Purpose
Enables or disables read-ahead operations for sequential table access.
Field Group
Performance
Valid Settings
| Setting | Description |
|---|---|
| TRUE | Read-ahead is enabled for sequential file access. |
| FALSE | No read-ahead is issued. |
Default
TRUE
Changes Take Effect
After the DBS Control Record has been written. Operations in progress at the time of the change are not affected.
Usage Notes
The File System can issue a read-ahead I/O to bring the next data block, or group of data blocks, into memory whenever a data block is read. Preloading data blocks in advance allows processing to occur concurrently with I/Os, and can improve processing time significantly when tables are being scanned sequentially, for example, when running commands such as SCANDISK.
When ReadAhead is enabled, the number of data blocks that are preloaded is determined by the Read Ahead Count, StandAloneReadAheadCount, and UtilityReadAheadCount fields.
If the Cylinder Read feature is enabled, and there are cylinder slots available, all data blocks on the cylinder are read into memory in one read operation. In this case, individual data blocks are not preloaded. For more information on Cylinder Read, see Control GDO Editor (ctl).