Availability- How many 9’s are enough?

Availability, and the number of nines

Availability is the conditional probability that the system delivers the required service at the time it’s called upon to do so. Availability considers that when the system fails, it can be repaired and restored for service. •  It includes the time to repair •  This is a critical aspect of availability. •  Availability is often expressed in terms of some number of nines. “Three

9’s,” for example, is the same as saying “99.9% availability.”

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Availability versus Reliability

Availability is different from “Reliability” •  Reliability is simply the probability of the system will perform its function for a

given period of time, under certain conditions. •  Reliability is often expressed in terms of Mean Time Between Failure (MTBF) or

Failure Rate.

Unlike availability, reliability is not a conditional probability, and does not include maintenance or repair.

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Availability in Real Life

99.9% availability (3 nines) means, for example:

•  44 minutes of unsafe drinking water per month

•  3 crash-landings per week at Heathrow

•  3,000 letters lost by the Postal Service every hour

•  2,000 surgical mistakes in the NHS every week.

•  9,000 incorrect banking debits per hour

•  36,000 missed heartbeats per year (9 hours)

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Availability and Downtime

These are all different scenarios, and all unacceptable, but perhaps surprisingly, all pertain to the same availability value.

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Nines, and amount of downtime

This table shows the amount of time annually that a system is down (or unavailable), given the number of nines availability that system has. We see here that even a 5-nines system has on average, over five minutes of downtime annually.    

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% Availability Time unavailable, annually 99% 88 hours

99.9% 8.8 hours 99.99% 53 minutes

99.999% 5.3 minutes 99.9999% 32 seconds

Availability Performance: Operations Ethic

It’s important to remember that the true availability performance of a data center is not based solely upon engineering and certifications, but also requires superior operational processes and discipline. A skilled and well trained data center operations team, mature MOPs & SOPs, disciplined security processes, and superior engineering combine to ensure superior availability performance.

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Downtime = Lost Revenue

In today’s always-on digital world, downtime translates into lost revenue. The average cost of downtime to a business is $7,900/minute, in 2015. •  This is up by 41% from 2010 Hypothetical tier ratings are interesting, but a demonstrated track record of strong availability performance, along with evidence of mature and disciplined data center operations, and sensible enterprise architecture approaches are important for minimizing risk to your business.

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