Self-adjusting two-failure tolerant disk arrays

We have presented a representation for a storage system with two failure tolerance based on flat XOR codes. We argue that this representation allows us to implement fast algorithm for the layout of very large, evolving disk arrays. Much needs to be done. Fast, but efficient algorithms for major changes in the disk array such as rack failure or insertion of new disks still need to be implemented and tested. Our goal is usually not to find an optimal layout (in a sense to be defined precisely), but one that is close to optimal. To assert that our algorithms perform at this level involves a more mathematical analysis of the consequences of failures in such an array to derive bounds on the robustness of optimal layouts, a task we have barely started. Nevertheless, the results we have indicate that the algorithms are quite effective and certainly fast and easy to implement. This presents definite progress over the true optimization (including looking for proven optimal designs) that can be done only for special, small cases and supports our pragmatic attitude.