The first thing to note is the general format of these configuration files. They are obviously
XML and contain multiple property specifications within a single configuration element.
The property specifications always contain name and value elements with the possibility for
optional comments not shown in the preceding code.

We set three configuration variables here:
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The dfs.default.name variable holds the location of the NameNode and is
required by both HDFS and MapReduce components, which explains why it's in
core-site.xml and not hdfs-site.xml.

The dfs.replication variable specifies how many times each HDFS block should
be replicated.

The mapred.job.tracker variable holds the location of the JobTracker just
like dfs.default.name holds the location of the NameNode. Because only
MapReduce components need know this location, it is in mapred-site.xml.