For each pool in the data caches, a configurable percentage of buffers can be read in by asynchronous prefetch and held until their first use. For example, if a 2K pool has 4000 buffers, and the limit for the pool is 10 percent, then, at most, 400 buffers can be read in by asynchronous prefetch and remain unused in the pool. If the number of nonaccessed prefetched buffers in the pool reaches 400, Adaptive Server temporarily discontinues asynchronous prefetch for that pool.
As the pages in the pool are accessed by queries, the count of unused buffers in the pool drops, and asynchronous prefetch resumes operation. If the number of available buffers is smaller than the number of buffers in the look-ahead set, only that many asynchronous prefetches are issued. For example, if 350 unused buffers are in a pool that allows 400, and a query’s look-ahead set is 100 pages, only the first 50 asynchronous prefetches are issued.
This keeps multiple asynchronous prefetch requests from flooding the pool with requests that flush pages out of cache before they can be read. The number of asynchronous I/Os that cannot be issued due to the per-pool limits is reported by sp_sysmon.