Its not just zfs/btrfs, ext3/4 has the same cow behavior when data=journal (not default but to note) And if you're on an ssd its likely doing wear leveling in

Its not just zfs/btrfs, ext3/4 has the same cow behavior when data=journal (not default but to note) And if you're on an ssd its likely doing wear leveling in a similar fashion. Really the better way to do this than overwriting files in place is to have encrypted files and keys and just reencrypt with a random key you don't retain, or some other method I suppose. But the ability to rewrite to the same spot in the filesystem, for a copy on write filesystem, is rather ludicrous. Hesitate all you want, I prefer cow fs's for most tasks.