High availability is an increasing requirement for SaaS vendors. Availability is often expressed at “nines”, or the percentage of availability in a year. The following handy table gives you a quick decode to the most common nines.
|“Number of Nines”||Availability as a Percentage||Max Time in a Year|
|Three Nines||99.9%||8.76 hours|
|Four Nines||99.99%||52.6 minutes|
|Five Nines||99.999%||5.26 minutes|
|Six Nines||99.9999%||31.5 seconds|
Note that “six sigma” is normally defined to allow a process variation of plus or minus 1.5 sigma so that it’s actually 4.5 sigma in a “normal distribution” which is 99.99966 which is pretty close to “six nines.” A true “six sigma” is “nine nines” or 99.9999999%
I was talking to a SaaS startup planning to launch this month and I suggested that they could achieve six nines if they launched at the right time this month:
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