Dependability

• How you decide what is operating properly
  • Infrastructure providers offer service legal agreements to guarantee the level of dependability
  • Service accomplishment is when that is delivered
  • Service interruption is when it is not
• In a system of components, the reliability of the whole system is less than the reliability of the individual components

Metrics

• Mean time to failure (MTTF)
  • Measures reliability
• Failures in time (FIT)
  • The rate of failures
  • 1/MTTF
  • FIT for a system = (number of component * (1/ Failure Rate of that component)) + …
  • Traditionally reported as failures per billion hours of operation
• Mean time to repair (MTTR)
  • Measures service interruption
• Mean time between Failures (MTBF)
  • MTTF + MTTR
• Module availability
  • MTTF / (MTTF + MTTR)

Improving

• Fault Avoidance
  • Prevent fault occurrence by construction
• Fault Tolerance
  • Using redundancy to allow the service to comply with the service specification despite the occurrence of faults
• Fault Prediction
  • Predict the faults and replace the component on time

Terms

• Fault
  • A component behaves incorrectly
  • Ex: An alpha particle hitting a memory cell and content is flipped
• Error
  • The fault is used
  • Does not have to impact outcome
    • Ex: memory is modified but A is still less than B in an if statement
  • Ex: modified memory content is used
• Failure
  • The fault changes the outcome
  • Ex: modified memory causes if statement to skip