The figure of merit here is really hard drives/square foot. Mainframes use the same hard drives as rack-mounted or blade servers, and it is unclear how much th

The figure of merit here is really hard drives/square foot. Mainframes use the same hard drives as rack-mounted or blade servers, and it is unclear how much the computational substrate matters in the context of storage capacity per square or cubic foot. Additionally, your wording seems to imply that you think a mainframe would be significantly more dense. While you may cut down on extra NICs, power supplies, cases, etc. by moving to mainframes, the high degree of redundancy in mainframes in general, and their storage subsystems in particular, may well cancel out this advantage as it pertains to density.