RA_PAGES and RA_THRESHOLD

The RA_PAGES configuration parameter indicates the number of pages that the database server brings into memory in a single I/O operation during sequential scans of data or index pages. The RA_THRESHOLD parameter indicates the point at which the database server issues an I/O request to bring in the next set of pages from disk. Because the greater portion of I/O wait time is involved in seeking the correct starting point on disk, you can increase efficiency of sequential scans by increasing the number of contiguous pages brought in with each transfer.

However, setting RA_PAGES too large or RA_THRESHOLD too high with respect to the value of buffers in the BUFFERPOOL configuration parameter can trigger unnecessary page cleaning to make room for pages that are not needed immediately.

Use the following formulas to calculate values for RA_PAGES and RA_THRESHOLD:

RA_PAGES = ((BUFFERS * bp_fract) / (2 * large_queries)) + 2
RA_THRESHOLD = ((BUFFERS * bp_fract) / (2 * large_queries)) - 2 

bp_fract

is the portion of data buffers to use for large scans that require read-ahead. If you want to allow large scans to take up to 75 percent of buffers, bp_fract would be 0.75.

large_queries

is the number of concurrent queries that require read-ahead that you intend to support.