Mean Time To failure, MTTF, is a commonly accepted metric for reliability. In this paper we present a novel approach to achieve the desired MTTF with minimum redundancy. We analyze the failure behavior of large scale systems using failure logs collected by Los Alamos National Laboratory. We analyze the root cause of failures and present a choice of specific hardware and software components to be made fault-tolerant, through duplication, to achieve target MTTF at minimum expense. Not all components show similar failure behavior in the systems. Our objective, therefore, was to arrive at an ordering of components to be incrementally selected for protection to achieve a target MTTF. We propose a model for MTTF for tolerating failures in a specific component, system-wide, and order components according to the coverage provided. Systems grouped based on hardware configuration showed similar improvements in MTTF when different components in them were targeted for fault-tolerance.