YARN is the Hadoop second generation that not use the jobtracker daemon anymore, and substitute it with resource manager. But why, on mapred-site.xml hadoop 2 there is an mapreduce.jobtracker.address property?
You are correct. In YARN, jobtracker no longer exists. So as part of client configuration you don't have to to specify the property mapreduce.jobtracker.address. In YARN, you should specify the property mapreduce.framework.name to yarn in the config file. Instead of setting up mapreduce.jotracker.address, you need to specify yarn.resourcemanager.address.
setting yarn.resourcemanager.address is not mandatory:
refer following link which explains migration from MR1 to MR2
https://www.cloudera.com/documentation/enterprise/5-4-x/topics/cdh_ig_mapreduce_to_yarn_migrate.html
来源：https://stackoverflow.com/questions/29680155/why-there-is-a-mapreduce-jobtracker-address-configuration-on-yarn