The Ave Disk Queue Length Counter is derived from the product of Avg.Disk sec/Transfer) times Disk Transfers/sec, the average response of the device times the I/O rate. This corresponds to a well-known theorem of Queuing Theory called Little’s Law, which states:
N = l * Sr
where N is the number of outstanding requests in the system, l is the arrival rate of requests, and Sr is the response time. So the Ave Disk Queue Length Counter is an estimate of the number of outstanding request to the (Logical or Physical) disk. This includes any requests that are currently in service at the device, plus any that requests that are waiting for service. If requests are currently waiting for the device inside the SCSI device driver layer of software below the diskperf filter driver, the Current Disk Queue Length Counter will have a value greater than 0. If requests are queued in the hardware, which is usual for SCSI disks and RAID controllers, the Current Disk Queue Length Counter will show a value of 0, even though requests are queued.
Since the Ave Disk Queue Length Counter value is a derived value, not a direct measurement, you do need to be careful how you interpret it. Little’s Law is a very general result that is often used in the field of computer measurement to derive a third result when the other two values are measured directly. However, Little’s Law does require an equilibrium assumption in order for it be valid. The equilibrium assumption is that the arrival rate equals the completion rate over the measurement interval. Otherwise, the calculation is meaningless. In practice, this means you should ignore the Ave Disk Queue Length Counter value for any interval where the Current Disk Queue Length Counter is not equal to the value of Current Disk Queue Length for the previous measurement interval.
Suppose, for example, the Ave Disk Queue Length Counter reads 10.3, and the Current Disk Queue Length Counter shows 4 requests in the disk queue at the end of the measurement interval. If the previous value of Current Disk Queue Length was 0, the equilibrium assumption necessary for Little’s Law does not hold. Since the number of arrivals is evidently greater than the number of completions during the interval, there is no valid interpretation for the value in the Ave Disk Queue Length Counter, and you should ignore the Counter value. However, if both the present measurement of the Current Disk Queue Length Counter and the previous value are equal, then it is safe to interpret the Ave Disk Queue Length Counter as the average number of outstanding I/O requests to the disk over the interval, including both requests currently in service and requests queued for service.
You also need to understand the ramifications of having a total disk round trip time measurement instead of a simple disk service time measure. Assuming M/M/1, a disk at 50% busy has one request waiting on average and disk response time is 2 * service time. This means that at 50% busy – assuming M/M/1 holds, an Ave Disk Queue Length value of 1.00 is expected. That means that any disk with an Ave Disk Queue Length value greater than 0.70 probably has a substantial amount of queue time associated with it. The exception of course is when M/M/1 does not hold, such as during a backup operation when there is only a single user of the disk. A single user of the disk can drive a disk to near 100% utilization without a queue!