I wrote:
> The fsync is associated with the write, which is not done with insert
> lock held. We're not quite that dumb.
But wait --- are you thinking of the call path where a write (and
possible fsync) is forced during AdvanceXLInsertBuffer because there's
no WAL buffer space left? If so, that's *exactly* the scenario that
can be expected to be less common with more buffer space.
regards, tom lane