Hardware failures will hit one twin and not the other. Same for some kinds of high load triggers. Major changes like OS upgrades can be applied to one twin and then the other.
How often to do hit actual hardware issues on dedicated instances for small to mid size applications? Usually you see those issues the first time you use it, or it get replaced data center wide and you get forewarning so you can move stuff over first, with minimal downtime.
Same with OS upgrades, I can't remember the last time a OS upgrade completely borked anything I have deployed. At worst, the kernel didn't boot but recover from a backup was fast enough.
Same with OS upgrades, I can't remember the last time a OS upgrade completely borked anything I have deployed. At worst, the kernel didn't boot but recover from a backup was fast enough.