fsck_ffs -X, panic: wapbl_register_deallocation: out of resources

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

I just tried to snapshot-fsck a 6T WAPBL filesystem (fsck -n -f -T ffs:-X ...).
For about one hour, the RAID the filesystem is on was nearly fully busy.
The fsck_ffs process was consuming (just a little) CPU time, waiting on 
biowait or tstile, but non-kill-able.
A ffsconfig -l was hanging in tstile.
After about one hour, the system panic'ed with "wapbl_register_deallocation: 
out of resources". Replaying that filesystem's log took 40 minutes.
What can I do? Running that fsck command on much smaller filesystems works fine.