>What XFS did (and I experienced first hand) was add some uninitialized sectors to the file, and afterwards write the new content to the sectors. This behavior is apparently allowed by POSIX, and yet extremely annoying to users, who moved to other filesystems in droves.
To be fair there was more wrong with XFS than that - it could also corrupt *entirely unrelated* files. Personally I stopped using it when a power cut trashed /etc/passwd (and presumably a load of other files that were less obvious) despite nothing having it open at the time.