But what I read, in-place upgrade has smaller outage, compared to dump/restore.
Correct, in fact if you do it with the link option, it will be very fast.
But so many articles on having bugs afterwards. Do you think it is a good idea to use pg_upgrade for critical database application?
It entirely depends, I have successfully used pg_upgrade many, many times. That is what -c is for, to work out all the kinks before you upgrade.
Or any other tool should I consider? For example - slony?
On at 2.5TB database, you very well be doing a lot more harm than good using a tool such as slony.
JD
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