Calculating parallel availability
How do you calculate parallel availability in N + 1 and N + M systems?

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Availability should be calculated on the basis of what matters to users -- application availability. Do users care more if a particular server is up or if their database is running? Since they care about their applications, that is what you calculate availability statistics against.

If a particular server is down for 24 hours out of a week, but the critical application that runs on it had failed over to another system with just two minutes of downtime (during the failover), how would you calculate the downtime? It's two minutes out of the week, not 24 hours.

Availability is simply the percentage of the time that the application is available out of all required time.

Hope this helps.

Evan L. Marcus

Evan L. Marcus

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This was first published in May 2004