When 99.999% Isn't Good Enough
When discussing availability of a service, it is common to hear the term "Five Nines" referring to a service being available 99.999% of the time but "Five Nines" are relative. If your time frame is a week, then your service can be unavailable for 6.05 seconds whereas a time frame of a year, allows for a very respectable 5.26 minutes.
In reality, none of those calculations are relevant because no one cares if a service is unavailable for 10 hours, as long as they aren't trying to use it. On the other hand, if you're handling 50,000 transactions per second, 6.05 seconds of unavailability could cost you 302,500 transactions and no one cares if you met your SLA.
This problem is one I've come up against a number of times in the past and recently even more and the issue is orders of magnitude in IT. The larger the volume of business you handle, the less relevant the Five Nines become.