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.Comment: Updated data analysis. 7 pages, 7 figures, presented at the 31st
International Symposium on Lattice Field Theory (Lattice 2013), 29 July - 3
August 2013, Mainz, German