For work under umbrella of HADOOP-8468, user can enable nodegroup layer between node and rack in some situations. We should document it after YARN-18 and YARN-19 is figured out.
Configurable Hierarchical Topology for YARN
4-layer topology (with NodeGroup layer) implementation of Container Assignment and Task Scheduling (for YARN)
Add document for enabling node group layer in HDFS
When YARN part of work is done, the whole feature is completed and we should document it well.