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What does four 9's mean?

 
Skip Cole
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my test has the phrase that says something to the effect of they will be available four 9's. What does this jargon mean? Anyone know?
Thanks,
Skip
 
John Wetherbie
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"Four nines" is a way of saying 0.9999. In the context I am used to it means that the availability (or reliability) will be 99.99% or that there will only be one failure per 10,000 units of measurement, whatever they may be.
John
 
Skip Cole
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Thanks!
 
Danl Thompson
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But it also tells you that there will be failures. On average 20 per day. So you must consider how your design will handle this.
Dan'l
 
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