Beginner hadoop, there is a small doubt

I looked hadoop time, saw the HDFS, have a question , why HDFS is a Namenode more Datanode, or that there is only one master?
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1. only one namenode, no namenode communication , fast !
2.namenode only keep a little information, you can put all that information on the memory, and query up fast !
3. because namenode only keep a small number of the most critical information , rather than the actual data stored , so sufficient !
4. Even namenode crashes, there are secondary namenode can recover
client to read and write files in a directory to the master request ; master find memory returned to it the address of the file where the datanode ; client interaction talk datanode go ! That would save the master network bandwidth , both in memory and its efficiency is also high
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example, one or more namenode namenode like our leadership mechanism , a centralized leadership and efficient ! 2 leadership stability , fault tolerance better !