or no impact upon the computing user community. By doing this, they can claim to have phenomenally high availability, which might give the illusion of continuous availability. Systems that exhibit truly continuous availability are comparatively rare and higher priced, and most have carefully implemented specialty designs that eliminate any single point of failure and allow online hardware, network, operating system, middleware, and application upgrades, patches, and replacements. For certain systems, scheduled downtime does not matter, for example system downtime at an office building after everybody has gone home for the night. Percentage calculation Availability is usually expressed as a percentage of uptime in a given year. The following table shows the downtime that will be allowed for a particular percentage of availability, presuming that the system is required to operate continuously. Service level agreements often refer to monthly downtime or availability in order to calculate service credits to match monthly billing cycles. The following table shows the translation from a given availability percentage to the corresponding amount of time a system would be unavailable. Uptime and availability can be used synonymously as long as the items being discussed are kept consistent. That is, a system can be up, but its services are not available, as in the case of a network outage. This can also be viewed as a system that is available to be worked on, but its services are not up from a functional perspective (as opposed to software service/process perspective). The perspective is important here – whether the item being discussed is the server hardware, server OS, functional service, software service/process...etc. Keep the perspective consistent throughout a discussion, then uptime and availability can be used synonymously. "Powers of 10 trick" A simple memory trick is to calculate , for , where is the number of desired nines. More concisely seconds/day of downtime. For example: 90% ("one nine") is , so and therefore seconds/day. 99% ("two nines") is , so and therefore seconds/day. 99.9% ("three nines") is , so and therefore seconds/day. 99.99% ("four nines") is , so and therefore seconds/day. 99.999% ("five nines") is , so and therefore seconds/day. 99.9999% ("six nines") is , so and therefore seconds/day. 99.99999% ("seven nines") is , so and therefore seconds/day. 99.999999% ("eight nines") is , so and therefore seconds/day. 99.9999999% ("nine nines") is , so and therefore seconds/day. "Nines" Percentages of a particular order of magnitude are sometimes referred to