pg_upgrade does that, kinda. I'd like to have something better, but in the absence of that, I think it's quite wrong to think about deprecating it, even if we had logical replication fully integrated into core today. Which we by no means do.
I don't see any problem with extending pg_upgrade to use logical replication features under the covers.
It seems very smooth to be able to just say
pg_upgrade --online
and then specify whatever other parameters that requires.
It would be much easier to separate out that as a use-case so we can be sure we get that in 10.0, even if nothing else lands.
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Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services