@Sreeviswa Athikala
Namenode doesn't have capabilities to merge both fsimage and edit logs. After setup the brand new hadoop cluster, namenode will have empty fsimage and one editlog file.All the changes are writing to edit log files and if namenode runs like this edit logs gets huge and in any case if namenode needs to restart then it will take longer time because more changes needs to be applied to the last state of the metadata.
To avoid this Checkpoint service has launched and it will work based on below two configurations
dfs.namenode.checkpoint.period, set to 1 hour by default, specifies the maximum delay between two consecutive checkpoints
or
dfs.namenode.checkpoint.txns, set to 1 million by default, defines the number of uncheckpointed transactions on the NameNode which will force an urgent checkpoint, even if the checkpoint period has not been reached.
if any one of the above conditions met then immediately checkpoint service will happen.
here checkpoint nothing but merging editlog files and generating new FS image file and uploads it to Namenode.
Like checkpoint node we have secondary node, the only difference is checkpoint node will directly upload fsimage to namenode where as secondary namenode doesn't have fsimage upload feature to namenode.
