Monday, June 8, 2015

NAME NODE FILE SYSTEM

1:08 PM - By ajay desai 0

The name  node file system maintains metadata in two of its files: -

1) fsImage
2) Edit log

                     


                            (1) FS IMAGE

This file is a checkpoint of the HDFS metadata.

All the metadata information of a file like: -

(i) No: of blocks allocated for a file.

(ii) Physical address of those blocks.

(iii) Which blocks are present on which data nodes.

The usual size of this of this file is GBs. Whenever the name node fails, the latest state of metadata stored in fsImage file can be re-constructed by loading that data from name node hard disk to main memory.

The secondary name node loads this metadata and the information about the recent write operations performed by the name node present in edit log into its memory and begins its job

                      (2) EDIT LOG

When the client performs a write operation like creating a file. This operation is first recorded in edit log. And then the information about this operation is updated into fsImage and the edit log's contents are flushed.

As writing metadata information to fsImage takes a lot of time because of its huge size, a file called edit log is maintained by name node, which contains information about the recent write operations performed like creating or moving a file.



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