write one line, or a couple words of a line
write a little more
enforcing a barrier between all of these writes would kill you
remember that you don't know the storage stack below you, what you submit as one write may be broken up into multiple writes, and you have no guarantee of what order those multiple writes could be done in (think a raid array where your write spans drives as one example)
as a result a barrier needs to prohibit merging across the barrier as well as just reordering across the barrier.