Group buffer pool castout threshold

The group buffer pool castout threshold determines the total number of changed pages that can exist in the group buffer pool before castout occurs. Db2 casts out enough class castout queues to bring the number of changed pages below the threshold. Db2 periodically determines whether the threshold is exceeded.

How Db2 determines the group buffer pool castout threshold for a duplexed group buffer pool: For duplexed group buffer pools, it is possible to have smaller secondary structure because of space constraints. In that case, Db2 uses the smaller of the number of data entries in the primary and secondary group buffer pools. For example, if the primary structure contains 5000 data entries and the secondary structure contains 1000 data entries, and GBPOOLT is 30%, Db2 sets GBPOOLT to 300 pages (30% of 1000 pages).

Default group buffer pool castout threshold

The default value for the group buffer pool castout threshold is 30, which means that when the group buffer pool is 30% full of changed pages, castout is initiated.

Recommended castout threshold for LOBs

You should set the group buffer pool castout threshold to zero, or a low value, to reduce the need to have a large group buffer pool for LOBs.