It??™s extremely important to be clear on the difference between physical and perceived
downtime when discussing the practical usage of mirroring or any other
high-availability technique. While the trend is definitely on the decline, it seemed for a
while that every IT department I worked with would demand uptime charts for their
mission-critical systems. However, those charts always left out the basis on which the
uptime was calculated. The charts often looked good when presented to management,
but the user base would often paint a completely different picture.
nNote Many people believe they can use statistics to prove whatever point they want to make. While
there is a certain amount of truth to that, a real statistical analysis always includes the methodology used
to collect the data and what that data represents. Upper management generally prefers a concise, graphical
representation of information, but you should always be prepared to present the methodology you use
in your analysis, as well as what each term represents.
Some also believe that user downtime cannot be measured with any assurance of
accuracy.
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