- The NameNode executes file system namespace operations like opening, closing, and renaming files and directories.It also determines the mapping of blocks to DataNodes.
- The DataNodes are responsible for serving read and write requests from the file system’s clients. The DataNodes also perform block creation, deletion, and replication upon instruction from the NameNode.
- 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.
a file is split into one or more blocks and these blocks are stored in a set of DataNodes.
all blocks in a file except the last block are the same size.
Files in HDFS are write-once and have strictly one writer at any time.
- The NameNode uses a transaction log called the EditLog to persistently record every change that occurs to file system metadata.
- The entire file system namespace, including the mapping of blocks to files and file system properties, is stored in a file called the FsImage. The FsImage is stored as a file in the NameNode’s local file system too.
Blockreport: DataNode 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
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