In IBM Db2 databases, data is physically stored on disk across multiple files. Each file is divided into fixed-size blocks known as pages, which serve as the smallest unit of data exchange between disk storage and main memory.
To optimize DB2 database performance, it is critical to keep as many of these pages in main memory (RAM) as possible, reducing the need for slow disk I/O. However, since memory is limited, DB2 uses a specialized memory area called the buffer pool to cache frequently accessed data.
The buffer manager subsystem controls this buffer pool, managing the allocation, replacement, and movement of pages between disk and memory to ensure efficient use of system resources.
How Buffer Pools Work in DB2
- Data Access Workflow: Db2 reads and writes pages between disk storage devices and buffer pools. All standard data and index keys are brought into the buffer pool before any operations—such as reads, scans, updates, inserts, or deletes—are performed.
- LOB Exception: Large Objects (LOBs), such as large binary files, are typically accessed directly from disk and bypass the buffer pool to avoid memory overload.
- Default Buffer Pool: When a new database is created, Db2 automatically generates a default buffer pool named IBMDEFAULTBP. This default buffer pool uses a 4 KB page size, unless otherwise specified. If a tablespace is created with a 4K page size and no buffer pool is explicitly assigned, Db2 automatically maps it to IBMDEFAULTBP.
Supported Buffer Pool Page Sizes in Db2
Db2 supports multiple page sizes to accommodate different workload patterns:
- 4 KB (default, IBMDEFAULTBP)
- 8 KB
- 16 KB
- 32 KB
Choosing the correct page size is a critical tuning decision. For transaction-heavy OLTP workloads, smaller pages (4K, 8K) are often more efficient. For analytical or large object workloads, larger page sizes (16K, 32K) may reduce I/O and improve scan performance.
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