Sequential prefetch, or large I/O

Adaptive Server‘s data caches can be configured by a System Administrator to allow large I/Os. When a cache is configured to allow large I/Os, Adaptive Server can choose to prefetch data pages.

Caches have buffer pools that depend on the logical page sizes, allowing Adaptive Server to read up to an entire extent (eight data pages) in a single I/O operation.

Since much of the time required to perform I/O operations is taken up in seeking and positioning, reading eight pages in a 16K I/O performs nearly eight times as fast as a single-page, 2K I/O, so queries that table scan should perform much better using large I/O.

When several pages are read into cache with a single I/O, they are treated as a unit: they age in cache together, and if any page in the unit has been changed while the buffer was in cache, all pages are written to disk as a unit.

For more information on configuring memory caches for large I/O, see Chapter 15, “Memory Use and Performance.”

NoteReference to Large I/Os are on a 2K logical page size server. If you have an 8K page size server, the basic unit for the I/O is 8K. If you have a 16K page size server, the basic unit for the I/O is 16K.