[anarazel]

> It also seemed to be calling fsync much more than once per transaction.

If it's called many more times than once per transaction the likely reason is that wal_buffers is sized small. Whenever generated WAL exceeds wal_buffers, postgres flushes the WAL, so it does not have to reopen the file later. At that point you already gotten most benefits from batching too.

Edit: A second reason is that data pages need to be written out due to cache pressure or such, and that requires the WAL to be flushed first.