tech/UnderstandingAvailability written at 01:27:33; Add Comment
Really understanding availability numbers
People like to talk about reliability and availability, and throw around terms like '5-nines availability' and so on. You may have heard this from server salescritters (alongside their attempts to sell you redundant power supplies).
At the same time, what the terms really imply is not intuitive and is often surprising, especially at the high end.
The following chart comes from my co-worker John Calvin:
Things to note:
Even with a good service contract, a single commodity server is exposed to multi-hour outages and is so at best 3-Nines available. In fact, anything that can ever require service is at most 3-Nines available.
(In practice you are cruising on the edge of even 3-Nines; you are betting on only a few service calls a year.)
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