Prefetch processing

Db2 prefetch processing for GBP-dependent page sets and partitions varies depending on the level of coupling facility (CFLEVEL) in which the group buffer pool is allocated.

If the group buffer pool is allocated in a coupling facility with CFLEVEL=0 or 1, Db2 reads and registers one page at a time in the group buffer pool.

If the group buffer pool is allocated in a coupling facility with CFLEVEL=2 or higher, Db2 can register the entire list of pages that are being prefetched with one request to the coupling facility. This can be used for sequential prefetch (including sequential detection) and list prefetch. On the list, Db2 does not include any valid pages that are found in the local buffer pool.

For those pages that are cached as changed in the group buffer pool, or those that are locked for castout, Db2 still retrieves the changed page from the group buffer pool one at a time. For large, sequential queries, changed pages most likely do not exist in the group buffer pool.

For pages that are cached as clean in the group buffer pool, Db2 can get the pages from the group buffer pool (one page at a time), or can include the pages in the disk read I/O request, depending on which is most efficient.