Five nines
Five nines is not a hard metric, but rather the result of a predictive calculation. When a company claims that its device is five-nines reliable, it is talking about an absurdly complicated mathematical calculation based on industry-standard formulas used to predict the reliability of the box. For every possible definition of "failure" - ranging from a hint of trouble to a total meltdown - these formulas take into account the extent of the failures, the probability with which they will occur, how quickly the failures can be diagnosed, and how soon service can be restored.
Five-nines discussions blur the line between availability and reliability. A five-nines claim could be referring to either availability or reliability, depending upon which predictive formula is used. It's important to understand the difference between the two ways vendors can spin these terms.
For any given product, availability equals the total amount of time the product was up. Reliability means the number of instances in which the product went down. So you can have one big outage, and the box will reflect high reliability, but low availability. Or you could have two dozen outages of 5 seconds or less, and the box could be accurately described as being highly available, but unreliable.
Confusing, but that's the point. When a vendor or marketeer says "five nines," they're probably using it as a catchall phrase that is probably devoid of any real meaning.
From Five Nines, by the book, Network World, 04/14/03.
Additional resources
Reality Check on Five-Nines
Business Communications Review article.
IP Telephony: The Five Nines Story
Cisco white paper that discusses five nines. Focuses on how Cisco claims it meets the standard, but includes considerable discussion of the standard itself, such as what sorts of failures would count in a calculation of five-nines reliability.
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