Dismount

Volume

Enhanced Definition

In the context of IBM z/OS, dismounting a volume refers to the process of logically detaching a storage volume (typically a magnetic tape cartridge or, historically, a removable disk pack) from its associated device, making it unavailable for further I/O operations by the current user and releasing the device for other uses. It signals to the operating system that the volume is no longer actively required by a specific job or system component. In z/OS, dismounting a volume refers to the process of logically or physically detaching a storage volume (most commonly a tape reel or a removable disk pack) from a device, making it unavailable for I/O operations by the system. This action typically follows the completion of all data access operations on that volume and often precedes its physical removal from the drive or logical release in a virtual environment.

Key Characteristics

- Logical Detachment: The primary action is a logical one, informing z/OS that the volume is no longer in active use by a specific allocation.
- Resource Release: A successful dismount releases the physical device (e.g., a tape drive) that was holding the volume, making it available for allocation to another job or volume.
- Manual or Automated: Dismounts can be initiated automatically by the system upon job completion (e.g., when a DD statement's DISP parameter dictates it) or manually by an operator using commands like VARY OFFLINE or via JES3's Dismount command.
- Volume Status Update: The system's volume management facilities update the status of the volume from MOUNTED or ALLOCATED to DISMOUNTED or UNALLOCATED.
- Data Integrity: Ensures that all pending write operations to the volume are completed and buffers are flushed before the logical detachment, safeguarding data integrity.
- Precursor to Physical Removal: For physical removable media like tape cartridges, a logical dismount is a prerequisite for the physical removal of the tape from the drive by an operator or an Automated Tape Library (ATL) system.

Use Cases

- Batch Job Completion: After a COBOL or Assembler batch job finishes processing a tape dataset (e.g., writing output or reading input), the system automatically dismounts the tape volume.
- Tape Library Management: Automated Tape Libraries (ATLs) and Virtual Tape Systems (VTS) use dismount operations to manage the movement of physical or virtual tapes between drives and storage slots.
- System Maintenance: Dismounting volumes may be required before performing maintenance on tape drives or other storage devices, or when moving volumes to offsite storage.
- Resource Optimization: Promptly dismounting volumes after use ensures that valuable tape drives or other removable media devices are freed up and available for other waiting jobs, improving system throughput.
- Data Archiving: Once data has been written to a tape for long-term archiving, the tape is dismounted and typically moved to a secure, off-line storage location.

Related Concepts

Dismount is the inverse operation of mounting a volume, which makes a volume accessible to the system. It is integral to volume management within z/OS, which tracks the status, location, and availability of all storage volumes. It interacts closely with JCL (Job Control Language), particularly the DISP parameter on DD statements, which dictates volume disposition. For tape systems, dismount operations are fundamental to the functionality of Tape Management Systems (TMS) like IBM RMM or CA 1, and Automated Tape Libraries (ATLs), which automate the physical handling and tracking of tapes.

Best Practices:

Timely Release: Design batch jobs and applications to release volumes as soon as they are no longer needed, using appropriate DISP parameters in JCL, to maximize device availability.
Leverage Automation: Utilize the automatic dismount capabilities of z/OS and your Tape Management System to minimize manual operator intervention and potential errors.
Verify Status: For critical operations, operators should verify that volumes have been successfully dismounted and that the associated device is free.
TMS Integration: Ensure your dismount procedures are fully integrated with your Tape Management System to accurately update volume status, retention, and location information.
Error Handling: Implement robust error handling in applications to gracefully manage scenarios where a dismount might fail, preventing resource deadlocks.