I/O Characteristics of NoSQL Databases

The advent of the so-called NoSQL databases has brought about a new model of using storage systems. While traditional relational database systems took advantage of features offered by centrallymanaged, enterprise-class storage arrays, the new generation of database systems with weaker data consistency models is content with using and managing locally attached individual storage devices and providing data reliability and availability through highlevel software features and protocols. This work aims to review the architecture of several existing NoSQL DBs with an emphasis on how they organize and access data in the shared-nothing locallyattached storage model. It shows how these systems operate under typical workloads (new inserts and point and range queries), what access characteristics they exhibit to storage systems. Finally, it examines how several recently developed key/value stores, schemafree document storage systems, and extensible column stores organize data on local filesystems on top of directly-attached disks and what system features they must (re)implement in order to provide the expected data reliability.