Download Time Analysis for Distributed Storage Codes with Locality and Availability

TL;DR

This paper analyzes the expected download time in distributed storage systems using availability codes, providing formulas and bounds for different traffic regimes and validating with simulations.

Contribution

It introduces analytical expressions and bounds for download times in systems with availability codes, including high-traffic approximations and extensive validation.

Findings

Availability codes can reduce download time in some scenarios.

The M/G/1 queue approximation is highly accurate for high-traffic analysis.

Bounds and formulas help optimize distributed storage system performance.

Abstract

The paper presents techniques for analyzing the expected download time in distributed storage systems that employ systematic availability codes. These codes provide access to hot data through the systematic server containing the object and multiple recovery groups. When a request for an object is received, it can be replicated (forked) to the systematic server and all recovery groups. We first consider the low-traffic regime and present the close-form expression for the download time. By comparison across systems with availability, maximum distance separable (MDS), and replication codes, we demonstrate that availability codes can reduce download time in some settings but are not always optimal. In the high-traffic regime, the system consists of multiple inter-dependent Fork-Join queues, making exact analysis intractable. Accordingly, we present upper and lower bounds on the download time, and an M/G/1 queue approximation for several cases of interest. Via extensive numerical simulations, we evaluate our bounds and demonstrate that the M/G/1 queue approximation has a high degree of accuracy.