Derrick: A Three-layer Balancer for Self-managed Continuous Scalability

Data arrangement determines the capacity, resilience, and performance of a distributed storage system. A scalable self-managed system must place its data efficiently not only during stable operation but also after an expansion, planned downscaling, or device failures. In this article, we present Derrick, a data balancing algorithm addressing these needs, which has been developed for HYDRAstor, a highly scalable commercial storage system. Derrick makes its decisions quickly in case of failures but takes additional time to find a nearly optimal data arrangement and a plan for reaching it when the device population changes. Compared to balancing algorithms in two other state-of-the-art systems, Derrick provides better capacity utilization, reduced data movement, and improved performance. Moreover, it can be easily adapted to meet custom placement requirements.