Introduction to Fault Tolerance

Fault Tolerance Techniques

• Introduction
  • Hardware Faults – Occurs due to a physical defect of a system like a broken wire or a logic struck at 0 in a gate.
  • Software faults – occurs due to a bug introduced in a system so the software misbehaves for a given set of inputs
  • Error – the manifestation of a fault is the error (Fault may occur anytime, but only the error manifests that fault)
  • Fault Latency – the time between the onset of fault and its manifestation as an error is the fault latency
  • Error Recovery
    • Forward Error Recovery – the error is masked without any computations having to be redone.
    • Backward Error Recovery - the system is rolled back to a moment in time before the error is believed to have occurred.

What Causes Failures?

There are three main causes of failures:

• Errors in the specification or design
  • Mistakes in the specification and Design are very difficult to guard.
  • Many hardware failures and all software failures occur due to such mistakes.
  • It is difficult to ensure that the specification is completely right.
• Defects in the components
  • Hardware components can develop defects.
  • Wear and tear of components
• Environmental effects
  • Devices can be subjected to whole array of stresses, depending on the application.
  • High ambient temperatures can melt components or otherwise damage them.

Fault Types

Faults are classified according to temporal and output behavior

• Temporal behavior classification

    A(t)    B(t)

    C(t)    D(t)

• Permanent faults
  • Does not die away with time, remains until it is repaired
  • Ex. Broken wires
  • From the above Diagram: A(t)>0; B(t) =C(t) = D(t)=0
• Intermittent Fault
  • It cycles between the fault active and fault benign states.
  • Eg. Caused by loose wires    
  • From the above Diagram: A(T)>0; B(t)>0; D(t)>0; C(t)=0
• Transient Fault
  • Dies away after some time
  • Ex: environmental effects
  • From the above Diagram: A(t)>0; C(t)>0; B(t)=D(t)=0

• Output behavior classification
  • Malicious
  • Non malicious