Measuring high availability

The need for high availability is determined by the business requirements, potential risks, and operational limitations (e.g. the more components you add to your infrastructure, the more complex and time-consuming it is to maintain).

The level of high availability depends on the following:

• how much downtime you can bear without negatively impacting your users and
• how much data loss you can tolerate during the system outage.

The measurement of availability is done by establishing a measurement time frame and dividing it by the time that it was available. This ratio will rarely be one, which is equal to 100% availability. At Percona, we don’t consider a solution to be highly available if it is not at least 99% or two nines available.

The following table shows the amount of downtime for each level of availability from two to five nines.

Availability %	Downtime per year	Downtime per month	Downtime per week	Downtime per day
99% (“two nines”)	3.65 days	7.31 hours	1.68 hours	14.40 minutes
99.5% (“two nines five”)	1.83 days	3.65 hours	50.40 minutes	7.20 minutes
99.9% (“three nines”)	8.77 hours	43.83 minutes	10.08 minutes	1.44 minutes
99.95% (“three nines five”)	4.38 hours	21.92 minutes	5.04 minutes	43.20 seconds
99.99% (“four nines”)	52.60 minutes	4.38 minutes	1.01 minutes	8.64 seconds
99.995% (“four nines five”)	26.30 minutes	2.19 minutes	30.24 seconds	4.32 seconds
99.999% (“five nines”)	5.26 minutes	26.30 seconds	6.05 seconds	864.00 milliseconds

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