num_iocleaners - Number of asynchronous page cleaners configuration parameter
This parameter allows you to specify the number of asynchronous page cleaners for a database.
- Configuration type
- Database
- Parameter type
- Configurable
- Configurable by member in a Db2® pureScale® environment
- Default [range]
- Automatic [0 - 255 ]
- Unit of measure
- Counter
The number of castout engines is equal to the value that is set for the num_iocleaners configuration parameter. However, the maximum number of castout engines is 128.
These page cleaners write changed pages from the buffer pool 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.
The page cleaners will also decrease recovery time from soft failures, such as power outages, because the contents of the database on disk will be more up-to-date at any given time.
If this parameter is set to AUTOMATIC, the number of page cleaners started will be based on the number of physical CPU cores configured on the current machine, as well as the number of local logical database partitions in a partitioned database environment. There will always be at least one page cleaner started when this parameter is set to AUTOMATIC.
number of page cleaners = max( ceil( # CPUs / # local logical DPs ), 1 )
This
formula ensures that the number of page cleaners is distributed almost evenly across your logical
database partitions, and that there are no more page cleaners than there are physical CPU cores.
- Workload
Environments with high update transaction rates might require more page cleaners to be configured. This is only applicable when DB2_USE_ALTERNATE_PAGE_CLEANING is OFF, which is also the default value.
- Buffer pool sizes
Environments with large buffer pools might also require more page cleaners to be configured. This is only applicable when DB2_USE_ALTERNATE_PAGE_CLEANING is OFF, which is also the default value.
- The parameter can be reduced if both of the following conditions
are true:
- pool_data_writes is approximately equal to pool_async_data_writes
- pool_index_writes is approximately equal
to pool_async_index_writes.Note: Reducing to a lower value, than what is computed via AUTOMATIC, is not recommended.
- The parameter should be increased if either of the following conditions
are true:
- pool_data_writes is much greater than pool_async_data_writes
- pool_index_writes is much greater than pool_async_index_writes.