When a query requires a sequential scan of data or index pages, most of the I/O wait time occurs while the database server seeks the appropriate starting page. If you bring in several contiguous pages with each I/O operation, performance for sequential scans improves dramatically. Bringing additional pages in with the first page in a sequential scan is called read-ahead.
The timing of the I/O operations that are required for a sequential scan is also important. If the scan thread must wait for the next set of pages to be brought in after it works its way through each batch, a delay results. Timing second and subsequent read requests to bring in pages before they are needed provides the greatest efficiency for sequential scans.
Adjust the number of pages to bring in and the frequency of read-ahead I/O requests as described in Tuning Read-Ahead Behavior.
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