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a module. System-level testing should measure reliability and validate as many cus-
tomer operational profiles as possible. It requires that most of the failure detection be
performed prior to system testing. System integration becomes the general functional
validation phase. 20
In this ICOV DFSS phase, reliability measurements and metrics are used to track
the number of remaining software defects, the software mean time to failure (MTTF),
and to anticipate when the software is ready for deployment. The test engineers in
the DFSS team can apply usage profiling mechanisms to emphasize test cases based
on their anticipated frequency of execution in the field. The major activities in this
phase are:
Software reliability measurements (after metrics definition)
Usage of profile-based testing
Software reliability estimation techniques
Software reliability demonstration tests
14.3
SOFTWARE AVAILABILITY
Reliability analysis concerns itself with quantifying and improving the reliability
of a system. There are many aspects to reliability, and the reliability profile of one
system may be quite different from that of another. Two major aspects of reliability
that are common to all systems are availability (i.e., the proportion of time that all
critical functions are available) and reliability (i.e., no transaction or critical data be
lost or corrupted). These two characteristics are independent. A system may have a
very high availability, but transactions may be lost or corrupted during the unlikely
occurrence of a failure. However, a system may never lose a transaction but might be
down often.
Let us make the following definitions:
MTBF
=
Mean time between failures (MTBF), or uptime.
MTTR
=
mean time to repair the system (MTR), or downtime.
A
=
software system availability.
System availability is the proportion of time that the system is up. Because the
system only can be either up or down, the following is true:
MTBF
MTBF + MTTR =
1
A
=
(14.1)
MTTR
MTBF
1
+
20 See Silverman and De La Fuente, http://www.opsalacarte.com/pdfs/Tech Papers/Software Design for
Reliability - Paper.pdf.
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