GreenDM: A Versatile Tiering Hybrid Drive for the Trade-Off Evaluation of Performance, Energy, and Endurance

Abstract : There are trade-offs among performance, energy, and device endurance for storage systems. These trade-offs become more complex in storage system comprising different storage technologies. Designs optimized for one dimension or workload often suffer in another. Therefore, it is important to study the trade-offs so as to be able to adapt the system to workloads. As different types of drives have different traits, tiering hybrid drives are studied more closely. However, previous tiering hybrids are often designed for high throughput, efficient energy consumption, or improving endurance---leaving empirical study on the trade-offs being unexplored. Past endurance studies also lack a concrete model and metric to help study the trade-offs. Lastly, previous designs are often based on inflexible policies that cannot adapt easily to changing conditions.

We designed and developed GreenDM, a versatile tiering hybrid drive that combines Flash-based SSDs with traditional HDDs; we present our endurance model to study the aforementioned trade-offs. GreenDM presents a block interface and requires no modifications to existing application software. GreenDM migrates hot data to the faster SSD and cold data to the slower HDD. GreenDM offers tunable parameters useful in adapting the system to many workloads. We have designed, developed, and carefully evaluated GreenDM with a variety of workloads using commodity SSD and HDD drives. We demonstrated the importance of versatility to be able to adapt to various workloads. Our thesis is that one must study the trade-offs among performance, energy, and endurance, especially in the ever more popular tiered storage systems, to enable adaptation to workloads. Our system is versatile so that it can adapt to different workloads to achieve certain trade-offs by adjusting the important system parameters. We also provide several interesting observations along the cost dimension. We developed a cost model for GreenDM and evaluated it under realistic cost metrics. Future storage system designs have to consider multiple optimizations dimensions: performance, energy, endurance, and dollar cost. We close with several interesting long-term future research directions. First, it will be interesting to provide automated control knobs for users to trade-off performance, energy efficiency, and endurance. Second, one could extend the two-tier system to three tiers and explore more tiering policies. Third, it would be useful to add security as an additional dimension to further explore these trade-offs. Forth, one could experiment with different storage devices and policies in the future, and help build more efficient storage systems to achieve high performance at minimum cost. Fifth and last, it would be interesting to provide control support at the CPU level as well to further justify the trade-offs among performance, energy, and endurance.