DescriptionIn this paper we propose DrAFTS -- a methodology for implementing probabilistic guarantees of instance reliability in the Amazon Spot tier. Amazon offers "unreliable" virtual machine instances (ones that may be terminated at any time) at a potentially large discount relative to "reliable" On-demand and Reserved instances. Our method predicts the "bid values" that users can specify to provision Spot instances which ensure at least a fixed duration of execution with a given probability. We illustrate the method and test its validity using Spot pricing data post facto, both randomly and using real-world workload traces. We also test the efficacy of the method experimentally by using it to launch Spot instances and then observing the instance termination rate. Our results indicate that it is possible to obtain the same level of reliability from unreliable instances that the Amazon service level agreement guarantees for reliable instances with a greatly reduced cost.