Determining the correct size and ratio of group buffer pools

One of the critical tuning factors in a Db2 data sharing configuration is the size of the group buffer pools.

Consider the following aspects of group buffer pool (cache structure) size:

Total structure size

The total structure size of a group buffer pool is specified in the coupling facility policy definition for the cache structure.

Number of directory entries

A directory entry is used by the coupling facility to determine where to send cross-invalidation signals when a page of data is changed or when that directory entry must be reused. A directory entry contains control information for one database page, no matter in how many places that page is cached. For example, if page P1 is cached in the group buffer pool and in the buffer pools of three members, that page still has only one directory entry.

Number of data entries

Data entries are the actual places where the data page resides. These are 4 KB, 8 KB, 16 KB, or 32 KB in size (the same size as the data page).

For GBPCACHE NO group buffer pools, no data entries exists.

The number of directory entries and data entries in the coupling facility structure is determined by the size specified in the coupling facility policy and the ratio of directory entries to data pages. The ratio is automatically defined for each group buffer pool at the time that the first member of the group is installed.

For secondary group buffer pools, the ratio is the same as the ratio used for the primary group buffer pools.

After installation, you can change the ratio with the ALTER GROUPBUFFERPOOL command. However, the change does not take effect until the next time the group buffer pool is allocated.

The following sections describe the symptoms and values that are not ideal for best performance, and discuss how you can fix the problems.