For sequential table or index scans, you can configure the database server to read several pages ahead while the current pages are being processed. A read-ahead enables applications to run faster because they spend less time waiting for disk I/O.
The database server performs a read-ahead whenever it detects the need for it during sequential data or index reads.
The RA_PAGES parameter in the ONCONFIG file specifies the number of pages to read from disk or the index when the database server performs a read-ahead.
The RA_THRESHOLD parameter specifies the number of unprocessed pages in memory that cause the database server to perform another read-ahead. For example, if RA_PAGES is 10, and RA_THRESHOLD is 4, the database server reads ahead 10 pages when 4 pages remain to be processed in the buffer.
For more information on using the onstat -p command to monitor the database server use of read-ahead, see Monitoring the Shared-Memory Profile and Latches. For an example of onstat -p output, see the IBM Informix Administrator's Reference.
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