How Db2 writes changed data to disk

Periodically, Db2 must write changed pages from the primary group buffer pool to disk. This process is called castout.

The member that is responsible for casting out the changed data uses its own address space because no direct connection exists from a coupling facility to disk, as shown in the following figure. The data passes through a private buffer, not through the Db2 buffer pools.

Begin figure summary. To write to the disk, the primary group buffer pool passes through the appropriate buffer. Detailed description available. — Figure 1. Writing data to disk. No direct connection exists between the coupling facility and the disk. The data must pass through the address space of DB2A before being written to disk.

When a group buffer pool is duplexed, data is not cast out from the secondary group buffer pool to disk. After a set of pages is written to disk from the primary group buffer pool, Db2 deletes those pages from the secondary group buffer pool.