(2)Availability of complex systems
For systems that are configured from multiple computers or equipment, the
formula used to calculate availability depends on whether a system is a
“serial system” or “parallel system.”
●Availability of serial systems
A “serial system” is a system that only operates when all of the devices
from which it is configured are operational. If there is a failure with even
one device, the system is unable to operate.
Availability = A1×A2
Chapter 8
Availability A1
Availability A2
If A1 = 0.9 and A2 = 0.8,
0.9 × 0.8 = 0.72
Device 1
Device 2
●Availability of parallel systems
A “parallel system” is a system that operates as long as at least one device
is operational. The system stops operating only when all of the devices
from which it is configured have failed.
Rate of failure from both device 1 and device 2
Reference
Failure rate
The “failure rate” is the rate representing the number of failures that occur
within a given time.
1
Failure rate =
MTBF
Computer system
the availability is
Availability = 1− (1−A1) × (1−A2)
Rate of failure
with device 1
Rate of failure
with device 2
Availability A1
Device 1
Availability A2
If A1 = 0.9 and A2 = 0.8,
the availability is
1 × (1−0.9) x (1−0.8) = 0.98
Device 2
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