>For comparison, public telephony systems aimed for five 9s. That was usually expressed as "20 minutes downtime over 40 years, combined hardware and software budget, for outages affecting more than 32 users." One software crash requiring human intervention would count for more than 20 minutes, so you were allowed <1 of these in 40 years system lifetime.

And all of that is total bogus (the "aim", not your information), as no public telephony system (and surely not in my country) ever had anything close to that.

A few hours of downtime a few times a year is much more like it, although it has been getting better over time.