Purpose
Specifies the number of data blocks that will be preloaded into memory in advance of the current data block when performing sequential table scans if the ReadAhead field is set to TRUE.
Field Group
Performance
Valid Range
1 through 100 data blocks
Default
1 data block
Changes Take Effect
After the DBS Control Record has been written.
Usage Notes
Increasing the Read Ahead Count field setting can reduce the amount of CPU time spent waiting for read operations to finish. Read Ahead Count should be set high enough to make the typical sequential scan limited only by the CPU, rather than by the read I/O.
The CPU must work harder when data blocks are large or row selection criteria are complex. Consequently, if either of these conditions exist, read ahead counts can be lower.
For example, if a large default data block size is used, most of the tables will consist of large data blocks, and the default ReadAheadCount will suffice. If the default data block size is made very small, most of the tables will consist of small data blocks and system performance might benefit by increasing the ReadAheadCount to 25 or higher.