Abstract: In modern multi-availability zone(Multi-AZ) cloud storage systems,failures exhibit significant hierarchical and correlated characteristics.Traditional erasure codes(e.g.,RS codes) or flat Fountain code schemes often overlook the physical topology,leading to high cross-AZ repair bandwidth for frequent local failures and poor flexibility in heterogeneous networks.To address this,this paper proposes a hierarchical AZ-Fountain coding model tailored for Multi-AZ environments.This model explicitly couples the rateless property of Fountain codes with the Multi-AZ failure domain structure,introducing a two-layer coding architecture:utilizing an AZ-optimized Bi-modal distribution(AZ-OBMD) locally to achieve zero cross-AZ traffic for local repairs,and an AZ-Raptor-link distribution(AZ-RLD) globally to ensure efficient recovery during AZ-level disasters.Experimental results show that,comparing to RS codes,LRC,and the recent AZ-Code,the proposed scheme maintains zero cross-AZ traffic for local block failures similar to LRC,while significantly reducing the number of participating blocks and local repair latency.Furthermore,it significantly lowers repair latency and transmission overhead during AZ-level disaster recovery,achieving a systematic trade-off between reliability and repair cost.

Key words: Multi-AZ, Distributed storage, Fountain codes, Hierarchical coding, Cross-AZ repair bandwidth