Group Buffer Pool

GBP

Enhanced Definition

A Group Buffer Pool (GBP) is a shared cache structure residing in the IBM z/OS Coupling Facility (CF) that manages data pages (table spaces and index spaces) for a DB2 data sharing group. Its primary purpose is to ensure data consistency and availability across all members of the data sharing group, facilitating concurrent access to shared data.

Key Characteristics

- Location: Resides exclusively within the IBM z/OS Coupling Facility (CF), leveraging its high-speed memory and inter-processor communication capabilities.
- Data Coherency: Maintains data consistency across all DB2 members in a data sharing group through mechanisms like page locks (P-locks) and cross-invalidation signals.
- Types: Can be defined as a data GBP (typically GBP0) to cache actual data pages, or a directory-only GBP (e.g., GBP1) used primarily for tracking page status and managing coherency.
- Recovery Facilitation: Plays a critical role in DB2 group restart and fast write processing, ensuring data integrity and rapid recovery after failures.
- Shared Access: Allows multiple DB2 subsystems (members) within a data sharing group to concurrently read and update the same data pages without compromising data integrity.
- Structure: Composed of data entries (for caching pages) and directory entries (for tracking the status and location of pages in member local buffer pools).

Use Cases

- High Availability and Scalability: Enables multiple DB2 members to access and update the same data concurrently, providing high availability and allowing workloads to be scaled out across multiple z/OS LPARs.
- Workload Balancing: Distributes transaction processing across various DB2 members, optimizing resource utilization and improving overall system throughput for shared data.
- Disaster Recovery: Facilitates group restart of a DB2 data sharing group after a member or Coupling Facility failure, ensuring rapid recovery and minimal downtime.
- Performance Enhancement: Reduces physical I/O to disk by caching frequently accessed data pages in the high-speed CF memory, improving transaction response times.

Related Concepts

The GBP is a fundamental component of DB2 Data Sharing, which relies heavily on the Coupling Facility (CF) for its shared memory and lock management services. It works in conjunction with each DB2 member's local buffer pools, acting as a higher-level, shared cache. Data pages flow between local buffer pools and the GBP. Furthermore, the GBP interacts closely with the Lock Structure in the CF to manage P-locks and maintain page-level data coherency across the entire data sharing group.

Best Practices:

Optimal Sizing: Carefully size GBPs based on workload characteristics, data access patterns, and available CF memory to minimize cross-invalidation and maximize the cache hit ratio.
Continuous Monitoring: Regularly monitor GBP activity, including read/write ratios, cross-invalidation rates, and false contention, using DB2 statistics (SMF type 100 records) and tools like OMEGAMON.
Isolation of Critical Data: Consider allocating separate GBPs for highly active or critical table spaces/index spaces to reduce contention and improve performance for specific workloads.
CF Connectivity and Redundancy: Ensure robust and redundant connectivity to the Coupling Facility, configuring multiple CF links and duplexing GBPs where appropriate, to prevent single points of failure.
Leverage Fast Write: For frequently updated data, configure fast write capability to allow DB2 to write updated pages to the GBP and log, deferring synchronous writes to disk and improving transaction commit times.