fault avoidance and tolerance technique

8/13/2019 Fault Avoidance and Tolerance Technique

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FAULT AVOIDANCE AND

TOLERANCE TECHNIQUE

BY

Nithiyanandham M.

Pavithra R.


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The fault intolerance (or fault-avoidance) approachimproves system reliability by removing the source

of failures (i.e., hardware and software faults) before

normal operation begins

The approach of fault-tolerance expect faults to be

present during system operation, but employs design

techniques which insure the continued correct

execution of the computing process

Fault-Intolerance and Fault-Tolerance


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Dependability

Dependability Includes

Availability

Reliability

Safety

Maintainability


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Availability: A measurement of whether a system is ready to be used

immediately. System is up and running at any given moment

Reliability: A measurement of whether a system can run continuously

without failure. System continues to function for a long period of time

A system goes down 1ms/hr has an availability of more than

99.99%, but is unreliable

A system that never crashes but is shut down for a week onceevery year is 100% reliable but only 98% available

Safety: A measurement of how safe failures are

System fails, nothing serious happens

For instance, high degree of safety is required for systems

controlling nuclear power plants Maintainability: A measurement of how easy it is to repair a system

A highly maintainable system may also show a high degree of

availability


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A faultis the cause of the error

An erroris part of a system state that may lead to a failure

A system failswhen it cannot meet its promises (specifications)

Faultscan be three categories

Transient (appear once and disappear)

Intermittent (appear-disappear-reappear behavior)

A loose contact on a connectorintermittent fault

Permanent (appear and persist until repaired)


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FAULTS

Any fault may be

fail-silent (fail-stop)

Byzantine

synchronous system vs. asynchronous systemE.g., IP packet versus serial port

transmission


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ACHIEVING FAULT TOLERENCE

Redundancy

information redundancy

Hamming codes, parity memory ECC memory

time redundancy

Timeout & retransmit

physical redundancy/replication

TMR, RAID disks, backup servers

Replication vs. redundancy:Replication: multiple identical units functioning concurrently

vote on outcome


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REDUNDANCY

Redundancy is key technique for hiding failures When one unit is functioning others are available to fill in in

case the unit ceases to work.

Redundancy types:

1. Information: add extra (control) information

Error-correction codes in messages

2. Time: perform an action persistently until it succeeds:

Transactions

3. Physical: add extra components (S/W & H/W)

Process replication, electronic circuits


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ACTIVE REPLICATION

Technique for fault tolerance through physical redundancy

No redundancy:

Triple Modular Redundancy (TMR):

Threefold component replication to detect and

correct a single component failure


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Availability: how much fault tolerance?

100 % fault-tolerance cannotbe achieved.

The closer we wish to get to 100%, the more expensive thesystem will be.

Availability: % of time that the system is functioning

five nines: system is up 99.999% of the time: 55.6 minutes

downtime per year

Three nines: system is up 99.9% of the time: 8.76 hours

downtime per year

Downtime includes all time when the system is unavailable.


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GOAL OF FAULT TOLERANT

COMPUTING

Dependability

Reliability

Availability

Safety Security

Performability

Maintainability

Testability

Goal of tolerance


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POINTS OF FAILURE

Points of failure: A system is k-fault tolerant if it can

withstand k faults.

Need k+1 components with silent faults

k can fail and one will still be workingNeed 2k+1 components with Byzantine faults

k can generate false replies: k+1 will provide a majority

vote

fault avoidance and tolerance technique

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