gbde: ufs & ffs

[RW]

Why does the gbde section of the Handbook recommend running fsck -p -t ffs, 
which calls  fsck_ffs, when ordinary filesystems are checked with fsck_ufs.

According to 
<http://sixshooter.v6.thrupoint.net/jeroen/faq.html#UFS-DIFF-FFS>, ffs runs 
on top of ufs "providing directory structure information, and a variety of 
disk access optimizations". Am I correct in thinking that soft-updates 
protect ffs from inconsistency, so that only a background ufs check is 
required?

Is it possible to background check gbde partitions? If so, can it be done 
after booting is complete?