Why is the siocActiveTimePercentage counter only ever 0% or 100%?

Last week I published a blog article about new performance counters in vSphere 5.1 for monitoring Storage I/O Control (SIOC). Soon after publishing, I received a question about the siocActiveTimePercentage counter, which only ever showed 0% or 100% values.

It seemed a bit strange that we would show this as a percentage, so I did some investigating. What I learnt was that the counter is there to basically to show if SIOC is active or not. If SIOC is active, the counter is set to 100%. If it is not active, it is set to 0%. The reason it is a percentage value is because we don’t have ON-OFF counters right now and percentage allows for a meaningful aggregation. For example, if SIOC is active during 1/3 intervals in a day, it will get 100% value in those 1/3 number of samples and 0 otherwise. So the percentage will turn out to be 33%, which basically says that SIOC is active 33% percent of the time during the roll up period.

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