A high-performance disk is capable of about 50 to 70 random or up to 160 sequential I/O operations per second. The components you are using, as well as the request size, bus speed, and other factors, determine your system's capacity. Judge the maximum acceptable usage that your system can sustain based on your experience. Disk-time values should not consistently exceed the rate you've established as your baseline for performance. Consistent values in the 70 percent to 85 percent range are a definite cause for concern. However, if a queue is developing, lower percentages might indicate a disk that is unable to handle the load. If you see extremely high rates of disk usage, investigate the factors that might be responsible. Monitoring Disk Transfers/sec (a counter with values equal to the sum of Disk Reads/sec and Disk Writes/sec) or the individual counters Disk Reads/sec and Disk Writes/sec can show you the number of requests for service by the disk; the values of these counters provide a measure of disk demand.
If your workloads consist of random bursts of high activity, you might see high activity rates followed by long periods of idle time. If you only look at the average counter values with these types of workload, it can appear that your disk isn't very busy even though it was bottlenecked during those bursts of high activity. To determine how well your disk system is handling these bursts, sample at short intervals when the activity occurs.
The disk time counters can yield inaccurate values when multiple disks are in use. You can compensate for this by monitoring % Idle Time and comparing its value with the values reported by the % Disk Time, % Disk Read Time, and % Disk Write Time counters.
Figure 8.7 depicts maximum disk usage and the development of a queue.
Figure 8.7 High Disk Time Values