Flashing Up the Storage Layer

In the near future, commodity hardware is expected to incorporate both flash and magnetic disks. In this paper we study how the storage layer of a database system can benefit from the presence of both kinds of disk. We propose using the flash and the magnetic disk at the same level of the memory hierarchy and placing a data page to only one of these disks according to the workload of the page. Pages with a read-intensive workload are placed on the flash disk, while pages with a write-intensive workload are placed on the magnetic disk. We present a family of on-line algorithms to decide the optimal placement of a page and study their theoretical properties. Our system is self-tuning,i.e., our algorithms adapt page placement to changing workloads. We also present a buffer replacement policy that takes advantage of the asymmetric I/O properties of the two types of storage media to reduce the total I/O cost. Our experimental evaluation shows remarkable I/O performance improvement over both flash-only and magnetic-only systems. These results, we believe, exhibit both the potential and necessity of such algorithms in future database systems.