I am concerned about the overhead of the diskperf measurements. What does this feature cost?

There is very little overhead involved with the collection of diskperf measurements.  We strongly recommend that you enable all disk performance data collection on any system where you care about performance.

Even if you don’t care that much about performance, you should turn on Logical Disk reporting at a minimum. The Logical Disk Object contains two Counters, Free Megabytes and % Free Space, that will alert you in advance to potential out-of-disk space conditions.

The diskperf measurement layer does add some code to the I/O Manager stack, so there is added latency associated with each I/O request that accesses a physical disk when measurement is turned on. However, the overhead of running the diskperf measurement layer, even twice on Windows 2000 machines, is trivial. In a benchmark environment where a 550 MHz 4-way Windows 2000 Server was handling 40,000 I/Os per second, enabling the diskperf measurements reduced its I/O capacity by about 5% to 38,000 I/Os per second. In that environment, we estimated that the diskperf measurement layer added about 3-4 microseconds to the I/O Manager path length for each I/O operation. (On a faster processor, the delay is proportionally less.) For a disk I/O request that you would normally expect to require a minimum of 3-5 milliseconds, this additional latency is hardly noticeable.

Besides, if you do not have disk performance statistics enabled and a performance problem occurs that happens to be disk-related (and many are), you won’t be able to gather data about the problem because loading the diskperf measurement layer requires a reboot!

In our view, you can only justify turning off the disk performance stats in a benchmark environment where you are attempting to wring out the absolute highest performance level from your hardware configuration. Of course, you will need to have the diskperf measurements enabled initially to figure out how to optimize the configuration in the first place. It is standard practice to disable disk performance monitoring prior to making your final measurement runs.