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.