> You use a cluster when your data and compute requirements are large and parallel enough that the tax paid on network latency trumps the 10-20X speedup you get

> You use a cluster when your data and compute requirements are large and parallel enough that the tax paid on network latency trumps the 10-20X speedup you get on SSD and 1000X speedup you get from just keeping data in RAM. Nowadays there's another reason to use clusters: to autoscale your expenditure wrt workload. A little inefficiency might be acceptable if you don't have to pay for a huge beefed up server idling at, say, 30%.