Worth Reading: The calculus of service availability

The vast majority of software services and systems should aim for almost-perfect reliability rather than perfect reliability—that is, 99.999% or 99.99% rather than 100%—because users cannot tell the difference between a service being 100% available and less than “perfectly” available. There are many other systems in the path between user and service (laptop, home WiFi, ISP, the power grid . . .), and those systems collectively are far less than 100% available. Thus, the marginal difference between 99.99% and 100% gets lost in the noise of other unavailability, and the user receives no benefit from the enormous effort required to add that last fractional percent of availability. Notable exceptions to this rule include antilock brake control systems and pacemakers! —Ben Treynor, Mike Dahlin, Vivek Rau, Betsy Beyer @The Communications of the ACM