The actual extension of a realm is started automatically by the DBH if space is needed to process a DML command in a realm where the number of free pages has fallen below the minimum requirement. The realm is extended within the user transaction of this DML. Although this increases the response time of the DML, the process does not significantly impact general operation.
Since online realm extension is started as a precaution if there are still a certain number of free database pages available in the realm, transactions requiring free space in the realm can continue to run in parallel with the realm extension operation. These transactions find the required storage space in the original part of the realm. For performance reasons, these transactions do not wait until realm extension is completed but adopt the optimistic strategy that they will always find space.
The time taken for online realm extension depends mainly on the number of pages to be formatted during extension (see section “Database page formatting during realm extension”).
Generally, the realm is logically extended by the number of database pages which were specified when online extensibility was activated, up to the maximum number of database pages possible in the realm (see table 17 in chapter "Estimating the extensibility of a realm" ). If one or several FPA extents are needed to manage the newly added pages, up to 64 PAM pages more may be required than was specified by the DAL command ACT INCR. New FPA extents are located contiguously at the beginning of the extension area, so that subsequent new free pages are created without fragmentation.
If the realm could be extended, the following message is output:
UDS0737 REALM HAS BEEN EXTENDED: (&00) DATABASE-PAGES