Scaling out the namespace using KV store

Currently the NN keeps the namespace in the memory. To improve the scalability of the namespace, users can scale up by using more RAM or scale out using Federation (i.e., statically partitioning the namespace).

We would like to remove the limitation of scaling the global namespace. Our vision is that that HDFS should adopt a scalable underlying architecture that allows the global namespace scales linearly.

We propose to implement the HDFS namespace on top of a key-value (KV) store. Adopting the KV store interfaces allows HDFS to leverage the capability of modern KV store and to become much easier to scale. Going forward, the architecture allows distributing the namespace across multiple machines, or storing only the working set in the memory (HDFS-5389), both of which allows HDFS to manage billions of files using the commodity hardware available today.