Hadoop Distributed File System (HDFS) is the underlying distributed file system that is the most common choice for running HBase. Many HBase features depend on the semantics of the HDFS to function properly. For this reason, it’s important to understand a little about how the HDFS works. In order to understand the inner working of HDFS, you first need to understand what a distributed file system is. Ordinarily, the concepts at play in the inner workings of a distributed file system can consume an entire semester’s work for a graduate class. But in the context of this appendix, we’ll briefly introduce the concept and then discuss the details you need to know about HDFS.