Clock2Q+: A Simple and Efficient Replacement Algorithm for Metadata Cache in VMware vSAN

TL;DR

Clock2Q+ is a new, simple cache replacement algorithm tailored for metadata caches in storage systems, outperforming existing algorithms by reducing miss ratios and maintaining efficiency in large-scale environments.

Contribution

The paper introduces Clock2Q+, a novel cache replacement algorithm with a correlation window, specifically designed for metadata caches, improving performance over state-of-the-art methods.

Findings

Clock2Q+ reduces metadata cache miss ratio by up to 28.5%.

It outperforms S3-FIFO and other algorithms on data traces.

Clock2Q+ is scalable, easy to tune, and has low overhead.

Abstract

Cache replacement algorithms are critical building blocks of storage systems. This paper examines the characteristics of metadata caches and argues that they inherently exhibit correlated references, even when the corresponding data accesses do not contain correlated references. The presence of correlated references reduces the effectiveness of cache replacement algorithms because these references are often mistakenly categorized as hot blocks. Clock2Q+ is specifically designed for metadata caches and has been implemented in vSAN and VDFS, two flagship storage products of VMware by Broadcom. Similar to S3-FIFO, Clock2Q+ uses three queues; however, Clock2Q+ introduces a correlation window in the Small FIFO queue, where blocks in this window do not set the reference bit. This simple enhancement allows Clock2Q+ to outperform state-of-the-art replacement algorithms. Compared to S3-FIFO, the second-best performing algorithm, Clock2Q+ achieves up to a 28.5% lower miss ratio on metadata traces. Clock2Q+ possesses the essential properties required for large-scale storage systems: it has low CPU overhead on cache hits, low memory overhead, scales efficiently to multiple CPUs, and is both easy to tune and implement. Additionally, Clock2Q+ outperforms state-of-the-art cache replacement algorithms on data traces as well.