Queries using large I/O during sequential scans may still need to perform 2K I/O:
When a scan enters a new allocation unit, it performs 2K I/O on the 7 pages in the unit that share space with the allocation page.
If pages from the allocation unit already reside in the 2K pool when the prefetch requests are issued, the pages that share that extent must be read into the 2K pool.
If the 2K pool has its asynchronous prefetch limit set to 0, the first 7 reads are performed by normal asynchronous I/O, and the query sleeps on each read if the pages are not in cache. Set the limits on the 2K pool high enough that it does not slow prefetching performance.
