No. Two non-conflicting transactions (where neither transaction reads the other's changes) can both wait for the other to commit, so they can be left hanging,

No. Two non-conflicting transactions (where neither transaction reads the other's changes) can both wait for the other to commit, so they can be left hanging, and the COMMIT/fsync can be batched. Depending on how you interpret ACID, even two conflicting transactions (B reads what A wrote) can be left hanging. Whoever submitted A can be left blocked at COMMIT, while B is processed assuming A had committed; then either both commit or both fail. This is ACID as long as you only consider data committed once the COMMIT actually succeeds, and don't produce observable effects (outside the system) before that's true.