Whenever we deal with a physical system constructed by man the term of availability is mandatory. The availability of a system represents the time for which the users have access to that system. The term is mostly used when speaking about servers or other type of hosts. If a user wants to access the server, but the server cannot respond to the request it is said that the server is down. The availability of the server is the total time for which that server is not down. The two notions are thus complementary.
The high availability is the system design protocol by which the server is kept up for as long as possible. It has the role of increasing availability of the data as much as possible. A good system these days has availability times way above 99%. A good server has availability of “five nines” – that is 99.999%. A very good system goes above “six nines”, which is 99.9999%.
Additional to the high availability, another protocol is used very often. That is the disaster recovery. Whenever the system crashes, or is down a disaster recovery takes place. It also has the role to describe the efficiency of that system. Sometimes, in case of extreme disasters, the recovery time is infinite. For example, if the server room is flooded or a fire burns up the place, the data on that server will never be recovered fully. The disaster recovery has the role to minimize this time. Often scheduled downtimes are used. They have the role to test these recovery times and analyze the procedures. The procedures get perfected and the overall efficiency goes up.
Whenever using a good system, either as a user buying or as an administrator it is best to take a severe look at the high availability protocol of the server. This will help you to decide if it works and will work as desired with the data or not.
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