onmode -C: Control the B-tree scanner
Use the onmode -C command to control the B-tree scanner and specify information about B-tree scanner threads.
| Element | Purpose | Key considerations |
|---|---|---|
| -C | Controls the B-tree scanner for cleaning indexes of deleted items | There is no limit to the number of threads that can run at one time. However, there is a limit of 128 threads that can be started at one time. If, for example, you wanted 150 threads to run, you could execute two commands: onmode -C 100 and onmode -C 50. |
| start count | Starts additional B-tree scanner threads. | If count is not specified, a count of 1 is assumed. There is no limit on the number of scanner threads that can be specified. |
stop count |
Stops B-tree scanner threads. | If count is not specified, a count of
1 is assumed. Stopping all index scanners prevents all index cleaning. Either of these commands stop the B-tree scanner. |
| threshold sizecount | Sets the minimum number of deleted items an index must encounter before an index is placed on the hot list. | Once all indexes above the threshold have been cleaned and there is no other work for the B-tree scanner to do, the indexes below the threshold are added to the hot list. |
| duration num | The number of seconds that the hot list is valid. | After this number of seconds expires, the hot list will be rebuilt by the next available B-tree scanner thread, even if unprocessed items are on the list. Scanners currently processing requests are not interrupted. |
| rangesize size | Determines the size of an index before index range cleaning is enabled. | A size of -1 can be used to disable
range scanning. |
| alice num | Sets the system's alice mode. | Valid num values range from 0 (OFF)
to 12. |
| compression value | For a database server instance, modifies the level at which two partially used index pages are merged. The pages are merged if the data on those pages totals a set level. | Valid values for the level are low, med (medium), high,
and default. The system default value is med. |
The B-tree scanner has statistical information which tracks index efficiency and how much extra work the index currently places on the server. Based on the amount of extra work the index has accomplished because of committed deleted index items, the B-tree scanner develops an ordered list of indexes that have caused the server to do extra work. This list is called the hot list. The index causing the highest amount of extra work is cleaned first and the rest of the indexes are cleaned in descending order. The DBA can allocate cleaning threads dynamically, thus allowing for configurable workloads.
This command has an equivalent SQL administration API function.