Decoupled SSD: Rethinking SSD Architecture through Network-based Flash Controllers

Modern NAND Flash memory-based Solid State Drives (SSDs) are designed to provide high-bandwidth for I/O requests through high-speed NVMe interface and increased internal flash memory bandwidth. In addition to providing high performance for incoming I/O requests, the flash translation layer (FTL) also handles other flash memory management processes including garbage collection that can negatively impact I/O performance. In this work, we address how the sharing of system resources (e.g., system-bus and DRAM) for I/O requests and garbage collection can cause interference and performance degradation. In particular, we propose to rethink SSD architecture through a Decoupled SSD (dSSD) system that decouples the front-end (i.e. cores, system-bus, DRAM) with the back-end (i.e. flash memory). A flash-controller network-on-chip (fNoC) that interconnects the flash controllers together is introduced to enable decoupling of the I/O path and garbage collection path to improve performance and reliability. dSSD enables advanced commands such as copyback command to be exploited for efficient garbage collection and we propose to extend copyback command with global copyback through the fNoC. To improve reliability, we propose to recycle superblocks through superblock recycle table within the flash controller. Without any modification to the FTL, a hardware-based offloading mechanism within the flash controller of the dSSD is proposed to dynamically re-organize a superblock. Our evaluations show that decoupled SSD results in up to 42.7% I/O bandwidth improvement and 63.8% GC performance improvement, while achieving approximately 31.4× improvement in tail-latency on average. Dynamic superblock management through the dSSD results in approximately 23% improvement in lifetime with minimal impact on performance and cost.