but instance store is transient! You may argue if you do triple replicated in different availability zones then you are ok. Well, in this case it would be very costly as you will end up with constantly spinning EC2 cluster. Even if you don't do any computation you must keep it all spinning. And see what happened to elasticity... you end up paying inflated cloud prices for constantly spinning fixed size EC2 cluster! Instead of being able to rapidly roll out large cluster, make the computation and fold it back and pay only for what you have used - isn't it the true promise of cloud?