In all honesty you should rarely (if ever) patch or reboot all Prod servers in a single wave. No matter how much testing you think you've done in your developme

In all honesty you should rarely (if ever) patch or reboot all Prod servers in a single wave. No matter how much testing you think you've done in your development or pre-production environments something will still surprise you in prod and that costs more. Always and without exception, no matter what OS or software you use, make sure that after a patching cycle (or impactful change) you have a chunk of your environment untouched and still fully functioning. There will be situations where you can't, special cases where you're left with few options. If the system is that critical have 2 of everything (clusters, databases, etc.) in a way where the mirror can stay untouched and keep your business going. And if it needs saying, something like updates or reboots should never happen truly automatically (as in the software decided when or if to do it). A human should decide how to automate that and have full control over when and if it happens.