Warped Mirrors for flash

Flash-based devices are cost-competitive to traditional hard disks in both personal and industrial environments and offer the potential for large performance gains. However, as flash-based devices have a high bit-error rate and a relatively short lifetime, reliability issues remain a major problem. One possible solution is redundancy; using techniques such as mirroring, data reliability and availability can be greatly enhanced. All standard RAID approaches assume that devices do not wear out, and hence distribute work equally among them; unfortunately, for flash, this approach is not appropriate as the life of flash cell depends on the number of times it is written and cleaned. Hence, identical write patterns to mirrored flash drives introduce a failure dependency in the storage system, increasing the probability of concurrent device failure and hence data loss. We propose Warped Mirrors as a solution to this endurance problem for mirrored flash devices. By carefully inducing a slight imbalance into write traffic across devices, we intentionally increase the workload of one device in the mirror pair, and thus increase the odds that it will fail first. Thus, with our approach, device failure independence is preserved. Our simulation results show that across both synthetic and traced workloads, little performance overhead is induced.