Efficiently Reclaiming Space in a Log Structured Store

TL;DR

This paper presents a new cleaning strategy for log structured storage systems that improves space reclamation efficiency by approximating an optimal garbage collection order, validated through simulation.

Contribution

It introduces a novel cleaning strategy that better prioritizes stale pages, significantly enhancing garbage collection performance in log structured stores.

Findings

The new strategy outperforms previous methods in cleaning efficiency.

Simulation confirms near-optimal cleaning performance.

Improves space reclamation in SSD and log-structured storage systems.

Abstract

A log structured store uses a single write I/O for a number of diverse and non-contiguous pages within a large buffer instead of using a write I/O for each page separately. This requires that pages be relocated on every write, because pages are never updated in place. Instead, pages are dynamically remapped on every write. Log structuring was invented for and used initially in file systems. Today, a form of log structuring is used in SSD controllers because an SSD requires the erasure of a large block of pages before flash storage can be reused. No update-in-place requires that the storage for out-of-date pages be reclaimed (garbage collected or "cleaned"). We analyze cleaning performance and introduce a cleaning strategy that uses a new way to prioritize the order in which stale pages are garbage collected. Our cleaning strategy approximates an "optimal cleaning strategy". Simulation studies confirm the results of the analysis. This strategy is a significant improvement over previous cleaning strategies.