The file system metadata section of HDFS.
The NameNode is the repository of all HDFS metadata.
The metadata are stored in two files:
- fsimage file which is the metadata store
- EditLog transaction log file which records every metadata transaction
The DataNode stores HDFS data in files in its local file system. The DataNode has no knowledge about HDFS files. It stores each block of HDFS data in a separate file in its local file system. The DataNode does not create all files in the same directory. Instead, it uses a heuristic to determine the optimal number of files per directory and creates subdirectories appropriately. It is not optimal to create all local files in the same directory because the local file system might not be able to efficiently support a huge number of files in a single directory. When a DataNode starts up, it scans through its local file system, generates a list of all HDFS data blocks that correspond to each of these local files, and sends this report to the NameNode. The report is called the Blockreport.
Extended attributes (abbreviated as xattrs) are a filesystem feature that allow user applications to associate additional metadata with a file or directory.