HybridStore: A Cost-Efficient, High-Performance Storage System Combining SSDs and HDDs

Unlike the use of DRAM for caching or buffering, certain idiosyncrasies of SSDs make their integration into existing systems non-trivial. Flash memory suffers from limits on its reliability, is an order of magnitude more expensive than the HDD, and can sometimes be as slow as the HDD (due to excessive garbage collection (GC) induced by high intensity of random writes). Given these trade-offs between HDDs and SSDs in terms of cost, performance, and lifetime, the current consensus among several storage experts is to view SSDs not as a replacement for HDD but rather as a complementary device within the high performance storage hierarchy. We design and evaluate such a hybrid system called Hybrid Store to provide: (a) Hybrid Plan: improved capacity planning technique to administrators with the overall goal of operating within cost-budgets and (b) HybridDyn: improved performance/lifetime guarantees during episodes of deviations from expected workloads through two novel mechanisms: write-regulation and fragmentation busting. As an illustrative example of HybridStore's efficacy, Hybrid Plan is able to find the most cost-effective storage configuration for a large scale workload of Microsoft Research and suggest one MLC SSD with ten 7.2K RPM HDDs instead of fourteen 7.2K RPM HDDs only. HybridDyn is able to reduce the average response time for an enterprise scale random-write dominant workload by about 71%as compared to a HDD-based system.