SelectiveEC: Towards Balanced Recovery Load on Erasure-Coded Storage Systems

Abstract

Erasure coding (EC) has been commonly used to offer high data reliability with low storage cost. Upon failures, the lost blocks are recovered in batches. Due to the limited number of stripes, the data layout within a batch is non-uniform. Together with the random selection of source and replacement nodes for recovery tasks, the recovery workload among live nodes is skewed within a batch, which severely slows down failure recovery. To solve this problem, We present SelectiveEC, a new recovery task scheduling module that provides provable network traffic and recovery load balancing for large-scale EC-based storage systems. It relies on bipartite graphs to model the recovery traffic among live nodes. Then, it intelligently selects tasks to form batches and carefully determines where to read source blocks or to store recovered ones, using theories such as a perfect or maximum matching and <inline-formula><tex-math notation="LaTeX">$k$</tex-math></inline-formula> -regular spanning subgraph. SelectiveEC supports single-node failure and multi-node failure recovery, and can be deployed in both homogeneous and heterogeneous network environments. We implement SelectiveEC in HDFS, and evaluate its recovery performance in a local cluster of 18 nodes and AWS EC2 of 50 virtual machine instances. SelectiveEC increases the recovery throughput by up to <inline-formula><tex-math notation="LaTeX">$30.68\%$</tex-math></inline-formula> compared with state-of-the-art baselines in homogeneous network environments. It further achieves <inline-formula><tex-math notation="LaTeX">$1.32\times$</tex-math></inline-formula> recovery throughput and <inline-formula><tex-math notation="LaTeX">$1.23\times$</tex-math></inline-formula> benchmark throughput of HDFS on average in heterogeneous network environments, due to the straggler avoidance by the balanced scheduling.