19.2. Percentages of Availability

A useful matrix for evaluating system availability refers to uptime as a number of nines. One nine is 90%, two are 99%, three are 99.9%, and so on. Most system vendors quote system availability as the percentage of time the system is up and running using nines. Some vendors guarantee uptime ranging from 99.9% all the way to 99.9999%.

Availability of 99% in a 24x7 operation translates to more than 87 hours of downtime a year. In a system that is 99% available, there can be 1 failure during the year that lasts 3.5 days, or there can be 10 failures, each lasting more than 8 hours, or there can be 87 outages of about 1 hour each. Although 87 different 1-hour outages over 1 year might be tolerable, an outage that shuts down a business for 3 consecutive days could be extremely costly.

Uptime %	Downtime %	Downtime per Year	Downtime per Week
98%	2%	7.3 days	3 hours, 22 minutes
99%	1%	3.65 days	1 hour, 41 minutes
99.8%	0.2%	17 hours, 30 minutes	20 minutes, 10 seconds
99.9%	0.1%	8 hours, 45 minutes	10 minutes, 5 seconds
99.99%	0.01%	52.5 minutes	1 minute
99.999%	0.001%	5.25 minutes	6 seconds
99.9999%	0.0001%	31.5 seconds	0.6 seconds

Note

Availability of a standalone server includes only the availability of the server itself, not the operating system, application, or network connections.