Nines of availability

"Nines" is shorthand for an availability level by the number of 9s in it: two nines is 99%, three nines 99.9%, four nines 99.99% and five nines 99.999%.

Each extra nine cuts the allowed downtime by ten. Over an average month two nines allow about 7 hours 18 minutes, three nines 43 minutes 48 seconds, four nines 4 minutes 23 seconds and five nines 26 seconds.

The cost rises much faster than the number. Going from three to four nines usually means moving from people fixing problems to systems recovering on their own, because four minutes a month is less than a person needs to respond.

A service is also limited by what it depends on. Three dependencies at 99.9% that are all required cap the whole at about 99.7%.

At a glance

2 nines  99%      7h 18m a month
3 nines  99.9%    43m 48s a month
4 nines  99.99%   4m 23s a month
5 nines  99.999%  26s a month

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Questions

How many nines do most SaaS products promise?

Three nines (99.9%) a month is the most common figure, sometimes 99.95% on higher plans.

Where does the term five nines come from?

From telephone networks, which were designed for 99.999% availability. It is now used as shorthand for very high availability in general.

UptimeUptime is the share of a period during which a service worked as expected, usually given as a percentage such as 99.9%.SLA (service level agreement)An SLA (service level agreement) is a promise to customers about the level of service, such as 99.9% monthly uptime, and what they receive if it is not met.Error budgetAn error budget is the amount of failure an SLO allows over its window: at 99.9% over a 30-day month, 43 minutes 12 seconds of downtime.AvailabilityAvailability is the share of time, or of requests, for which a service can be used when someone needs it.

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术语表

几个 9（Nines）

“几个 9”是按百分比中 9 的个数来简称可用性级别：2 个 9 是 99%，3 个 9 是 99.9%，4 个 9 是 99.99%，5 个 9 是 99.999%。

每多一个 9，允许的停机时间就缩小到十分之一。按平均月计算，2 个 9 约 7 小时 18 分，3 个 9 为 43 分 48 秒，4 个 9 为 4 分 23 秒，5 个 9 为 26 秒。

成本的上升远快于数字的变化。从 3 个 9 到 4 个 9，通常意味着从“人来修复问题”变成“系统自己恢复”，因为每月 4 分钟比一个人响应所需的时间还短。

服务还受限于它的依赖。三个都必不可少、各为 99.9% 的依赖，会把整体上限压到约 99.7%。

速览

2 个 9  99%      每月 7 小时 18 分
3 个 9  99.9%    每月 43 分 48 秒
4 个 9  99.99%   每月 4 分 23 秒
5 个 9  99.999%  每月 26 秒

查看任意目标的停机时间 →

常见问题

大多数 SaaS 产品承诺几个 9？

按月 3 个 9（99.9%）最常见，高级套餐有时会承诺 99.95%。

“5 个 9”这个说法从哪里来？

来自电话网络，它们按 99.999% 的可用性设计。现在泛指极高的可用性。

可用时间（Uptime）Uptime 指在一段时间内，服务按预期正常工作的时间所占的比例，通常用百分比表示，例如 99.9%。SLA（服务等级协议）SLA（服务等级协议）是对客户的服务水平承诺，例如每月 99.9% 的可用率，以及未达成时客户能得到什么补偿。错误预算（Error budget）错误预算是 SLO 在其时间窗口内允许的失败量：以 30 天的月份计，99.9% 对应 43 分 12 秒的停机时间。可用性（Availability）可用性是指在用户需要时，服务能够正常使用的时间（或请求）所占的比例。

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