential Delays: While efficient, if a deferred tape is needed, the job will pause until the operator mounts the correct volume, potentially introducing a delay during execution.
Use Cases
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- Conditional Tape Usage: In jobs where a tape dataset might only be processed under certain conditions (e.g., an
IF/THEN/ELSEJCL construct), deferring the mount prevents an unnecessary mount if the condition is not met. - Multiple Datasets on Same Volume: When a job step accesses multiple datasets that reside on the same physical tape volume, deferring mounts for subsequent datasets on that volume avoids redundant mount requests.
- Shared Tape Drives: In environments with limited tape drives, deferred mounts help ensure that a drive is not tied up with an unneeded tape, making it available for other jobs.
- Tape Backup and Recovery: For jobs that might read from a recovery tape only if an error occurs, deferring the mount ensures the tape is only requested when truly necessary.
- Conditional Tape Usage: In jobs where a tape dataset might only be processed under certain conditions (e.g., an
Related Concepts
Deferred mount is closely related to JCL (Job Control Language), specifically the UNIT parameter and its options like DEFER or AFF. It interacts with Device Allocation in z/OS, influencing when and how tape drives are assigned to a job. It's a strategy used by Tape Management Systems (TMS) to optimize tape library operations and reduce Operator Intervention. Understanding deferred mounts is crucial for efficient Job Scheduling and Resource Management in a mainframe environment, as it impacts job turnaround time and overall system throughput.
- Strategic Application: Use deferred mounts strategically for tape datasets that are not immediately required or might not be used at all within a job step to optimize resource usage.
- Avoid Overuse: Do not defer mounts for critical, frequently accessed tapes, as the delay for operator intervention can negatively impact job turnaround time.
- Coordinate with TMS: Ensure your JCL and deferred mount strategies align with your site's Tape Management System policies and procedures.
- Monitor Performance: Regularly monitor job performance and tape drive utilization to ensure that deferred mounts are indeed improving efficiency and not introducing unexpected bottlenecks.
- Clear JCL Comments: For complex jobs using deferred mounts, include clear JCL comments to explain the logic and intent, aiding future maintenance and troubleshooting.