Fundamentals of Quantitative Design and Analysis - Computer Architecture: A Quantitative Approach - page 35

Assume a disk subsystem with the following components and MTTF:
■ 10 disks, each rated at 1,000,000-hour MTTF
■ 1 ATA controller, 500,000-hour MTTF
■ 1 power supply, 200,000-hour MTTF
■ 1 fan, 200,000-hour MTTF
■ 1 ATA cable, 1,000,000-hour MTTF
Using the simplifying assumptions that the lifetimes are exponentially distrib-
uted and that failures are independent, compute the MTTF of the system as a
whole.
Answer
The sum of the failure rates is
or 23,000 FIT. The MTTF for the system is just the inverse of the failure rate:
or just under 5 years.
The primary way to cope with failure is redundancy, either in time (repeat the operation to
see if it still is erroneous) or in resources (have other components to take over from the one
that failed). Once the component is replaced and the system fully repaired, the dependability
of the system is assumed to be as good as new. Let's quantify the benefits of redundancy with
an example.
Example
Disk subsystems often have redundant power supplies to improve dependab-
ility. Using the components and MTTFs from above, calculate the reliability of