本文计算了系统在一年内达到99.99%（四九）和99.999%（五九）可用性时的停机时间。对于四九可用性，每年的停机时间为约1小时；对于五九可用性，每年的停机时间减少到仅5.256分钟。
1: 99.99%
4个9 (four nines) = 99.99% availability
a year has 3600*12*365 = 31536000 seconds.
(a year) * 99.99% = 31536000 * 99.99% = 31532846.4 seconds.
the rest value = 31536000 - 31532846.4 = 3153.6 seconds = 52.56 minutes ~= 1 hour.
2: 99.999%
5个9 (four nines) = 99.999% availability
a year has 3600*12*365 = 31536000 seconds.
(a year) * 99.999% = 31536000 * 99.999% = 31535684.64 seconds.
the rest value = 31536000 - 31535684.64 = 31535684.64 seconds = 5.256 minutes.
FYI
http://en.wikipedia.org/wiki/Myth_of_the_nines
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