an alternative to the application barrier() call

barriers do not force write ordering. If you do "write 1, barrier, write 2", all that is required is that anybody looking at the file will see *one* of these three states: nothing, write 1, or write 1+2. But that does not imply that all three states have to somehow exist, if only nothing,write1+2 ever exist, you have fulfilled the requirements.

An actual implementation may be "allocate temporary space, write 2, write 1, make the file point at temporary space". Notice that write 2 is done BEFORE write 1, but we have fulfilled the requirements of barrier.