>> Sure. I should have written: Ability to switch jornaling on or off
>> without affecting data stored on the file system.  (Maybe via
>> tunefs(8).)
> So, tunefs would need:
> 1- Journal reply (for "turn journal off").
> 2- A way to allocate space for the journal in the file system [...]
>    (for "turn journal on").

Not necessarily.

If it's acceptable for these to work only when the filesystem was
created with journaling support, then (2) is unnecessary (because it's
already allocated), and (1) is not really necessary either (the replay
can happen when the filesystem is next mounted/checked; disabling
journaling doesn't need to do more than disable writing to the
journal).

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