Integrating flash-based SSDs into the storage stack

Over the past few years, hybrid storage architectures that use high-performance SSDs in concert with high-density HDDs have received significant interest from both industry and academia, due to their capability to improve performance while reducing capital and operating costs. These hybrid architectures differ in their approach to integrating SSDs into the traditional HDD-based storage stack. Of several such possible integrations, two have seen widespread adoption: Caching and Dynamic Storage Tiering. Although the effectiveness of these architectures under certain workloads is well understood, a systematic side-by-side analysis of these approaches remains difficult due to the range of design alternatives and configuration parameters involved. Such a study is required now more than ever to be able to design effective hybrid storage solutions for deployment in increasingly virtualized modern storage installations that blend several workloads into a single stream. In this paper, we first present our extensions to the Loris storage stack that transform it into a framework for designing hybrid storage systems. We then illustrate the flexibility of the framework by designing several Caching and DST-based hybrid systems. Following this, we present a systematic side-by-side analysis of these systems under a range of individual workload types and offer insights into the advantages and disadvantages of each architecture. Finally, we discuss the ramifications of our findings on the design of future hybrid storage systems in the light of recent changes in hardware landscape and application workloads.