“Have you ever wondered how to determine the logical disk size looking at Performance Monitor performance metrics?”

It is easy, but requires some calculation.  First, look at the logical disk object and add the ‘’% Free Space” and “Free Megabytes” counters to the Perfmon display.  Then switch to ‘Report View’. Figure 1.


 Figure 1.

 Simply divide Free Megabytes by Free Space (remember this is a percentage, so the displayed value must be divided by 100 to get the true numeric value.  In the above example, the calculation will be 5,541 / .02338 = 236,997.43.  So the disk size is 236,997MB or (dividing by 1024) 231.44GB.  This logical drive is 231GB!

 However, customers using Performance Sentry get this information automatically by collecting ‘base’ or ‘denominator’ counters.

 Some compound counters contain a Denominator/Base value that is used in calculating the current counter value. By default Performance Sentry writes the Denominator/Base value in the SMF file, unless the user chooses not to. This parameter is found in Performance Sentry Administration under the “Parameter Administration” pane under “Write Denominator Counters? (Y/N)”. See Figure 2.


 Figure 2.

 A good example of a compound counter that uses a denominator/base value is the Logical Disk/% Free Space counter. This counter is a PERF_RAW_FRACTION counter type. The counter type defines a compound structure supplying the numerator (Free Megabytes) and denominator/base (Free Space Base).

 Denominator/Base counters are identified by a unique counter name, constructed by appending the characters “Base” to the counter name. The counter named % Free Space Base is the denominator counter associated with the % Free Space counter.

 So the calculation formula is defined as:

 % Free Space = Free Megabytes / % Free Space Base (Total Available Mbytes in System)

 The Collection Service is responsible for calculating the fraction, which is the value, reported for the counter each interval. The Write Denominator Counters option instructs the Collection Service to supply the denominator value used in the calculation, too, which is otherwise a null value. In this case, the denominator value is a useful metric to collect because it supplies the capacity of the Logical Disk in bytes.

 The Summarization Utility of Performance Sentry makes use of the associated Denominator counters to summarize several Hit % counters in the Cache object correctly over the summarization interval. If denominator values for these PERF_SAMPLE_FRACTION counters are present, the Summarization Utility correctly produces the weighted average Hit % for the interval. If denominator values are not present; the Summarization Utility calculates the simple arithmetic mean, which is subject to error.

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