alt Hacker NewsI know how checkpointing works in Postgres (which isn't very different from how it works in most other redo-log implementations). It still does not change that you need to actually update the heap at some point.
Postgres allows a group commit to try to combine multiple transactions to avoid the multiple fsyncs, but it adds delay and is off by default. And even so, it reduces fsyncs, not writes.
But it turns those multiplied writes into two more sequential streams of writes. Yeah, it duplicates things, but the purpose is to allow as much sequential IO as possible (along with the other benefits and tradeoffs).