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[src/UPDATING]

20080731:
WAPBL (metadata journaling support) has been added, but at this
time isn't backwards compatible with pre-WAPBL aware kernels
and userland (fsck_ffs in particular). Please make sure you
don't use a journaled filesystem with an older kernel/userland,
especially an uncleanly mounted journaled filesystem. WAPBL
also requires the super block to be in the UFS2 format. You
can use fsck_ffs -c 4 to update the superblock format.