Fast upgrade of highly available cluster is a vital part of being industry-acceptable solution for any data management system. Because the cluster is required to be highly available.
Without this documented technique upgrade of 1Tb cluster would last many hours, not seconds.
There are industry concerns about scalability beyond tens of terabytes per cluster, but such downtime would significantly lower that boundary.
If somebody
wants to write a reliable tool for this to ship as part of PostgreSQL,
well and good.
IMV that's a good idea. We could teach pg_upgrade or some new tool to do that reliably. The tricky part is that the tool must stop-start standby remotely...
Best regards, Andrey Borodin.