Hard disk controllers and disk drives are the two primary components of the disk subsystem. The two objects which gauge hard disk performance are Physical and Logical Disk. Despite the disk subsystem becoming more an powerful, they are still the most common performance bottleneck as their speeds are exponentially slower than other system resources.
In the Windows Server Resource Monitor’s Disk tab, in Windows Server 2008 R2 the physical and logical disk counters are enabled by default . The Disk section in Resource Monitor, shown below, gives a decent high-level overview of the current combined physical and logical disk activity. For more fine-grained monitoring of the disk activity, you should consider using the Performance Monitor component with the desired counters in the Physical Disk and Logical Disk sections.
Monitoring using the Physical and Logical Disk objects comes with a small price however as each object uses a small amount of system resources when they are used for monitoring. As such, they should be disabled unless you are using them for monitoring purposes.
The most useful counters to monitor the disk subsystem are the % Disk Time and Avg. Disk Queue Length counters.
% Disk Time monitors the time that a certain physical or logical drive uses in servicing the read and write requests.
Avg. Disk Queue Length counts the number of requests which have not yet been serviced on the physical or logical drive. The Avg. Disk Queue Length is an interval average and therefore is a numerical representation of the number of delays the disk drive is having. In general, if the delay is often higher than 2, the disks are inadequate to service the system workload and performance may be compromised.