Availability
What the “nines” of availability mean, how redundancy raises it and why a single point of failure brings it down. Simple calculations, scenarios and failures, every answer explained.
- 33 cards
- 3 topics
- Free
The outline, topic by topic
Each topic is weighted the way the exam weighs it.
- The nines
- Redundancy
- Points of failure
A few cards from the deck
What is “availability”?
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The share of time a service answers correctly
It is expressed as a percentage over a period: 99.9% over a month means about 43 minutes of downtime allowed.
How much downtime per year does 99.9% availability allow?
- About 8 h 45 min
- About 3 and a half days
- About 52 minutes
- About 5 minutes
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About 8 h 45 min
0.1% of an 8,760-hour year is about 8.76 hours. 99% would give 3.65 days, 99.99% about 52 minutes, 99.999% about 5 minutes.
The share of time a service answers correctly: what is this called?
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availability
It is expressed as a percentage over a period: 99.9% over a month means about 43 minutes of downtime allowed.
How much downtime per year does 99.99% availability allow?
- About 52 minutes
- About 8 h 45 min
- About 5 minutes
- About 1 day
Show the answer
About 52 minutes
0.01% of 525,600 minutes is about 52.6 minutes. Each extra nine divides the allowed downtime by ten.