
Ask HN: What does 99.999% reliably actually mean? - isuckatcoding
I always thought of it as some marketing thing but apparently it maps to uptime&#x2F;downtime. Anyone care to explain the math?
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greenyoda
If a service is up 99.999% of the time, it's available for (0.99999 x 365) or
364.99635 days per year. That means that it can be down for 0.00365 days or a
total of 5.256 minutes per year.

In comparison, if a service is only 99% reliable, it can be down for a total
of 3.65 days per year.

The availability of your service depends on several factors, such as the
availability of your servers, the availability of network connectivity,
whether you have an uninterruptible power supply, etc.

Reliability is important to different people for different reasons. If Google
search goes down for an hour, it's a mild inconvenience for any given user,
but Google would lose a lot of revenue. If the phone system goes down for an
hour, lots of people could die because they couldn't call for emergency
services.

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SixSigma
Also a Service Level Agreement doesn't mean that you will really get 99.999%
reliability, usually it just details what financial recompense you will
receive when that level of service is not achieved.

e.g. "We will give you $1 per minute for the first 5 minutes of downtime, $10
per minute for the next 50, $100 per hour thereafter"

Or something, I just made that rate up but that's the idea.

