def getChunkSuffix(chunk_id):
    """ given a chunk_id (e.g.: c-12345678-1234-1234-1234-1234567890ab_6_4)
    return the coordinates as a string. In this case 6_4
    """
    # go to the first underscore
    n = chunk_id.find('_')  + 1
    if n == 0:
        raise ValueError("Invalid chunk_id: {}".format(chunk_id))
    suffix = chunk_id[n:]
    return suffix