Wikipedia

Five nines, commonly taken to mean “99.999%”, may refer to:
• High availability of services, when the downtime is less than 5.26 minutes per year

Availability % Downtime per year Downtime per month Downtime per day
90% (“one nine”) 36.5 days 72 hours 2.4 hours
99% (“two nines”) 3.65 days 7.20 hours 14.4 minutes
99.9% (“three nines”) 8.76 hours 43.8 minutes 1.44 minutes
99.99% (“four nines”) 52.6 minutes 4.38 minutes 8.64 seconds
99.999% (“five nines”) 5.26 minutes 25.9 seconds 864 milliseconds
99.9999% (“six nines”) 31.6 seconds 2.59 seconds 86.4 milliseconds
99.99999% (“seven 9s”) 3.16 seconds 262 milliseconds 8.64 milliseconds
99.999999% (“eight 9s”) 316 milliseconds 26.2 milliseconds 0.864 milliseconds
99.9999999% (“nine 9s”) 31.6 milliseconds 2.62 milliseconds 0.0864 milliseconds
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One Response to Wikipedia

  1. shinichi says:

    High availability

    Wikipedia

    https://en.wikipedia.org/wiki/High_availability

    High availability is a characteristic of a system, which aims to ensure an agreed level of operational performance, usually uptime, for a higher than normal period.

    Modernization has resulted in an increased reliance on these systems. For example, hospitals and data centers require high availability of their systems to perform routine daily activities. Availability refers to the ability of the user community to obtain a service or good, access the system, whether to submit new work, update or alter existing work, or collect the results of previous work. If a user cannot access the system, it is – from the users point of view – unavailable. Generally, the term downtime is used to refer to periods when a system is unavailable.

    Percentage calculation

    Availability is usually expressed as a percentage of uptime in a given year. The following table shows the downtime that will be allowed for a particular percentage of availability, presuming that the system is required to operate continuously. Service level agreements often refer to monthly downtime or availability in order to calculate service credits to match monthly billing cycles. The following table shows the translation from a given availability percentage to the corresponding amount of time a system would be unavailable.

    Availability % Downtime per year Downtime per day
    90% (“one nine”) 36.5 days 2.4 hours
    99% (“two nines”) 3.65 days 14.4 minutes
    99.9% (“three nines”) 8.76 hours 1.44 minutes
    99.99% (“four nines”) 52.6 minutes 8.64 seconds
    99.999% (“five nines”) 5.26 minutes 864 milliseconds
    99.9999% (“six nines”) 31.6 seconds 86.4 milliseconds
    99.99999% (“seven 9s”) 3.16 seconds 8.64 milliseconds
    99.999999% (“eight 9s”) 316 milliseconds 0.864 milliseconds
    99.9999999% (“nine 9s”) 31.6 milliseconds 0.0864 milliseconds

    Uptime and availability can be used synonymously, as long as the items being discussed are kept consistent. That is, a system can be up, but its services are not available, as in the case of a network outage. This can also be viewed as, a system can be available to work on, but its services are not up from a functional perspective (as opposed to software service/process perspective). The perspective is important here, whether the item being discussed is the server hardware, server OS, functional service, software service/process…etc. Keep the perspective consistent throughout a discussion, then uptime and availability can be used synonymously.

    “Nines”

    Percentages of a particular order of magnitude are sometimes referred to by the number of nines or “class of nines” in the digits. For example, electricity that is delivered without interruptions (blackouts, brownouts or surges) 99.999% of the time would have 5 nines reliability, or class five. In particular, the term is used in connection with mainframes or enterprise computing, often as part of a service-level agreement.

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