I apologize in advance, but whenever people claim to use a in-memory big-data system, how exactly does this end up working?<p>You can only stuff so much into memory, so you can scale up vertically in-terms of memory, unless you buy a massive big-iron POWER box, you scale out horizontally. But with each of these in-memory appliances, what happens when you need to spill out to disk?<p>In essence why should one bother with these in-memory appliances as opposed to buying boxes with fast SSD's instead? Sure you spill out to disk, but do you take that big of a hit compared to the enormous cost of keeping everything in memory?