Administration Guide

Managing Multiple Database Buffer Pools

Each database requires at least one buffer pool. However, depending on your needs you may choose to create several buffer pools, each of a different size, for a single database. The CREATE, ALTER, and DROP BUFFERPOOL statements allow you to create, change, or remove a buffer pool. You can specify which data is cached in a buffer pool with the CREATE TABLESPACE and ALTER TABLESPACE statements.

The buffpage configuration parameter specifies the size of any buffer pool, if the buffer pool's size is specified as -1 in the SYSCAT.BUFFERPOOLS catalog view. (Otherwise this parameter is ignored.) A buffer pool's size can be set with the DDL statements ALTER BUFFERPOOL or CREATE BUFFERPOOL.

A new or migrated database has a default buffer pool called IBMDEFAULTBP. Migrated databases have a default buffer pool with a size determined by the buffpage configuration parameter (because the SIZE of the buffer pool in the SYSCAT.BUFFERPOOLS catalog view is set to -1 for migration). New databases have a default buffer pool with a size determined by the platform. Once a database is created or migrated, then other buffer pools can be created for it.

When working on your database design, you may have determined that tables with 8 KB page sizes were best. As a result, you should create a buffer pool with an 8 KB page size (along with one or more table spaces with the same page size).

In a partitioned database environment, each buffer pool for a database has the same default definition on all database partitions (unless it was otherwise specified in the CREATE BUFFERPOOL statement, or the buffer pool's size was changed for a particular database partition with the ALTER BUFFERPOOL statement).

When you create a table space with a page size of 4K and do not assign it to a specific buffer pool, the table space is assigned to the default buffer pool. If you create a table space with a page size of 8K, you should assign it to a buffer pool that uses 8K pages. If this buffer pool is currently not active, DB2 will attempt to assign the table space to an active buffer pool that uses 8K pages (if one is available). This assignment, if made, is temporary. When the database is activated again, and the originally specified buffer pool is active, then DB2 assigns the table space to that buffer pool.

You cannot use the ALTER TABLESPACE statement to add the table space to a buffer pool that uses a different page size.

When creating or altering buffer pools, the total memory that is required by all buffer pools must be available to the database manager so that all of the buffer pools can be allocated when the database is started. Should this memory not be available when a database is started, the database manager attempts to start the default buffer pool (IBMDEFAULTBP), but only with a minimal size. A warning message is returned with each failed attempt to start a buffer pool; the database continues in this operational state until its configuration is changed and the database can be fully restarted.

Note:

Although the size and attributes associated with the default buffer pool can be changed, it cannot be dropped. Also, there is a minimum size for each buffer pool that is based on the platform being used.

There are advantages to having a large amount of memory allocated to buffer pools. For example, larger buffer pool sizes:

• Enable often-requested data pages to be kept in the buffer pool, allowing for quicker access. Fewer I/O operations can reduce I/O contention, thereby providing better response time and reducing the processor resource needed for I/O operations.

• Provide the opportunity to achieve higher transaction rates with the same response time.

• Prevent I/O contention for frequently used disk storage devices such as catalog tables and frequently referenced user tables and indexes. Sorts required by queries also benefit from reduced I/O contention on the disk storage devices containing the temporary table spaces.

Choosing One or Many Buffer Pools

If any of the following conditions apply to your system, you should use only a single buffer pool:

• The total buffer space is less than 10 000 4-KB pages.
• People with the application knowledge to do specialized tuning are not available.
• You are working on a test system.

If your system is not constrained by these conditions, then consider using more than one buffer pool for the following potential performance improvements:

• You can put temporary table spaces into a separate buffer pool to provide better performance for queries that require temporary storage, especially sort-intensive queries.

• If you have data that must be accessed repeatedly and quickly by many short update transaction applications, then you should consider moving the table space containing the data into a separate buffer pool. If this buffer pool is sized appropriately, its pages have a better chance of being found, contributing to a lower response time and a lower transaction cost.

• You can isolate data into separate buffer pools to favor certain applications, data, and indexes. For example, you might want to put tables and indexes that are updated frequently into a buffer pool that is separate from those tables and indexes that are frequently queried but infrequently updated. This change will reduce the impact of the frequent updates (on the first set of tables) on the frequent queries (on the second set of tables).

• You can use smaller buffer pools for the data accessed by applications that are seldomly used, especially in the case where an application requires very random access into a very large table. In such a case, there is no need to keep the data in buffer pool memory for longer than a single query. It is better to keep a small buffer pool for this data, and free up the extra memory for other uses (for example, for other buffer pools).

• After separating different activities and data into separate buffer pools, good and relatively inexpensive performance diagnosis data can be produced from statistics and accounting traces.

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