Shared DASD

Enhanced Definition

Shared DASD (Direct Access Storage Device) refers to a configuration where one or more physical DASD volumes or storage subsystems are concurrently accessible by multiple z/OS systems (LPARs or standalone systems). Its primary purpose is to enable data sharing, enhance high availability, and facilitate workload balancing across an enterprise mainframe environment.

Key Characteristics

- Multi-System Access: Multiple z/OS images can read from and write to the same physical DASD volumes simultaneously, often within a Parallel Sysplex or a multi-LPAR environment.
- Data Integrity Criticality: Requires sophisticated software mechanisms (like Global Resource Serialization - GRS, or MIM/CA-MII) to manage concurrent access and prevent data corruption due to conflicting updates.
- Hardware Foundation: Typically implemented using high-end storage controllers (e.g., IBM DS8000 series) that provide advanced features like caching, mirroring, and multiple host connections.
- I/O Path Management: Involves careful planning of channel paths, control units, and device addresses to ensure efficient and redundant access from all connected systems.
- Performance Considerations: Can introduce I/O contention if not properly managed, requiring monitoring and tuning of dataset placement, caching, and serialization mechanisms.

Use Cases

- DB2 Data Sharing: Multiple DB2 subsystems within a Parallel Sysplex accessing and updating the same set of DB2 databases stored on shared DASD, providing high availability and scalability.
- IMS Data Sharing: Similar to DB2, multiple IMS systems sharing access to IMS databases residing on shared DASD, often coordinated via the Coupling Facility.
- Shared System Files: Housing common system datasets like catalogs, PDS/PDSE libraries, and spool files that need to be accessible by multiple z/OS systems for consistent operation.
- Application Data Sharing: Custom applications that require multiple z/OS batch jobs or CICS regions to access and update the same application data files (e.g., VSAM KSDSs) for coordinated processing.
- Disaster Recovery: Utilizing shared DASD configurations with replication technologies (e.g., GDPS, Metro Mirror) to maintain synchronized data copies for rapid recovery in a disaster scenario.

Related Concepts

Shared DASD is a foundational component of an IBM Parallel Sysplex, enabling the sharing of data and resources among cooperating z/OS systems. It heavily relies on Global Resource Serialization (GRS) or third-party alternatives (like MIM/CA-MII) to ensure data integrity by serializing access to shared resources. The Coupling Facility (CF) often works in conjunction with shared DASD, providing high-speed caching, locking, and list structures that enhance performance and coordination for data sharing products like DB2 and IMS. The underlying DASD Subsystem (e.g., IBM DS8000) provides the physical storage and advanced features necessary for robust shared DASD implementations.

Best Practices:

Implement Robust Serialization: Always use GRS (STAR or Ring mode) or a third-party serialization product (e.g., MIM/CA-MII) to protect shared datasets from concurrent update conflicts.
Optimize I/O Configuration: Design redundant I/O paths with sufficient capacity (multiple channels, control units, and paths) to minimize contention and ensure high availability.
Monitor Performance and Contention: Regularly monitor I/O rates, queue depths, and GRS enqueue activity to identify and resolve potential bottlenecks or serialization issues.
Plan for Recovery and Backup: Ensure that backup and recovery procedures are designed to handle shared datasets consistently across all accessing systems, especially for critical data.
Utilize Dataset Sharing Options: For VSAM datasets, use SHAREOPTIONS(3 3) or (4 3) for cross-system sharing, and understand the implications for data integrity and performance.