I've always heard that as "scheduled uptime" or "unscheduled outages". When I worked in a mainframe shop, they used to IPL (reboot) the mainframe every Sunday morning. That down time was never considered as part of the SLA.

Wow, even Windows boxes can run longer than a week without having to be rebooted. I figured a mainframe would be able to last almost indefinitely.

weekly reboots force the business to build their processes around the system being offline at the same time each week.

That way you don't have to try and organize downtime if maintenance is required, you know every sunday morning is available when needed.

Not disagreeing, just commenting that I would have assumed that taking the entire system offline for a reboot on a weekly basis would be untenable from a business perspective. It seems at odds with the concept of all the redundancy built into mainframes to ensure high availability and uptimes. Given that many mainframe-based systems (e.g. airline reservations) generally need to be available 24/7/365, I would have assumed that while one part of the system is being rebooted, others are still available so the overall application can continue to run uninterrupted.

Looks like they've updated the article to correct this. It now says, "a bit over five minutes' worth of outage per year".

I would guess that they did the calculations, but interpreted it as "5 nines after the decimal point", when it's really "5 nines in total".

0.99999/1 (no %)