A Graph-Assisted Out-of-Place Update Scheme for Erasure Coded Storage Systems

Abstract

Erasure Codes (ECs) have widely been used in distributed storage systems to ensure data availability because of its low storage cost and high reliability. However, the update operations in erasure coded storage systems can bring extremely high I/O latency and load imbalance due to the complexity of relationships between data and parity blocks. Although several methods such as Parity Logging (PL) and Log-Structured Array (LSA) have been proposed to improve the performance of updates, they either bring extra I/O operations or decrease the performance of file access. To address the above problems, we propose a novel Graph-assisted Out-of-place Update (GOOD) scheme to improve the performance of the update, which avoids extra I/O operations and maintains the parallelism of the file access. The key idea of GOOD is to write the updated blocks into new places while maintaining blocks of the same file distribute among different nodes. The update and garbage collection processes are guided by the graphs constructed in advance. To demonstrate the effectiveness of GOOD, we conduct several experiments in a Hadoop cluster. The results show that GOOD reduces the average response time by up to 55.33% and number of I/O operations by up to 20.28% compared to the existing methods.