In earlier versions of Adaptive Server, the maximum value allowed for number of pre-allocated extents was 31. For the Cluster Edition the maximum values for number of pre-allocated extents is 32.
Using a value of 32 for number of pre-allocated extents has a special significance for configuration and impacts the space allocations Adaptive Server performs internally. If you set number of pre-allocated extents to 32, Adaptive Server reserves an entire allocation unit worth of extents for utility operations like BCP-in and select into, both of which use the large-scale allocation scheme of space reservation. This greatly improves the performance of these utilities, particularly when you run them concurrently on multiple nodes. Consequently, using a value of 32 guarantees that each node of a cluster is able to work independently on its own allocation unit without interference from the other nodes.
In earlier versions of Adaptive Server, the number of pre-allocated extents parameter specified the number of extents reserved in a single allocation call for tables of all sizes.
With this version of Adaptive Server, the value of number of pre-allocated extents is ignored for large tables with 240 or more pages for these commands only:
alter table table_name add column_name . . .
alter table table_name modify column_name . . .
alter table table_name drop column_name . . .
alter table lock . . .
reorg rebuild
When you run these command on tables larger than 240 pages, Adaptive Server reserves an entire allocation unit (32 extents), which greatly improves performance, particularly when you run them concurrently on multiple nodes.
The value of number of pre-allocated extents continues to be observed for the above commands for tables with fewer than 240 pages, and for all commands (such as select into, bcp, alter table partition) for tables of all sizes.
