Hadoop 1.x started with the architecture of a single NameNode. All DataNodes used to send their block reports to that single NameNode. There was a secondary NameNode in the architecture, but its sole responsibility was to merge all edits to FSImage. With this architecture, the NameNode became the single point of failure (SPOF). Since it has all the metadata of all the DataNodes of the Hadoop cluster, in the event of NameNode crash, the Hadoop cluster becomes unavailable till the next restart of NameNode repair. If the NameNode cannot be recovered, then all the data in all the DataNodes would be completely lost. In the event of shutting down NameNode for planned maintenance, the HDFS becomes unavailable for normal use. Hence, ...
During Hadoop 1.x
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