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If I don’t actually want durable and consistent data, I could also turn off fsync in Postgres …

The tradeoff here is that Kafka will still work perfectly if one of its instances goes down. (Or you take it down, for upgrades, etc.)

Can you lose one Postgres instance?

AIUI Postgres has high-availability out of the box, so it's not a big deal to "lose" one as long as a secondary can take over.

Only replication is built-in, you need to add a cluster manager like Patroni to make it highly-available.