The following list describes the most commonly used logical disk counters in simple terms. (To see the complete list, scroll through the Physical Disk and Logical Disk counters listed in Performance Monitor and read the explanatory text for each counter.)
Logical Disk and Physical Disk counters | Description |
% Disk Time | How often is the disk busy? If it's busy almost all of the time, and there is a large queue, the disk might be a bottleneck. This value is displayed as a percentage and is capped at 100%. |
Avg. Disk Queue Length | How often is disk busy? If it's busy almost all of the time, and there is a large queue, the disk might be a bottleneck. This counter displays %Disk Time as a decimal with no defined maximum. (A %Disk Time of 100% equals an Avg. Disk Queue Length of 1.0.) This counter is recommended for disk sets where the combined activity of the disks can exceed 100% of a single disk. |
Current Disk Queue Length (Known in previous versions as Disk Queue Length) | Are disk requests waiting? If more than two request are waiting over time, the disk may be a bottleneck. Unlike the other queue measures, this one measures requests, not time. It includes the request being serviced as well those waiting and is an instantaneous value, not an average. |
Avg. Disk sec / Transfer | How fast is data being moved (in seconds)? This counter measures the average time of each data transfer, regardless of the number of bytes read or written. It shows the total time of the read or write, from the moment it leaves the Disk Performance Statistics Driver to the moment it is complete. Avg. Disk sec / Read is usually in multiples of the time it takes for one rotation of the disk. Any variation usually represents time for driver processing or time in the queue. For example, on a 3600 rpm disk, the actual Avg. Disk sec/Read would be in multiples of 16 milliseconds. |
Disk Bytes / sec | How fast is data being moved (in bytes)? This is the primary measure of disk throughput. |
Avg. Disk Bytes / Transfer | How much data is being moved on each transfer? This counter measures throughput, an indication of disk efficiency. The disk is efficient if it is transferring large amounts of data relatively quickly (Avg. Disk sec/Transfer). This is the counter to watch when measuring maximum throughput. |
Disk Transfers / sec | How fast are data requests being serviced? This measures the number of read and writes completed per second, regardless of how much data they involve. Transfers/sec is not the inverse of sec/Transfer. Transfers/sec counts the number of completed I/O requests in each second. Sec/Transfer times the transfer from its inception to completion. |