Hybrid Block Storage for Efficient Cloud Volume Service

The migration of traditional desktop and server applications to the cloud brings challenge of high performance, high reliability and low cost to the underlying cloud storage. To satisfy the requirement, this paper proposes a hybrid cloud-scale block storage system called Ursa. Trace analysis shows that the I/O patterns served by block storage have only limited locality to exploit. Therefore, instead of using SSDs as a cache layer, Ursa proposes an SSD-HDD-hybrid storage structure that directly stores primary replicas on SSDs and replicates backup replicas on HDDs. At the core of Ursa’s hybrid storage design is an adaptive journal that can bridge the performance gap between primary SSDs and backup HDDs for random writes, by transforming small backup writes into journal appends which are then asynchronously replayed and merged to backup HDDs. To efficiently index the journal, we design a novel range-optimized merge-tree (ROMT) structure that combines a continuous range of keys into a single composite key {offset,length}. Ursa integrates the hybrid structure with designs for high reliability, scalability, and availability. Experiments show that Ursa in its hybrid mode achieves almost the same performance as in its SSD-only mode (storing all replicas on SSDs), and outperforms other block stores (Ceph and Sheepdog) even in their SSD-only mode while achieving much higher CPU efficiency (IOPS and throughput per core).