Dependability
! !
Qualitative term for the ability of the system to perform properly encapsulates reliability, availability, safety, maintainability, performability, testability
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Reliability - Unreliability
!
! ! !
R(t) is the probability that the system performs as specified without interruption over the entire interval [0,t] R(t) is conditioned on the system being operational at time t=0. Unreliability F(t) is the probability that the system fails at any time in the interval [0,t]. F(t) = 1 - R(t)
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Reliability - Unreliability
! ! !
time t can be very long, e.g. years in case of space applications Notation 0.9i = .99999999 i 9s This notation is often used for reliability
e.g. Q(t) = 10-x R(t) = 0.9x = (1 - 10-x)
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Safety S(t)
!
S(t) is the probability that the system does not fail in the interval [0,t] in such a manner as to cause unacceptable damage or other catastrophic effects. Safety is a measure of the fail-safe capability of the system
system can be unreliable, yet safe bias towards safe failure e.g. duplex system (detector) e.g. babbling driver (not safe)
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Availability A(t)
! A(t)
is the probability that the system is up and running correctly at time t is different from reliability.
! This
Reliability considers the interval [0,t] Availability takes an instance of time
! examples:
transaction processing systems, e.g. reservation systems
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Performability
!
! !
P(L,t) is the probability that the system performance will be at or above some level L at time t Measure of the likelihood that some subset of the function is performed correctly This differs from reliability, which dictates that all functions are performed correctly
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CS449/549 Fault-Tolerant Systems
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Graceful Degradation
!
The ability of system to automatically decrease its level of performance to compensate for hardware failure and software errors.
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Maintainability
M(t) is the probability that a failed system will be restored within a specified period of time t. ! Restoration process
!
locating problem, e.g. via diagnostics physically repairing system bringing system back to its operational condition
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Fault - Error - Failure
!
Fault = physical defect or flow occurring in some component (hardware or software) Error
= incorrect behavior caused by a fault
manifestation of fault
Failure = inability of the system to perform its specified service
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CS449/549 Fault-Tolerant Systems
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Fault - Error - Failure
Fault
physical universe bit stuck-at
Error
informational universe incorrect data to ALU
Failure
external universe system crash, incorrect bank balance
Note: presents of fault does not ensure that error will occur, e.g. memory stuck-at-0
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Characteristics of faults
! Cause
specification errors
very dangerous generic fault very hard to formally verify random, not manufacturing defects noise, EMP, radiation much like random component
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implementation
random component faults
external disturbance
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Characteristics of faults
! Origin
software or hardware dont care, except:
hardware can be analog indeterminate voltage level
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Characteristics of faults
!
Duration
permanent fault
once component fails, it never works correctly again easiest to diagnose
transient fault
1 time only 10 times as likely as permanent fault
intermittent fault
re-occurring may appear to be transient (if long period) hard and expensive to detect
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Avoidance - Masking - Tolerance
from Johnson 1989, Fig 2.12
Specification Mistakes Implementation Mistakes External Disturbances
Software Faults System Failure
Errors
Hardware Faults Component Defects
Fault Avoidance
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Fault Masking
Fault Tolerance
CS449/549 Fault-Tolerant Systems Sequence 2
