SRDF/Metro: R1 devise is host accessible, R2 is red-only SRDF/S: R1 is host accessible, R2 is write disabled

SRDF/Metro: R1 and R2 devices are host accessible at the same time SRDF/S: R1 devices are host accessible, R2 devices are write disabled

SRDF/Metro: R2 can be larger than R1 SRDF/S R2 cannot be larger than R1

SRDF/Metro: Requires a third array for any witness functionality SRDF/S: Does not require a third array