Tiered Replication: A Cost-effective Alternative to Full Cluster Geo-replication.

Cloud storage systems typically use three-way random replication to guard against data loss within the cluster, and utilize cluster geo-replication to protect against correlated failures. This paper presents a much lower cost alternative to full cluster geo-replication. We demonstrate that in practical settings, using two replicas is sufficient for protecting against independent node failures, while using three random replicas is inadequate for protecting against correlated node failures. We present Tiered Replication, a replication scheme that splits the cluster into a primary and backup tier. The first two replicas are stored on the primary tier and are used to recover data in the case of independent node failures, while the third replica is stored on the backup tier and is used to protect against correlated failures. The key insight of our paper is that, since the third replicas are rarely read, we can place the backup tier on separate physical infrastructure or a remote location without affecting performance. This separation significantly increases the resilience of the storage system to correlated failures and presents a low cost alternative to georeplication of an entire cluster. In addition, the Tiered Replication algorithm optimally minimizes the probability of data loss under correlated failures. Tiered Replication can be executed incrementally for each cluster change, which allows it to supports dynamic environments in which nodes join and leave the cluster, and it facilitates additional data placement constraints required by the storage designer, such as network and rack awareness. We have implemented Tiered Replication on HyperDex, an open-source cloud storage system, and demonstrate that it incurs a small performance overhead. Tiered Replication improves the cluster-wide MTTF by a factor of 20,000 compared to random replication and by a factor of 20 compared to previous non-random replication schemes, without increasing the amount of storage.