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High availability is important in most database systems. Having data available 24 hours a day, 7 days a week, every day of the year is important in many environments. Every database administrator aims at achieving the elusive "five nines," which means the system is up, running, and available to users 99.999% of the time.
In addition to being available, a system must perform well. High availability also means that the server, its hardware, and the database configuration perform well within the system. Response times must be sufficient. Transaction speeds must be maintained.
A truly available environment must achieve five-nines reliability while receiving a passing mark in performance.