For objects participating in data sharing, the thresholds of VDTH and DWTH must be viewed differently as the updated pages are moved to the CF at or before commit, and not when the thresholds are reached. But most important is that the write thresholds are not changed in the individual subsystems. The movement of pages to the CF when updated only occurs for those objects that have inter-DB2 interest. This also means that the normal buffer pool thresholds should not be changed. One of the important impacts of data sharing on buffer pools occurs when the choice is made to cache not only updated pages, but all pages of an object.
Performance tuning any system never ends.
The notes in this column just highlight some of the issues involved with subsystem buffer pools and data sharing. Defining the GBP is a story for another time. Next issue we will take a look at some of the SQL impacts on performance in Version 4.