Do you this tuber'koorka' or Chinese potato
This is a small tuber eaten in s.india and Sri Lanka. I have heard it called Koorka in Malayalam or Siri kizhngu in Tamil. Chinese potato seems to be commonly used but I am not sure if Chinese actually eat it.Wikipedia calls it a kind of coleus with edible tubers
It looks kind of small and dark and the skin is tough to clean.
koorka or Chinese potato