CRAID: online RAID upgrades using dynamic hot data reorganization
FAST'14 Proceedings of the 12th USENIX conference on File and Storage Technologies
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Under the severe energy crisis and the fast development of cloud computing, nowadays sustainability in large data centers receives much more attention than ever. Due to its high performance and reliability, RAID, particularly RAID-5, is widely used in these data centers. However, a challenge on the sustainability of RAID-5 is its scalability, or how to efficiently expand/reduce the disks. The main reason causing this problem is the special layout of RAID-5 with parity blocks. To address this problem, in this paper, we propose a novel redistribution approach to accelerate RAID-5 scaling, called Global Stripe-based Redistribution (GSR). The basic idea is to maintain the layout of most stripes while sacrificing a small portion of stripes according to a global view of all stripes. GSR has four main advantages: 1) It supports bi-directional RAID-5 scaling (both scale-up and scale-down), 2) GSR minimizes the overhead of scaling process, including the data migration cost, parity modification and computation cost, and the operations of metadata, 3) Different from previous approaches, GSR provides high flexibility and high availability for the write requests, 4) A disk array can achieve higher capacity, performance and storage efficiency by extending more disks via GSR. In our mathematical analysis, GSR maintains uniform distribution, saves up to 81.5% I/O operations and reduces the data migration time by up to 68.0%, which speeds up the scaling process by a factor of up to 3.13.