Availability is usually expressed as a percentage of system uptime in a given year.

The table below shows the downtime for different availability levels:

Availability                        Downtime per year
90%  (one nine)                    36.5 days
99%   (two nines)                  3.65 days
99.5%                                  1.83 days
99.9%  (three nines)              8.73 hours
99.95%                                 4.38 hours
99.99%  (four nines)              52.56 minutes      
99.999% (five nines)              5.26 minutes
99.9999   (six nines)              31.5 seconds

Five nines has become a fashionable term, but the average website hovers around 99.6% and only a fraction of the top Internet websites get past three-nines*. The truth is getting to five-nines is a much much harder task than what we can possibly imagine. As an example, to get to five-nines, it’s not enough to simply have a failover cluster of servers in a data center. One must have redundant power and internet connectivity to the data center, and even to have replication between multiple geographical locations in case of disasters. With the exception of large enterprises, almost no-one could afford such a setup in the past.

With the advent of IaaS Cloud providers, however, the costs of building such a service have decreased dramatically. Although a provider like Amazon provides an SLA of 99.95%, it is possible to have infrastructure rely on two or more providers to get higher availability. However, cloud hosting is a double-edged sword. SLAs have proven to be empty promises, as illustrated by the spectacular Amazon downtime** of April 2011. IT departments have to design for cloud failures and stay in control of their own destiny. 

Although it might be very hard to get to five nines, which is the benchmark for telco reliability, it is possible to get close. Here at Severalnines, high availability is part of our DNA. The team has an established background helping telecom service providers build some of their most critical databases. Our objective is to make complex clustering tasks easy and even enjoyable for infrastructure folks out there. Five nines is very hard, but getting close to five nines does not have to cost a telco price tag.

* according to statistics from e.g. Gomez or Pingdom
** http://aws.amazon.com/message/65648/