The correct answer is D. all of the above.
When a client contacts the namenode for accessing a file, the namenode responds with the following information:
- The size of the file requested.
- The block id and hostname of all the data nodes containing that block.
- The block id of the file requested.
The namenode maintains a map of all the blocks in the file system and the data nodes that store them. When a client requests a file, the namenode looks up the file in its map and returns the block id and hostname of all the data nodes that store the file. The client then contacts each of the data nodes to read the blocks of the file.
The block id is a unique identifier for each block in the file system. The block id is used to locate the block on the data node. The hostname is the name of the data node that stores the block. The client uses the hostname to connect to the data node.
The size of the file is the number of bytes in the file. The client uses the size of the file to determine how many blocks it needs to read from the data nodes.