ProCode: A Proactive Erasure Coding Scheme for Cloud Storage Systems

Common distributed storage systems use data replication to improve system reliability and maintain data availability, but at the cost of disk storage. In order to lower storage costs, data may instead be stored according to erasure codes, but this results in greater network and disk traffic when data blocks are reconstructed following an erasure. These methods are also passive, i.e., they only reconstruct data after failures occur. In this paper, we present a proactive erasure coding scheme (ProCode). We monitor the health of disks via drive failure prediction and automatically adjust the replication factor of data blocks on at-risk disks to ensure data safety. In this way, we achieve fast recovery after disk failures without significantly increasing the storage overhead. ProCode is implemented as an extension to HDFS-RAID used by Facebook. Compared with replication storage and erasure coding, ProCode improves system reliability and availability. Specifically, experimental results show 2 or more orders of magnitude reduction in the average number of data loss events over a 10- year period, a 63% or greater drop in degraded read latency, and a 78% drop in recovery time.