% Disk Time may exceed 100 percent in Controller
Posted: April 21st, 2008 | Author: TnT Admin | Filed under: Analyze | Tags: Analyze, Disk | No Comments »When you are running a load test and you may have encounter that the % Disk Time exceeds 100% in Controller or when you are analyzing the graphs in Analysis. Over 100%? Weird right? And when you open perfmon to verify, and it’s still more than 100%. Is it a bug of Windows? What’s the cause behind this?
Usually, this happens when the the disk is very busy such as copying of large amount of files out copying multiple files, etc. We must first understand that whatever monitoring data Controller is displaying for Windows is based on Windows Perfmon. You can refer to the article titled, “How does the monitoring work in LoadRunner?” if you need some explanation. This is actually by design of the Windows OS.
The behavior can occur because the OS uses overlapping input/output operations for multiple outstanding requests. The disk performance counters time the responses by using a 100 nanosecond precision counter, and then report the cumulative statistics for a given sample time. This sample time could go over 100 percent if, for example, you have 10 requests that completed in 2 milliseconds each in a 10 millisecond sampling interval. If you have multiple disks in a Raid arrangement, the overlapped input/output happens because the operating system can read and write to multiple disks, and this could show values that are higher than 100 percent for this counter.
(Source: Microsoft Help and Support)
There is no quick fix to this as of this writing and the most accurate way to determine disk utilization is to use % Idle Time in the below formula:
100% – % Idle Time = Actual % Disk Utilization
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