Replichard: Towards Tradeoff between Consistency and Performance for Metadata

Abstract

Metadata scalability is critical for distributed systems as the storage scale is growing rapidly. Because of the strict consistency requirement of metadata, many existing metadata services utilize a fundamentally unscalable design for the sake of easy management, while others provide improved scalability but lead to unacceptable latency and management complexity. Without delivering scalable performance, metadata will be the bottleneck of the entire system. Based on the observation that real file dependencies are few, and there are usually more idempotent than non-idempotent operations, we propose a practical strategy, Replichard, allowing a tradeoff between metadata consistency and scalable performance. Replichard provides metadata services through a cluster of metadata servers, in which a flexible consistency scheme is adopted: strict consistency for non-idempotent operations with dynamic write-lock sharding, and relaxed consistency with accuracy estimations of return values where consistency for idempotent requests is relaxed to achieve high throughput. Write-locks are dynamically created at subtree-level and designated to independent metadata servers in an application-oriented manner. A subtree metadata update that occurs on a particular server is replicated to all metadata servers conforming to the application "start-end" semantics, resulting in an eventually consistent namespace. An asynchronous notification mechanism is also devised to enable users to deal with potential stale reads from operations of relaxed consistency. A prototype was implemented based on HDFS, and the experimental results show promising scalability and performance for both micro benchmarks and various real-world applications written in Pig, Hive and MapReduce.