FUNDAMENTALS OF COMPUTER

Detailed explanation-1: -During start up the NameNode loads the file system state from the fsimage and the edits log file. It then waits for DataNodes to report their blocks so that it does not prematurely start replicating the blocks though enough replicas already exist in the cluster. During this time NameNode stays in Safemode.

Detailed explanation-2: -Hadoop FS-Image Editlogs. FsImage is a file stored on the OS filesystem that contains the complete directory structure (namespace) of the HDFS with details about the location of the data on the Data Blocks and which blocks are stored on which node. This file is used by the NameNode when it is started.

Detailed explanation-3: -In this code /var/lib/hadoop-0.20/cache/ is the location of fsimage, fstime and edits log. If in case of namenode failure for some particular time fsimage data will be stored in secondynamenode temporarily, and after namenode get recovered temporary data will be stored in fsimage. Save this answer.

Detailed explanation-4: -NameNode is used when the Primary NameNode goes down. Explanation: Secondary namenode is used for all time availability and reliability.

Detailed explanation-5: -Secondary NameNode The NameNode stores modifications to the file system as a log appended to a native file system file, edits. When a NameNode starts up, it reads HDFS state from an image file, fsimage, and then applies edits from the edits log file.