A hybrid filesystem for hard disk drives in tandem with flash memory

The traditional hard disk drive (HDD) is often a bottleneck in the overall performance of modern computer systems. With the development of solid state drives (SSD) based on flash memory, new possibilities are available to improve secondary storage performance. In this work, we propose a new hybrid SSD–HDD storage system and a selection of algorithms designed to assign pages across an HDD and an SSD to optimise I/O performance. The hybrid system combines the advantages of the SSD’s fast random seek speed with the sequential access speed and large storage capacity of the HDD to produce significantly improved performance in a variety of situations. We further improve performance by allowing concurrent access across the two types of storage devices. We show the drive assignment problem is NP-complete and accordingly propose effective heuristic solutions. Extensive experiments using both synthetic and real data sets show our system with a small SSD can outperform a striped dual HDD and remain competitive with a dual SSD.