Molar: A Cost-Efficient, High-Performance Hybrid Storage Cache

This paper proposes a deMOtion-based, fLAsh-awaRe hybrid storage cache model, named MOLAR, to effectively integrate Flash-based Solid State Disks (SSDs) into traditional DRAM-based memory storage systems. In MOLAR, a flash-aware I/O path structure is designed to adapt the asymmetric read and write performance of SSD and moreover to reduce useless write operations. A new control metric, demotion count, is proposed to wisely select the evicted data blocks from DRAM to reside in SSD. Besides, for SSD can improve internal data placement from data access hints, the Logical Block Addresses (LBAs) in SSD are grouped into the long-lived region and the short-lived region self-adaptively via a heuristic control algorithm based on the change of data block demotion count. Through trace-driven simulations, the overall hit ratio in MOLAR outperforms two traditional policies from 1.44% to 5.34%. The average write latency in SSD is reduced by 3.5 X. Moreover, write amplification is effectively reduced by about 36% in two typical flash address mapping policies.