How Db2 writes changed data to disk in a data sharing environment

Periodically, Db2 must write changed pages from the group buffer pool to disk. This process is called castout. The castout process runs in the background without interfering with transactions.

Suppose that DB2A is responsible for casting out the changed data. That data must first pass through DB2A's address space because no direct connection exists between the coupling facility and disk. (See the following figure.) This data passes through a private buffer, not through the Db2 buffer pools.

Figure 1. Writing data to disk
Begin figure description. Data from the primary group buffer pool passes through a buffer belonging to the responsible member. From there, it is written to disk. End figure description.

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