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That gets tricky if one of the writes fails.

Generally you get two phase commit involved here, or an asynch mirror that you can restore over if there's failure.

Two phase commit makes a promise it can't keep. CAP theorem is not amused.

It's slow, complex, a leaky abstraction, and ahould be avoided for better consistency models.

I generally couldn't agree more, but yet its the first tool in the toolbox for a lot of databases.

That sort of flexibility does not sound as though it would scale well as the data get big.

Define "big." Double writing in some form is the only option for live migration and you either bolt it on later or plan for it now.

When the overhead of the double writing becomes substantial enough affect the architecture.

Ah. So bigger than Facebook or Google.

wat. particularly today with logical replication this is pretty much unnecessary.