SuperCopyback: Revisiting Copyback on Modern High-Performance NAND Flash-based SSDs

NAND flash-based SSDs have emerged as a critical storage solution due to their exceptional performance and cost-effectiveness. However, the sequential write limitation of NAND flash blocks necessitates garbage collection (GC) to reclaim space occupied by stale data. Nevertheless, the extensive data migration involved in GC significantly impacts performance and Quality of Service (QoS) of SSDs. To mitigate this issue, copyback has been proposed as a means to accelerate GC by eliminating off-chip data movements. Specifically, copyback reads data into on-plane latches and immidiately re-writes it into another page on the same plane. However, in the case of modern high-performance SSDs, copyback is rarely utilized due to the following challenges: (1) Copyback operates at the page-level and thus fails to effectively reclaim invalid data within the context of subpage-level mapping; (2) Copyback eliminates off-chip data movements, preventing data pages from being read out for Redundant Array of Independent NAND (RAIN) parity computation, thereby compromising SSD reliability. In this study, we introduce SuperCopyback as a solution that efficiently addresses these issues for modern SSDs. Firstly, we propose a Multiple-Read-One-Write (MROW) copyback approach through lightweight latch circuit modifications to enable subpage-level copyback implementation; additionally, we propose an orchestrated GC method to effectively utilize MROW copyback. Furthermore, we present a novel copyback-based RAIN scheme that conceals data pages readout latency in write operations and relocates parities to support efficient copyback. The experimental results on both synthetic and real traces demonstrate that SuperCopyback achieves a performance comparable to an ideal scenario where data movement of $\mathbf{4 K B}$ takes only 1ns.