Group buffer pool class castout threshold

Each group buffer pool contains a fixed number of castout class queues. This number is an internal value that is set by Db2.

Db2 internally maps updated pages that belong to the same page sets or partitions to the same castout class queues. Because of a limited number of castout class queues, it is possible that more than one page set or partition gets mapped into the same castout class queue. This internal mapping scheme is the same across all sharing subsystems.

When Db2 writes changed pages to the group buffer pool, it determines how many changed pages are on a particular class castout queue. If the number of changed pages on a specified castout class queue exceeds the threshold, Db2 casts out a number of pages from that queue.

How Db2 determines the class castout threshold for a duplexed group buffer pool

For duplexed group buffer pools, Db2 uses the smaller of the number of data entries in the primary and secondary structures. For example, if the primary structure contains 5000 data entries, the secondary structure contains 1000 data entries, and CLASST is 10,0, Db2 sets the class castout threshold to 100 pages (10% of 1000 pages).

Default group buffer pool class castout threshold

The default for the class castout threshold (CLASST) is 5,0, which means that castout is initiated for a particular page set or partition when 5% of the group buffer pool contains changed pages for the class.