Efficient Use of Low Cost SSDs for Cost Effective Solid State Caches

Conventional Hard Disk Drives (HDDs) and emerging Solid State Drives (SSDs) each has strengths and limitations in terms of latency, price, and lifetime. To mitigate the limitations and amplify their advantages, the Solid State Cache (SSCs), which use flash based SSD as a data cache, have been introduced in previous studies [4, 5]. These studies have focused only on improving the performance and endurance of the storage system with SSCs that comprises only of Single-Level Cell (SLC) or a Multi-Level Cell (MLC) flash memory chips. Our focus is on issues of Hybrid SSC that combines SLC, MLC, and TLC SSCs for performance and price efficiency. Figure 1 shows the motivation of our work. The xaxis represents the capacity of SSCs per price ($) and the y-axis represents the performance and endurance of SSCs. For clarity, we simply visualize the characteristics of each SSC without absolute values in the figure. In the figure, SLC-based SSC shows the best performance and write endurance at the expense of price. In contrast, MLCand TLC-based SSCs are cheaper but they suffers from low performance and endurance. To meet the performance and price requirements, we consider hybrid approaches such as SLC+MLC, SLC+TLC, and MLC+TLC SSC combinations that are indicated by the shaded triangle in Figure 1. Specifically, we propose a management scheme of Hybrid Solid State Cache, namely HySSC, that integrates high performance SLC SSCs and low cost TLC SSCs.