Back in the days of 16-bit systems and early 32-bit, a company called Platypus used to sell SCSI boards that had gangs of RAM instead of electromechanical discs and worked as superfast (for the time) swap files. As I remember, at the time, their boards were said to offer the equivalent of up to 8Gb RAM, and that going higher exceeded the data pipeline exchange rate. It's quite likely that the recent silicon hard drives have access times that would allow similar operation. I don't think that Platypus is in business any longer, though. They were fearsomely expensive.