Fault Tolerance

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Fault Tolerance Reliability CDA 5140 Spring
2006

• Chapter 1
• Overview Definitions

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Topics

• basic concepts of Fault Tolerance (FT)
• reliability availability of systems, both
  hardware software
• tools to compare contrast FT designs

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What is FT?

• Computing in presence of errors
• Some techniques from analog systems of 1940s -
  1960s
• Digital technology adds to these to be faster,
  better cheaper
• Investigate architecture keeping in mind tradeoff
  of cost, weight volume
• Becoming more important as digital systems become
  more more prevalent

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Why Have FT?

• Needed more in 21st century since
• Harsher environments
• Many novice users
• Increasing repair costs
• Larger systems
• Digital systems more prevalent
• More users dependent on digital systems from
  business to government to home to school

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How is FT Obtained?

• Add redundancy in form of
• Hardware, e.g. RAID
• Software, e.g. 2 algorithms for same task
• Information, e.g. coding theory
• Time, e.g. on Internet if fault, then new route

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Definitions Terminology

• Failure - departure from correct operation
• Fault - flaw in hardware or software resulting in
  failure, e.g. physical problems, design flaws,
  defects in hardware design or implementation for
  software
• Error - incorrect response from module leading to
  system failure if no FT
• Type - hardware or software
• Cause - improper design, hardware failure,
  external disturbance

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Definitions continued

• Permanent Fault - always present, needs repair to
  remove
• Intermittent fault - not always present but still
  needs repair to remove
• Transient fault - will disappear without repair
• Fault latency - fault can go undetected does
  not cause error
• Fault-avoidance - use of high quality components
  careful design to avoid faults
• Fault-tolerance - use of redundancy (hardware,
  software, information or time) to correct system
  operation after fault occurs

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Definitions continued

• Graceful degradation - system still performs but
  with degraded but correct performance after
  faults
• Fail-safe - system can fail but only to safe
  state to avoid catastrophes
• Reliability - probability of not failing within
  time t given operating correctly at time 0
• Availability - probability system operating
  correctly at time t
• Maintainability - probability that system can be
  restored to operation by time t given not
  operational at time 0

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Definitions continued

• Mean-time-to-failure (MTTF) - expected value of
  system failure time
• Mean-time-to-repair (MTTR) - expected value of
  system repair time
• Mean-time-between-failure (MTBF) - expected value
  between successive system failure, MTTF MTTR
• Fault detection - method used to detect presence
  of fault
• Fault confinement - technique to confine damage
  of fault to as small an area as possible

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Definitions continued

• Fault diagnosis - automatic identification of
  faulty modules
• Recovery - system put into operating state,
  possibly degraded
• Hardware redundancy - extra hardware to detect,
  mask or diagnose faults
• Passive hardware redundancy - fault masking to
  hide faults prevent faults from resulting in
  errors no action by system

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Definitions continued

• Information redundancy - use of coding theory
  techniques (addition of bits)
• Software redundancy - use of diagnostic software
  or extra modules, each with distinct algorithm
• Temporal redundancy - repeating bus cycles or
  whole programs, new route on Internet

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Microelectronic Growth

• Density of chips dramatically increased
  concomitantly, use of digital systems
• Obvious need for FT in space shuttle, nuclear
  power plants, but with increased use in homes,
  more faults likely so will need FT there too
• Interesting observations
• 1999 typical home had 40-60 microprocessors
• 2004 expected to be 280

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Reliability Availability

• Goal high reliability availability based on
  sound analysis not conjecture!
• Use both reliability availability as measures

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Air Traffic Control Example

• ATC fails once/year, so MTTF 8766 hours
• Airline Reservation System (ARS) down 5
  times/year, so MTTF1753 hours
• Availability (A) uptime/(uptime downtime)
• ATC down 1 hour, so
• A 8765/(8765 1) 0.999886
• ARS down for 1 minute, 5 times, or 0.083333 hours
• A 8765.91666/(87666) 0.999905

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Air Traffic Control Example contd

• Unavailability U 1-A
• So, comparing the two systems for U
• (1-0.999886)/(1-0.999905) 12
• The ARS is 12 times better than the ATC in terms
  of availability.
• Homework 1 1.13, 1.14, 1.17 (3 examples)