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Computing Infrastructures > Dependability - Theory > Flashcards

Dependability - Theory Flashcards

1
Q

What are the basic steps on building reliable systems?

A

Error detection, error containment, error masking

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2
Q

What dependability is?

A

A measure of how much we trust a system

The ability of a system to perform its functionality while exposing reliability, availability, maintain ability, safety, security

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3
Q

What is reliability?

A

Continuity of correct service

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4
Q

What is availability?

A

Readiness for correct service

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5
Q

What is maintainability?

A

Ability for easy maintenance

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6
Q

What is safety?

A

Absence of catastrophic consequences

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7
Q

What is security?

A

Confidentiality and integrity of data

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8
Q

When do we think about dependability?

A

During design time and runtime

Failures in development should be avoided, failures in operation cannot be avoided, they must be dealt with

Design should take failures into account and guarantee that control and safety are achieved when failures occur. Effects of such failures should be predictable and deterministic not catastrophic.

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9
Q

How can we provide dependability?

A

Through failure avoidance, and tolerance partum

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10
Q

What are some of the failures avoidance procedures we can take?

A

Conservative design

Design validation

Detailed test

Infant mortality screen

Error avoidance

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11
Q

What techniques can we implement in order to increase tolerance?

A

Error detection/error masking during system operations

Online monitoring

Diagnostics

Self recovery and self repair

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12
Q

Define reliability and how it is calculated

A

Ability of a system or components to perform its required functions under stated conditions for a specified period of time

It is therefore the Probably that the system will operate correctly in specified operating environment until time T

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13
Q

Define availability in how it is calculated

A

The degree to which system or component is operational and accessible when required for use

Is calculated by dividing the uptime by the sum of the uptime with the downtime (total time)

It is the probability that the system will be operating at time T

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14
Q

Is it possible to have systems with low reliability that have high availability? What about the opposite?

A

Yes, system failures can be repaired quickly and do not damage data, low reliability may not be a problem

The opposite is generally more difficult

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15
Q

What is MTTF?

A

Meantime to failure it is the meantime before any failure will occur

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16
Q

What is the MTBF?

A

Meantime between failures is the meantime between two failures

17
Q

How can we calculate MTBF?

A

By dividing the total operating time by the number of failures

18
Q

What are the types of failures according to time?

A

Infant mortal, random failures, wear out

19
Q

Define fault

A

A defect within the system

20
Q

Define error

A

A deviation from the required operation of the system or sub system

21
Q

Define failure

A

The system fails to perform its required function

22
Q

What are reliability block diagrams?

A

An inductive model where a system is divided into blocks that represent distinct elements such as components or subsystems

Every element in the RBD has its own reliability (previously calculated or modeled)

Blocks are then combined together to model all the possible success paths

23
Q

How do we calculate the reliability of components in the RBD?

A

Series components we multiply the reliabilities

Parallel components we multiply the chances of each components to fail simultaneously, and from this overall failure probability, we obtain the reliability by subtracting one by this failure probability

24
Q

What is triple modular redundancy and what is the MTTF of the system?

A

System works properly if two out of three components work properly and the voter works properly

The MTF of the system is equal to five times MTTF of simple component divided by six

25
Q

Triple modular redundancies, good or bad

A

The MTTF of the system is shorter than the MTTF of the single component. But it has higher reliability if the mission time is shorter than 70% of the mean time to failure of the component

26
Q

What is standby redundancy

A

System composed of two parallel replicas, the primary replica working all time, and the redundant replica that is activated when the primary replica fails

27
Q

What is necessary in order to have a standby redundancy?

A

A mechanism to determine whether the primary replica is working properly or not

A dynamic switching mechanism to disable the primary replica and activate the redundant one

28
Q

If nothing is sad in the exercise of the exam, what can we assume about the distribution of the failure? What is the value of the reliability?

A

This is an exponential distribution of the failure. Which means that the failure rate is constant across the period.

Elder elevators to minus lambda times time

Computing Infrastructures (13 decks)

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