Capacity Planning - The Practice of Cloud System Administration - page 366

• Timetable: For each component, what is the lead time from ordering to delivery,
and from delivery until it is in service? Are there specific constraints for bringing
new capacity into service, such as change windows?
Math Terms
Correlation Coefficient: Describes how strongly measurements for different data
sources resemble each other.
Moving Average: A series of averages, each of which is taken across a short time
interval (window), rather than across the whole data set.
Regression Analysis: A statistical method for analyzing relationships between
different data sources to determine how well they correlate, and to predict
changes in one based on changes in another.
EMA: Exponential moving average. It applies a weight to each data point in the
window, with the weight decreasing exponentially for older data points.
MACD: Moving average convergence/divergence. An indicator used to spot
changes in strength, direction, and momentum of a metric. It measures the differ-
ence between an EMA with a short window and an EMA with a long window.
Zero Line Crossover: A crossing of the MACD line through zero happens when
there is no difference between the short and long EMAs. A move from positive to
negative shows a downward trend in the data, and a move from negative to posit-
ive shows an upward trend.
MACD Signal Line: An EMA of the MACD measurement.
Signal Line Crossover: The MACD line crossing over the signal line indicates
that the trend in the data is about to accelerate in the direction of the crossover. It
is an indicator of momentum.
From that information, you can calculate the amount of capacity you expect to need for
each resource by the end of the following year with a simple formula:
Future Resources = Current Usage × (1 + Normal Growth + Planned Growth) + Head-
room
You can then calculate for each resource the additional capacity that you need to pur-
chase:
Additional Resources = Future Resources - Current Resources