On Mon, 23 Oct 2006, Michael van Elst wrote:
>> Why? If you are up-to-date enough to have a journaled file system, why=20
>> don't you have up-to-date tools?
>
> You may need to transport the disk to an older system and you
> may want to run the same filesystem without journaling.
>
> This is not a necessary feature, it is just nice to have. With
> ext3 I can promote/demote from/to ext2 easily. With Solaris UFS
> the journal ist just a mount option.
>
>> Old tools don't know how to tell if a journal is dirty.

The downgrade from journaled to plain-old-ffs is something that should be 
done by a tool on a not-currently-mounted filesystem.  After checking that 
the filesystem is in a clean state w.r.t. the journal, it can rewrite the 
type flag in the superblock.