Sequential prefetch, or large I/O

Adaptive Server data caches can be configured to allow large I/Os. When a cache allows large I/Os, Adaptive Server can 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 takes nearly the same amount of time as a single page, 2K I/O. Reading eight pages using eight 2K I/Os is nearly eight times more costly than reading eight pages using a single 16K I/O. Table scans perform much better when you use large I/Os.

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.

See Chapter 5, “Memory Use and Performance,” in Performance and Tuning Series: Basics.