Codes with Locality for Two Erasures

TL;DR

This paper introduces locally 2-reconstructible codes that recover from two erasures sequentially using local parity checks, providing bounds and optimal constructions based on Turán graphs.

Contribution

It generalizes codes with all-symbol locality to handle two erasures sequentially, deriving universal bounds and constructing optimal codes based on Turán graphs.

Findings

Derived upper bounds on minimum distance for locally 2-reconstructible codes.

Constructed optimal codes based on Turán graphs.

Established a new bound on minimum distance for single-erasure locality codes.

Abstract

In this paper, we study codes with locality that can recover from two erasures via a sequence of two local, parity-check computations. By a local parity-check computation, we mean recovery via a single parity-check equation associated to small Hamming weight. Earlier approaches considered recovery in parallel; the sequential approach allows us to potentially construct codes with improved minimum distance. These codes, which we refer to as locally 2-reconstructible codes, are a natural generalization along one direction, of codes with all-symbol locality introduced by Gopalan \textit{et al}, in which recovery from a single erasure is considered. By studying the Generalized Hamming Weights of the dual code, we derive upper bounds on the minimum distance of locally 2-reconstructible codes and provide constructions for a family of codes based on Tur\'an graphs, that are optimal with respect to this bound. The minimum distance bound derived here is universal in the sense that no code which permits all-symbol local recovery from $2$ erasures can have larger minimum distance regardless of approach adopted. Our approach also leads to a new bound on the minimum distance of codes with all-symbol locality for the single-erasure case.