EDTENTRY
An EDT Entry (Eligible Device Table Entry) is a data structure within the z/OS operating system that describes a specific tape device type or a pool of identical tape devices available for dynamic allocation. It contains information about the device's characteristics and its eligibility criteria, enabling the system to select an appropriate tape drive when a job requests one without specifying a unit address. An `EDTENTRY` (Event Definition Table Entry) is a CICS resource definition that specifies an **event specification** for **CICS Event Processing (CEP)**. It defines the characteristics of a business or system event that CICS should detect, capture, and emit, enabling event-driven architectures within the mainframe environment.
Key Characteristics
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- Device Description: Each EDT Entry defines a specific type of tape device, such as
3490,3590, or a genericTAPE, along with its associated features and capabilities (e.g., density, recording format). - Dynamic Allocation Enabler: It is primarily used by the z/OS system to facilitate the dynamic allocation of tape drives, allowing jobs to request a generic device type rather than a specific physical unit address.
- HCD Definition: EDT Entries are defined as part of the I/O configuration using the Hardware Configuration Definition (HCD) utility, which generates the IODF (I/O Definition File) loaded during an IPL.
- Unit Control Block (UCB) Association: While an EDT Entry describes a *type* of device, individual physical tape drives are represented by Unit Control Blocks (UCBs), which are linked to the appropriate EDT Entry.
- Eligibility Criteria: The entry specifies criteria that make a device eligible for selection, such as whether it's a specific device type or part of a generic pool.
- System-Wide Scope: Once defined and active via an IPL or dynamic I/O activation, EDT Entries are available system-wide for all address spaces requiring tape device allocation.
- Device Description: Each EDT Entry defines a specific type of tape device, such as
Use Cases
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- JCL Tape Allocation: A common use is in JCL where a
UNITparameter specifies a generic device type (e.g.,UNIT=TAPE,UNIT=3490,UNIT=CART) instead of a specific device address. The system uses EDT Entries to find an eligible, available drive. - Dynamic Allocation APIs: Programs using dynamic allocation services (e.g.,
SVC 99orDYNALLOCmacros) can request tape devices based on generic names defined in the EDT, allowing for flexible resource management. - Tape Management Systems Integration: Tape Management Systems (TMS) often interact with EDT Entries to manage tape volumes and assign them to available drives, ensuring proper device utilization and tracking.
- Device Pooling: Facilitates the creation of pools of identical tape devices, allowing the system to pick any available drive from that pool, improving throughput and reducing contention.
- JCL Tape Allocation: A common use is in JCL where a
Related Concepts
EDT Entries are a crucial component of the z/OS I/O configuration, working closely with the Hardware Configuration Definition (HCD) process, which is used to define them within the IODF. They are intrinsically linked to Unit Control Blocks (UCBs), as each physical tape drive has a UCB that references an EDT Entry. The EDT is fundamental to Dynamic Allocation, enabling the system to assign resources based on generic requests rather than fixed addresses. It also interacts with Tape Management Systems by providing the system's view of available tape device types.
- Define Generic Unit Names: Use meaningful generic unit names (e.g.,
TAPE,CART,3590) in HCD to allow for flexibility and future hardware upgrades without requiring JCL changes. - Optimize Device Pools: Group similar tape devices into appropriate EDT pools to maximize resource utilization and minimize job waiting times for tape drives.
- Regular HCD Review: Periodically review and update the HCD definitions for EDT Entries to accurately reflect changes in hardware, device types, or operational requirements.
- Consistent Naming Conventions: Establish and adhere to consistent naming conventions for generic unit names across your z/OS environment to improve clarity and reduce operational errors.
- Consider Future Capacity: When defining EDT Entries, anticipate future tape device needs and define sufficient generic pools to accommodate growth and new applications.