Prior to learn the concepts of Hadoop 2.x Architecture, I strongly recommend you to refer the my post on Hadoop Core Components, internals of Hadoop 1.x Architecture and its limitations. It will give you the idea about Hadoop2 Architecture requirement. And we have already learnt about the basic Hadoop components like Name Node, Secondary Name Node, Data Node, Job Tracker and Task Tracker.
Hadoop 1.x Architecture is a history now because in most of the Hadoop applications are using Hadoop 2.x Architecture. But still understanding of Hadoop 1.x Architecture will provide us the insights of how hadoop has evolved over the time.