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XOR-based erasure codes are a computationally-efficient means of generating redundancy in storage systems. Some such erasure codes provide irregular fault tolerance: some subsets of failed storage devices of a given size lead to data loss, whereas other subsets of failed storage devices of the same size are tolerated. Many storage systems are composed of heterogeneous devices that exhibit different failure and recovery rates, in which different placements- mappings of erasure-coded symbols to storage devices-of a flat XOR-based erasure code lead to different reliabilities. We have developed redundancy placement algorithms that utilize the structure of flat XOR-based erasure codes and a simple analytic model to determine placements that maximize reliability. Simulation studies validate the utility of the simple analytic reliability model and the efficacy of the redundancy placement algorithms.