Understanding AIO
Using the vmstat command with an interval and count value, you can determine if the processor is idle waiting for disk I/O. The wa column details the percentage of time the processor was idle with pending local disk I/O.
If there is at least one outstanding I/O to a local disk when the wait process is running, the time is classified as waiting for I/O. Unless AIO is being used by the process, an I/O request to disk causes the calling process to block (or sleep) until the request is complete. After a process's I/O request completes, it is placed on the run queue.
A wa value consistently over 25 percent might indicate that the disk subsystem is not balanced properly, or it might be the result of a disk-intensive workload.
Note: AIO does not relieve an overly busy disk drive. Using the iostat command with an interval and count value, you can determine
if any disks are overly busy. Monitor the %tm_act column
for each disk drive on the system. On some systems, a %tm_act of 35.0 or higher for one disk can cause noticeably slower performance.
The relief for this case could be to move data from more busy to less
busy disks, but simply having AIO does not relieve an overly busy
disk problem.