This section provides data about Adaptive Server prefetch requests for large I/O. It reports statistics on the numbers of large I/O requests performed and denied.
“Large I/Os Performed” measures the number of times that a requested large I/O was performed. “% of total” reports the percentage of large I/O requests performed as a percentage of the total number of requests made.
“Large I/Os Denied” reports the number of times that large I/O could not be performed. “% of total” reports the percentage of large I/O requests denied as a percentage of the total number of requests made.
Adaptive Server cannot perform large I/O:
If any page in a buffer already resides in another pool.
When there are no buffers available in the requested pool.
On the first extent of an allocation unit, since it contains the allocation page, which is always read into the 2K pool.
If a high percentage of large I/Os were denied, it indicates that the use of the larger pools might not be as effective as it could be. If a cache contains a large I/O pool, and queries perform both 2K and 16K I/O on the same objects, there will always be some percentage of large I/Os that cannot be performed because pages are in the 2K pool.
If more than half of the large I/Os were denied, and you are using 16K I/O, try moving all of the space from the 16K pool to the 8K pool. Re-run the test to see if total I/O is reduced. Note that when a 16K I/O is denied, Adaptive Server does not check for 8K or 4K pools, but uses the 2K pool.
You can use information from this category and “Pool Turnover” to help judge the correct size for pools.
“Total Large I/O Requests” provides summary statistics for large I/Os performed and denied.