  We report on our progress to obtain the decay constants f_B and f_Bs from
lattice-QCD simulations on the RBC-UKQCD Collaborations 2+1 flavor domain-wall
Iwasaki lattices. Using domain-wall light quarks and relativistic b-quarks we
analyze data with several partially quenched light-quark masses at two lattice
spacings of a approx 0.11 fm and a approx 0.08 fm.
