1. Fault Tolerance In Operating System
Zihao Mao

2. Fault-Tolerance Definition
Refers to the ability of a system or component to continue normal operation despite the presence of hardware or software faults
A fault management technique which build a components in such a way that it can meet its specifications in the presence of faults.

3. Properties Reliability Availability Safety Maintainability
R(t) : Probability of a system operating correctly up to time t given that the system was operating correctly at time t=o
Mean Time To Failure(MTTF)
Mean Time To Repair(MTTR)
Availability
Defined as the fraction of time the system is available to service users’ requests
Safety
The ability to minimize the impacts of small failures.
Maintainability
How easy is it to repair the faults?
High reliability doesn’t mean high availability

5. Fault Definitions Fault Failure
An erroneous hardware or software state resulting from component failure, operator error, physical interference from the environment, design error, program error, or data structure error.
A defect in a hardware device or component
An incorrect step, process, or data definition in a computer program
Failure
When the system failed to meet its promises. Caused by faults.

6. Typical Failure Types Crash failure Omission failure Timing failure
When the system halts, but it behaves correctly before halting
Omission failure
fails to respond
Timing failure
correct output, but the time taken to respond has exceeded the specification.
Response failure
Wrong output
Arbitrary/Byzantine failure:
Arbitrary/Malicious output
The severity raises from top to bottom in the list.

7. Fault Categories
Temporary: A fault that is not present all the time for all operating conditions
Transient: A fault that occurs only once.
Intermittent: A fault that occurs at multiple, unpredictable times.
Permanent
A fault that, after it occurs, is always present.

8. Fault Detection Techniques
Fail-stop
Detects the crashing
Fail-silent
Detects when the system remain silent.(after crashing)
Fail-safe
Detects wrong outputs
Byzantine failure
Hard to detect

9. Fault Tolerant Techniques
Redundancy
Hide the effects of the faults.
Recovery
Bring the system to a fault-free state to remove the effect of faults.

10. Redundancy Physical Redundancy Temporal Redundancy
Involves the use of multiple components that either perform the same function simultaneously or are configured so that one component is available as a backup in case of the failure of another component
Ex: extra CPUs, multiple parallel circuitry, multi-versions software, backup name server.
Temporal Redundancy
repeating a function or operation when an error is detected.
EX: re-execution, execute backup copy, retransmission
Information Redundancy
replicating or coding data in such a way that bit errors can be both detected and corrected.
Ex: Parity, Hamming codes.

11. Triple Modular Redundancy(TMR)
If A2 fails a V1: majority vote a all B get good result
What if V1 fails?

13. Redundancy level How man faults can be tolerated in the system?
k-fault tolerant system : handles k number of faults.
TMR: 1-fault tolerant
For silent faults: (k + 1) components required.
For Byzantine faults: (2k + 1) components required.

14. Recovery Forward Recovery Backward Recovery
Move the system to a new state from which system continue operating
Ex: Error-corrections
Backward Recovery
Bring the system back into a previous fault-free state
Ex: Checkpoints, Message Logging, Unix Targon/32 System

15. Checkpoints
Periodically store system states on stable storage when system is operating suffering the effects from faults.
At recovery, bring the system back to the last state stored in checkpoints.
Problem: inconsistent cut.

16. Independent Checkpoints
Each processes periodically checkpoints independently
Fix the problem of inconsistency.

17. Message logging Checkpointing is expensive Message logging:
Periodically saving states.
Restart from the last consistent state.
Message logging:
Take infrequent checkpoints
Log all messages between checkpoints to local stable storage
At recovery: simply relay messages from previous checkpoint.
Avoid re-computations
Problem: inconsistency

18. Summary
Fault-tolerance is the ability of a system or component to continue normal operation despite the presence of hardware or software faults
Reliability
Mean Time To Failure(MTTF)
Mean Time To Repair(MTTR)
Availability
Techniques
Redundancy
Triple Modular Redundancy(TMR)
Recovery
Checkpoints
Independent checkpoints
Message logging