time.To.recover.(MTTR):.Achieving high availability is also
a factor of how quickly a system can be repaired when failures occur. Reparability is
the measure of how quickly a failed device can be restored to service, expressed as a
Mean Time To Repair (MTTR). The industry and the academic world today concentrate
on finding solutions to reduce this time and to make it transparent to the users.
??? Reliability??”Mean.time.between.failures.(MTBF):.Individual components have
their own MTBFs. The MTBF of a system can be calculated by considering the failure
rate of each component in the system. As systems have more and more components,
the system MTBF will always go down. Today??™s industry has made significant progress
in achieving very good MTBF through redundant hardware and software (e.g.,
clustered nodes to avoid single point of failure, application availability standpoint,
etc.). Another way of expressing availability is referred to as number of Nines. This is
expressed as a percentage of uninterrupted service per year and hence the downtime.
A system is considered highly available when its availability is 99.9%, also called ???3-
Nines??? (Arora, 2005; Bauer, 2001; Brien, 2000). For example, 528 minutes of system
Rada deh & Al-Ameed
Copyright ?© 2007, Idea Group Inc.
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