GBP - Group Buffer Pool

Enhanced Definition

A Group Buffer Pool (GBP) is a shared data cache residing in an IBM Coupling Facility (CF), primarily used by DB2 for z/OS Data Sharing Groups. Its main purpose is to enable multiple DB2 members within a data sharing group to access and share common data pages efficiently and consistently, reducing I/O to disk.

Key Characteristics

- Shared Memory Structure: GBPs are allocated in a Coupling Facility, making them accessible to all DB2 members in a data sharing group for high-speed data access.
- Data Consistency: It maintains data consistency across all members by tracking page states (e.g., changed, invalid) and facilitating cross-member invalidation of pages in local buffer pools.
- Page Caching: Caches data pages and directory pages from DB2 table spaces and index spaces, significantly reducing physical I/O to DASD and improving transaction response times.
- GBP Types: Can be defined as CACHE (for both data and directory information) or NO (for directory-only, typically used for read-only data where local caching is sufficient).
- Structure and Size: Defined by a STRUCTURE statement in the Coupling Facility Resource Management (CFRM) policy and sized in terms of 4KB pages.
- Cross-Invalidation: When a page is updated by one DB2 member, the GBP mechanism invalidates that page in the local buffer pools of other members, ensuring they retrieve the latest version from the GBP or disk.

Use Cases

- DB2 Data Sharing Environments: Essential for any DB2 for z/OS data sharing group to allow multiple DB2 subsystems to read and write to the same data concurrently and consistently.
- High-Volume OLTP Workloads: Improves performance for online transaction processing by caching frequently accessed data and reducing disk I/O, especially when transactions span multiple DB2 members.
- Batch Processing with Shared Data: Facilitates efficient access to shared data by batch jobs running on different DB2 members, ensuring data integrity and consistency across concurrent batch streams.
- Disaster Recovery (GDPS): Plays a critical role in disaster recovery solutions like GDPS, where data sharing groups can span multiple sites, leveraging GBPs for cross-site data consistency and recovery.

Related Concepts

GBPs are intrinsically linked to the Coupling Facility (CF), where they physically reside, providing high-speed shared memory. They are the cornerstone of a DB2 Data Sharing Group (DSG), enabling multiple DB2 subsystems (members) to concurrently access and update the same DB2 table spaces and index spaces. GBPs work in conjunction with Local Buffer Pools (LBP) on each DB2 member, managing the flow of data pages between the shared cache and the local caches, and coordinating with the Internal Resource Lock Manager (IRLM) for global locking and data integrity.

Best Practices:

Optimal Sizing: Carefully size GBPs based on workload analysis, aiming for high GBP hit ratios to minimize physical I/O. Too small can lead to excessive I/O; too large can waste CF resources.
Monitoring and Tuning: Regularly monitor GBP activity using DB2 statistics (e.g., DISPLAY GROUPBUFFERPOOL command) and performance tools to identify contention, low hit ratios, or excessive write activity, and tune accordingly.
Separate GBPs: Consider using separate GBPs for different types of data (e.g., frequently updated OLTP data, read-only reference data, index data) to optimize caching strategies and reduce contention.