I think in actuality they go for what is known as "five-nines" reliability. Up 99.999% of the time. Or down about 5 minutes/year. When I worked on "big iron"

I think in actuality they go for what is known as "five-nines" reliability. Up 99.999% of the time. Or down about 5 minutes/year. When I worked on "big iron" (Not sure if the old hp superdomes counts mainframes..), the solution proposed for redundancy was to have two and be able to switch from one to the other.. Our development machines had to be rebooted from time to time as we turned off interrupts for some processes (quasi-realtime) and when under development some bugs could cause the processes to fail and be uninterruptible. Things stabilized quickly though. (edit: apparently there is 4 nines, 6 nines....) https://en.wikipedia.org/wiki/High_availability https://en.wikipedia.org/wiki/High_availability