checkarenas -f didn't mend an inconsistency that was reparable,
or rather it did, but forgot that.
in my case, clumps had been written but
where the writes to the clump info had not completed.
syncarenas kept a set of summary errors that noted
a fault before deciding whether it could repair it.
this change helped me recover my venti without fuss.
there might be more along these lines.