chngpgs_thresh - Changed pages threshold configuration parameter
This parameter specifies the level (percentage) of changed pages at which the asynchronous page cleaners will be started, if they are not currently active.
- Configuration type
- Parameter type
- Configurable by member in a Db2® pureScale® environment
- Default [range]
- 60 [5 - 99
]Note: The default value is subject to change by the Db2 Configuration Advisor after initial database creation.
- Unit of measure
Asynchronous page cleaners will write changed pages from the buffer pool (or the buffer pools) to disk before the space in the buffer pool is required by a database agent. As a result, database agents should not have to wait for changed pages to be written out so that they might use the space in the buffer pool. This improves overall performance of the database applications.
When the page cleaners are started, they will build a list of the pages to write to disk. Once they have completed writing those pages to disk, they will become inactive again and wait for the next trigger to start.
When the DB2_USE_ALTERNATE_PAGE_CLEANING registry variable is set (that is, the alternate method of page cleaning is used), the chngpgs_thresh parameter has no effect, and the database manager automatically determines how many dirty pages to maintain in the buffer pool.
Recommendation: For databases with a heavy update transaction workload, you can generally ensure that there are enough clean pages in the buffer pool by setting the parameter value to be equal-to or less-than the default value. A percentage larger than the default can help performance if your database has a small number of very large tables.