Optimizing data robustness in large-scale storage systems

Storage systems capacity provided by data centers do not cease to increase, currently reaching the exabyte scale using thousands of disks. In this way, the question of the resiliency of such systems becomes critical, to avoid data loss and reduce the impact of the reconstruction process on the data access time. We propose SD2S, a method to create a placement scheme for declustered RAID organizations, based on a shifting placement. It consists in the calculation of degree matrices, which represent the distance between the source sets of each couple of physical disks, thus the number of data blocks which will be reconstructed in case of a double failure. The scheme creation is made by the computation of a score function for all possible shifting offsets and the selection of the one ensuring the reconstruction of the highest percentage of data. Results show the data reconstruction distribution against the number of double failure events. Also, the overhead generated by the calculation of the shifting offsets is compared to greedy SD2S and CRUSH without replicas for systems reaching the hundred of disks. These results confirm that the selection of the best offset can lead to a complete data reconstruction giving a small overhead, especially for large systems.