% 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%.
This is the definition from Microsoft. However, as I saw from the % Disk Time data of my machine's performance monitor, it was always around 300 and 400. Why this does not match the definition of % Disk Time?
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.
In addition, I heard someone said that "If the Avg. Disk Queue Length is larger than the number of CPU cores of the machine then it will cause problem" I don't know if this is true or not because I cannot find anything about this on the Internet.