What is the other namenode which we use in highavailabilty of namenode is known as and what are the other namenodes used in hadoop federation i am confused
1 Answers
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Hope this information solves your confusion
1)Namenode::
Namenode holds the meta data for the HDFS Namespace information, block information etc. Namenode stores the HDFS filesystem information in a file named fsimage. Updates to the file system (add/remove blocks) are not updating the fsimage file , it will be appended to the edit log, Fsimage contains mapping of blocks to files and other file system properties called snapshot of namenode. When restaring, the namenode reads the fsimage and then applies all the changes from the log file to bring the filesystem state up to date in memory.
2)Secondary Namenode::
The secondary namenode periodically pulls these two(Edits nd fsimage) files, and the namenode starts writing changes to a new edits file. Then, the secondary namenode merges the changes from the edits file with the old snapshot from the fsimage file and creates an updated fsimage file. This updated fsimage file is then copied to the namenode.
3)FailOver NameNode or Passive Namenode(Only if HA enabled):: Prior to Hadoop 2.0.0, the NameNode was a single point of failure (SPOF) in an HDFS cluster. Each cluster had a single NameNode, and if that machine or process became unavailable, the cluster as a whole would be unavailable until the NameNode was either restarted or brought up on a separate machine. the above problems is addressed by providing the option of running two redundant NameNodes in an HA cluster, the Standby/Passive NameNode also performs checkpoints of the namespace state, and thus it is not necessary to run a Secondary NameNode, CheckpointNode, or BackupNode in an HA cluster. In fact, to do so would be an error.
