Ursa: Scalable Load and Power Management in Cloud Storage Systems

Enterprise and cloud data centers are comprised of tens of thousands of servers providing petabytes of storage to a large number of users and applications. At such a scale, these storage systems face two key challenges: (1) hot-spots due to the dynamic popularity of stored objects; and (2) high operational costs due to power and cooling. Existing storage solutions, however, are unsuitable to address these challenges because of the large number of servers and data objects. This article describes the design, implementation, and evaluation of Ursa, a system that scales to a large number of storage nodes and objects, and aims to minimize latency and bandwidth costs during system reconfiguration. Toward this goal, Ursa formulates an optimization problem that selects a subset of objects from hot-spot servers and performs topology-aware migration to minimize reconfiguration costs. As exact optimization is computationally expensive, we devise scalable approximation techniques for node selection and efficient divide-and-conquer computation. We also show that the same dynamic reconfiguration techniques can be leveraged to reduce power costs by dynamically migrating data off under-utilized nodes, and powering up servers neighboring existing hot-spots to reduce reconfiguration costs. Our evaluation shows that Ursa achieves cost-effective load management, is time-responsive in computing placement decisions (e.g., about two minutes for 10K nodes and 10M objects), and provides power savings of 15%--37%.