Adaptive Server and the operating system place limits on the number of outstanding I/Os for the server as a whole and for each engine. The configuration parameters max async i/os per server and max async i/os per engine control these limits for Adaptive Server. See your operating system documentation for more information on configuring them for your hardware.
The configuration parameter disk i/o structures controls the number of disk control blocks that Adaptive Server reserves. Each physical I/O (each buffer read or written) requires one control block while it is in the I/O queue.
See the System Administration Guide.
If Adaptive Server tries to issue asynchronous prefetch requests that would exceed max async i/os per server, max async i/os per engine, or disk i/o structures, it issues enough requests to reach the limit and discards the remaining requests. For example, if only 50 disk I/O structures are available, and the server attempts to prefetch 80 pages, 50 requests are issued, and the other 30 are discarded.
sp_sysmon reports the number of times these limits are exceeded by asynchronous prefetch requests. See “Asynchronous prefetch activity report”.