Shifted Element Arrangement in Mirror Disk Arrays for High Data Availability during Reconstruction

The mirror method and its variants are widely used in storage systems. However, they suffer from low data availability during on-line reconstruction because sequential reads are inevitable under the current element arrangements. In this paper, we propose a shifted element arrangement method in mirror disk arrays to provide high data availability during reconstruction, which can be used in both the mirror method and its variants. We design this novel element arrangement to minimize the maximum number of elements that are read out from one disk, thus minimize the number of read accesses during the reconstruction process. Theoretically, we prove that our shifted element arrangement improves data availability by a factor of n or (2n+1)/4 (n is the number of disks in the data disk array), respectively, in the mirror methods without or with parity, while still enjoying the theoretical optimal write efficiency. Our experimental evaluation with n equal to three to seven shows that the shifted element arrangement achieved an improvement of a factor of 1.54 to 4.55 in data availability during reconstruction, and compatible write efficiency.