Each DataNode sends a Heartbeat message to the NameNode periodically.
The NameNode marks DataNodes without recent Heartbeats as dead and does not forward any new IO requests to them.
The time-out to mark DataNodes dead is conservatively long (over 10 minutes by default) in order to avoid replication storm caused by state flapping of DataNodes.
<property>
<name>dfs.heartbeat.interval</name>
<value>3</value>
</property>
hdfs getconf -confKey dfs.heartbeat.interval