Why do AWS spot-instance prices spike above the "on demand" pricing?

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http://serverfault.com – Amazon Pricing on Spot Instance Inconsistencies This is something which will be best explained through screenshots of a historical chart of instance pricings. If you look at a lot of the instance prices for spot instances, you will notice regular patterns of spikes. See here: As you can see, the price for this compute medium instance, regularly spikes above the on demand price. A c1.medium instance (on demand), would only cost $0.186 per hour. But for a period of a few weeks, in zone B, the price would regularly spike to $1.20. This is some 6 times the actual on demand price. It's al (HowTos)