The output of statistics io reports the number physical reads performed by asynchronous prefetch and the number of reads performed by normal asynchronous I/O. In addition, statistics io reports the number of times that a search for a page in cache was found by the asynchronous prefetch without holding the cache spinlock.
See Chapter 1, Using the set statistics Commands” of the Performance and Tuning Series: Improving Performance with Statistical Analysis.
The sp_sysmon report contains information on asynchronous prefetch in both the “Data Cache Management” section and the “Disk I/O Management” section.
If you use sp_sysmon to evaluate asynchronous prefetch performance, you may see improvements in other performance areas, such as:
Much higher cache hit ratios in the pools where asynchronous prefetch is effective.
A corresponding reduction in context switches due to cache misses, with voluntary yields increasing.
A possible reduction in lock contention. Tasks keep pages locked during the time it takes for perform I/O for the next page needed by the query. If this time is reduced because asynchronous prefetch increases cache hits, locks are held for a shorter time.
See Performance and Tuning Series: Monitoring Adaptive Server with sp_sysmon.