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 data pages to read from the table on disk when the database server performs a read-ahead.
The RA_THRESHOLD parameter specifies the number of unprocessed data pages in memory that cause the database server to do another read-ahead. For example, if RA_PAGES is 10, and RA_THRESHOLD is 4, the database server reads ahead 10 data pages when 4 pages remain to be processed in the buffer.
For an example of the output that the onstat -p command produces to enable you to monitor the database server use of read-ahead, refer to Monitoring the Shared-Memory Profile and to the utilities chapter in the IBM Informix: Administrator's Reference.
The IDX_RA_PAGES parameter specifies the number of index pages to read from disk when the database server does a read-ahead of the index. The IDX_RA_THRESHOLD parameter specifies the number of unprocessed index pages in memory that cause the database server to do another read-ahead.
You can tune the values of RA_PAGES, RA_THRESHOLD, IDX_RA_PAGES and IDX_RA_THRESHOLD while the server is on-line using the onutil SET command.
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