Combo Drive: Optimizing Cost and Performance in a Heterogeneous Storage Device

We propose a new type of heterogeneous storage device called Combo Drive, which comprises of a smaller-capacity low-latency solid-state disk drive (SSD) concatenated with a larger-capacity high-throughput hard disk drive (HDD). The overall cost of a Combo Drive, similar to a Hybrid Drive, is still dominated by the more capacious HDD. With Combo Drive, the performance advantages of both the SSD and the HDD are readily utilized by assigning the lower portion of the address space, which is already considered by many file systems as faster than the higher portion, to the SSD. Performance can be optimized further on file system level on the host side. In contrast, existing Hybrid Drives utilize non-volatile memory hierarchically as a cache transparent to the environment requiring complex cache coherence algorithms. We built a Combo Drive prototype and propose multiple heuristic optimization algorithms implemented in file-system-level optimizers. Performance measurements on the host side show that the prototype achieves system start up time and application launch time similar to an SSD alone while offering large capacity and low cost of an HDD.