Evaluation Criteria to Run Scientific Applications in the Cloud - Cloud Computing with e-Science Applications - page 13

be used to verify the scalability of the candidate system. If the number of
machines increases and the cost efficiency factor is constant, the candidate
system has the same scalability rate as the base system.

1.7.2 Break-Even Point

The break-even point, represented in Figure 1.2, represents the point at which
the cost to use both the base and the candidate systems is the same, on a
yearly basis. In a cloud computing environment, with its pay-per-use model,
this metric is important. It represents the number of days in a year when it
is cheaper to use a cloud instead of buying a server. Figure 1.2 represents
the break-even point and is represented by the vertical bold line. If the user
needs to use the system for fewer days than the break-even point (left side of
the line), it is better to use a cloud, but if the usage is higher, it is more cost
efficient to buy a server.
To calculate the break-even point, it is necessary to obtain the yearly cost
of the base system. The yearly cost BS YC represents the cost to maintain
the system during a year; it is composed of the acquisition cost Acq $ of the
machines themselves plus the maintenance costs Ymn $ . To obtain the cost
of the machines on a yearly basis, it is necessary to determine the usable
lifetime LT of the machine, normally 3 to 5 years. It is necessary to divide the
acquisition costs of the machines by the usage time; this calculation results
in the cost per year of the machines. In the yearly cost, it is also necessary
to include the maintenance, personnel, and facilities costs of the machines.
The following equation calculates the yearly cost:

Acq
LT

$

BS

YC =

+ Ymn

$

Using this value and the cost efficiency factor, we can determine the
break-even point. The cost efficiency factor represents the cost on an hourly

Use a cloud Buy a server

Jan/1º Break Even Point Dec/31

FIGURE 1.2
Break-even point.