Nemo: A Low-Write-Amplification Cache for Tiny Objects on Log-Structured Flash Devices

TL;DR

Nemo is a novel cache design for flash storage that reduces application-level write amplification while maintaining high memory efficiency and low miss ratios, especially for tiny-object workloads.

Contribution

It introduces a set-associative cache with increased hash collision probability, a bloom filter-based index, and hybrid hotness tracking to optimize flash cache performance.

Findings

Significantly reduces application-level write amplification.

Achieves high memory efficiency with low miss ratios.

Maintains low write amplification even with advanced log-structured SSDs.

Abstract

Modern storage systems predominantly use flash-based SSDs as a cache layer due to their favorable performance and cost efficiency. However, in tiny-object workloads, existing flash cache designs still suffer from high write amplification. Even when deploying advanced log-structured flash devices (e.g., Zoned Namespace SSDs and Flexible Data Placement SSDs) with low device-level write amplification, application-level write amplification still dominates. This work proposes Nemo, which enhances set-associative cache design by increasing hash collision probability to improve set fill rate, thereby reducing application-level write amplification. To satisfy caching requirements, including high memory efficiency and low miss ratio, we introduce a bloom filter-based indexing mechanism that significantly reduces memory overhead, and adopt a hybrid hotness tracking to achieve low miss ratio without losing memory efficiency. Experimental results show that Nemo simultaneously achieves three key objectives for flash cache: low write amplification, high memory efficiency, and low miss ratio.