iSCSI drive running at 100% but SAN and Network are not [closed]

We have a virtual server running on VMware. This virtual server has a number of iSCSI drives attached to it (E, F & G drives). It has 2 virtual NICs – 1 for normal network traffic and 1 for iSCSI. Both NICs are connected at 1Gbps. We are running Windows 2012.

Every half hour we run a process that is quite intensive on 1 drive (say F drive). In PerfMon we are seeing that the % Disk Read Time is at 100% while this process runs. Looking at task manager the throughput on the iSCSI connection rarely exceeds 500Mbps, and normally sits around 200-300Mbps while this process runs.

The graph of the disk activity in the SAN and on the switch between the host and SAN also shows that they are running well under capacity.

I saw this question, however I don’t understand the answer and it doesn’t seem relevant to me (I could be wrong): iSCSI SAN – Network adapter bottleneck

What should I look at to see why Windows thinks the disk is running at 100%, yet the hardware is running under 50%?

I’m a dba/programmer, not a network guy so it’s quite possible I’m missing something simple


% Disk Read Time doesn’t mean what you think it means. It doesn’t mean that your disk is at 100% utilization, which makes the rest of your question somewhat irrelevant, since it’s based on a misunderstanding of this counter.

From the linked article:

The “% Disk Time” counter is nothing more than the “Avg. Disk Queue
Length” counter multiplied by 100. It is the same value displayed in a
different scale. If the Avg. Disk queue length is equal to 1, the
%Disk Time will equal 100. If the Avg. Disk Queue Length is 0.37, then
the %Disk Time will be 37. This is the reason why you can see the %
Disk Time being greater than 100%, all it takes is the Avg. Disk Queue
length value being greater than 1. The same logic applies to the %
Disk Read Time and the % Disk Write Time. Their data comes from the
Avg. Disk Read Queue Length and Avg. Disk Write Queue Length,

Source : Link , Question Author : Greg , Answer Author : MDMarra

Leave a Comment