VIA records that are stored on the owner page are allocated when the owner record is allocated. If a record is stored at a displacement from its owner page or if it is stored in a different page range than its owner, the VIA record is allocated after all primary allocations are made.
VIA records are also allocated in logical set order (the physical and logical order are the same). This can reduce the number of I/Os required to walk the set.
In an active online environment where buffer use is low, the number of I/Os to walk a VIA set is the same as the number of page changes encountered when walking the set. The simple example in the following figure illustrates that prior to reorganization, the set was spread across 3 pages and it took 6 page changes (6 I/Os) to walk the set. After CA IDMS/DB Reorg, the set is still spread across three pages, but it only takes three page changes (3 I/Os) to walk the set. The net result is a 50 percent savings in I/O.
|
Copyright © 2014 CA.
All rights reserved.
|
|