I was hinting at that, which is why we should define an uptime of a system rather than a machine because with distributed systems that uptime of the system isn'

I was hinting at that, which is why we should define an uptime of a system rather than a machine because with distributed systems that uptime of the system isn't dependent on the uptime of a single "machine" and a mainframe is a distributed system even if it's in a single rack. The question is then where you define the boundaries of a system and it's uptime. At least from my recollection for mainframes they defined uptime based on the execution of batch jobs and availability of services not the OS/Hardware which if it crashed often involved Big Blue coming to investigate WTF happened and how it happened since System Z machines are designed with so much redundancy that you can swam RAM modules without interrupting the workflow. Today with RAIM (RAID for Memory) IBM System Z machines even support an entire memory channel dying without interruption.