Ts'o: Delayed allocation and the zero-length file problem

Conversely, if the file data hasn't been written to disk then they expect that the rename over the old data won't occur.

There is no expectation that either of these operations will occur immediately, which is why they don't request that happen via fsync().

If the current method applications use when expecting this behaviour, then it'd be nice to define an API that does provide the desired semantics. That said, I can't think of any cases where you wouldn't want the new data blocks written out before renaming over an existing file.