Martijn van Oosterhout <> writes:
> The hard part going to be making sure that even if
> the power fails halfway through an upgrade that your data will still be
> readable...

I think we had that problem solved too in principle: build the new
catalogs in a new $PGDATA directory alongside the old one, and hard-link
the old user table files into that directory as you go.  Then pg_upgrade
never needs to change the old directory tree at all.  This gets a bit
more complicated in the face of tablespaces but still seems doable.
(I suppose it wouldn't work in Windows for lack of hard links, but
anyone trying to run a terabyte database on Windows deserves to lose

The stuff that needed rethinking in the old pg_upgrade code, IIRC, had
to do with management of transaction IDs and old WAL log.

                        regards, tom lane

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