def get_data_transfer_vols(conn):
	"""
	Retrieve the volume of data transferred per datanode

	:param conn: Database connection
	:return: datanodes [list], volumes [list]
	"""

	query = conn.cursor()
	query.execute(
		"select data_node, cast(sum(size)/(1024*1024*1024*1024.) as float) from file where status='done' group by data_node;")
	results = query.fetchall()

	nodes = []
	vols = []
	for i in results:
		nodes.append(i[0])
		vols.append(i[1])

	return nodes, vols