Fast Reconstruction for Degraded Reads and Recovery Process in Primary Array Storage Systems

Abstract

Baegjae SUNG†a) , Nonmember and Chanik PARK †b) , Member SUMMARY RAID has been widely deployed in disk array storage systems to manage both performance and reliability simultaneously. RAID conducts two performance-critical operations during disk failures known as degraded reads/writes and recovery process. Before the recovery process is complete, reads and writes are degraded because data is reconstructed using data redundancy. The performance of degraded reads/writes is critical in order to meet stipulations in customer service level agreements (SLAs), and the recovery process affects the reliability of a storage system considerably. Both operations require fast data reconstruction. Among the erasure codes for fast reconstruction, Local Reconstruction Codes (LRC) are known to offer the best (or optimal) trade-off between storage overhead, fault tolerance, and the number of disks involved in reconstruction. Originally, LRC was designed for fast reconstruction in distributed cloud storage systems, in which network traffic is a major bottleneck during reconstruction. Thus, LRC focuses on reducing the number of disks involved in data reconstruction, which reduces network traffic. However, we observe that when LRC is applied to primary array storage systems, a major bottleneck in reconstruction results from uneven disk utilization. In other words, underutilized disks can no longer receive I/O requests as a result of the bottleneck of overloaded disks. Uneven disk utilization in LRC is due to its dedicated group partitioning policy to achieve the Maximally Recoverable property. In this paper, we present Distributed Reconstruction Codes (DRC) that support fast reconstruction in primary array storage systems. DRC is designed with group shuffling policy to solve the problem of uneven disk utilization. Experiments on real-world workloads show that DRC using global parity rotation (DRC-G) improves degraded performance by as much as 72% compared to RAID-6 and by as much as 35% compared to LRC under the same reliability. In addition, our study shows that DRC-G reduces the recovery process completion time by as much as 52% compared to LRC. key words: array storage systems, RAID, erasure codes, fast reconstruction IntroductionThe data protection technique of RAID [1] has been widely deployed in primary disk array storage systems to manage both performance and reliability simultaneously. RAID recovers data when a disk failure occurs by using redundant data (e.g., erasure codes recovery process reconstructs data of the failed disk and rebuilds data onto a replacement disk. Simultaneously, RAID serves reads and writes from applications using data reconstruction (i.e., degraded reads/writes). Therefore, fast reconstruction is the main operation that improves both the performance of degraded reads/writes and the recovery process.In primary array storage systems, the performance of degraded reads/writes during disk failure is critical in order to meet stipulations in customer service level agreements (SLAs). In addition, the perfor...