Place red beans into a large container and cover with several inches of cool water; let stand 3 hours to overnight. Drain and rinse.
Combine red beans and barley in a pot; add enough water to fill 1/2 the pot. Bring to a boil, reduce heat to low, cover pot, and simmer until beans and barley are tender, about 1 hour.
Mix rice into bean-barley mixture; add more water if it has all been absorbed. Cook rice mixture for 20 minutes. Add mung beans; cook for 20 minutes.
Mix oats, millet, and tapioca into rice mixture; cook, stirring every 5 minutes, until all the grains are softened, about 20 minutes. Add more water to reach desired thickness of porridge. Stir brown sugar and raisins into porridge; cook for 5 minutes more. Remove pot from heat, stirring occasionally to prevent sticking. Serve hot or chilled.