Disk Latency information from cdm probe looks incorrect

Document ID : KB000092324
Last Modified Date : 14/05/2018
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Question:
Looking at the Disk Latency information monitored by the cdm probe on a Windows system, the Disk Write Latency and Total Latency values look too similar. Is this expected?
Answer:
The Total Latency value is not a sum of Disk Read and Disk Write Latency. It represents the total latency of the disk and it is not unusual for it to look similar to the Disk Write Latency.

On Windows servers the cdm probe relies on Windows performance counters for disk latency information.

The performance counters are mapped as follows: 

Total Latency (QOS_DISK_LATENCY) > PhysicalDisk Object\Avg. Disk sec/Transfer 
Disk Read Latency (QOS_DISK_READ_LATENCY) > PhysicalDisk Object\Avg. Disk sec/Read 
Disk Write Latency (QOS_DISK_WRITE_LATENCY) > PhysicalDisk Object\Avg. Disk sec/Write 

The probe relies on numbers provided by the Operating System, these are not calculated by the probe. 

 
Additional Information:
The following URL contains a list and description of the PhysicalDisk Object performance counters.

https://msdn.microsoft.com/en-us/library/ms804035.aspx