SOML Read: Rethinking the Read Operation Granularity of 3D NAND SSDs

NAND-based solid-state disks (SSDs) are known for their superior random read/write performance due to the high degrees of multi-chip parallelism they exhibit. Currently, as the chip density increases dramatically, fewer 3D NAND chips are needed to build an SSD compared to the previous generation chips. As a result, SSDs can be made more compact. However, this decrease in the number of chips also results in reduced overall throughput, and prevents 3D NAND high density SSDs from being widely-adopted. We analyzed 600 storage workloads, and our analysis revealed that the small read operations suffer significant performance degradation due to reduced chip-level parallelism in newer 3D NAND SSDs. The main question is whether some of the inter-chip parallelism lost in these new SSDs (due to the reduced chip count) can be won back by enhancing intra-chip parallelism. Motivated by this question, we propose a novel SOML (Single-Operation-Multiple-Location) read operation, which can perform several small intra-chip read operations to different locations simultaneously, so that multiple requests can be serviced in parallel, thereby mitigating the parallelism-related bottlenecks. A corresponding SOML read scheduling algorithm is also proposed to fully utilize the SOML read. Our experimental results with various storage workloads indicate that, the SOML read-based SSD with 8 chips can outperform the baseline SSD with 16 chips.