GC bypass: decoupling GC from the flash translation layer to eliminate GC-induced long-tail latency inside SSD

Abstract

NAND flash-based solid-state drives (SSDs) have been adopted by many data centers due to their high performance and low power consumption. However, the physical characteristics of the underlying flash memory necessitate garbage collection (GC) operations. Valid page migration during GC contributes significantly to latency overhead while competing for flash channel bandwidth and controller resources with user I/O requests through shared physical paths, leading to path conflicts and elevated long-tail latency. The existing Venice scheme introduces a low-cost interconnected network with path reservation mechanisms to provide substantial path diversity for SSDs. Nevertheless, its fair scheduling policy lacks priority differentiation between I/O and GC requests. In this paper, we propose GC bypass, which leverages Venice’s path diversity while enforcing GC request transmission through dedicated controllers. GC bypass decomposes GC requests into sub-requests and assigns low priority to valid page writes, enabling high-priority operations including user I/O, valid page reads, and block erases, to preempt paths reserved by low-priority requests. Valid pages failing to secure reserved paths are temporarily buffered for retry. Experimental results demonstrate that GC bypass reduces the 99.99^th percentile long-tail latency by up to 25% compared to Venice. GC bypass effectively mitigates interference between critical I/O operations and background maintenance tasks while maintaining the architectural benefits of path diversity.

Keywords

Solid-state drive (SSD) / NAND flash / Garbage collection (GC) / Interconnected network / Flash channel

Rights & permissions

The Authors. Published by Zhejiang University Press Co., Ltd.