I'm not sure this is necessarily true. 2 x 40Gbps is bandwidth which is typically not the limiting factor. If it was you can go with higher bandwidth like FC. RDS is a database service, not SAN. Look at SAN boxes from storage companies, you can get things like 60 SSDs in a single box. To match the IOPs of that you'd need a lot of servers with local storage. I think in general dis-aggregating compute and storage is the optimal approach, whether some particular solution is better/price effective is a different question. Having all your drives in a box means they're easier to share, easier to service, easier to replace the servers as well ... lots of wins there.