NAME NODE起保存DATA NODE上文件的位置信息用，主要有兩個保存文件：FsImage和EditLog，FsImage保存了上一次NAME NODE啟動時的狀態，EditLog則記錄每次成功后的對HDFS的操作行為。當NAME NODE重啟時，會合并FsImage和EditLog成為一個新的FsImage，清空EditLog，如果EditLog非常大的時候，則NAME NODE啟動的時間會非常長。因此就有SECOND NAME NODE。
SECOND NAME NODE會以HTTP的方式向NAME NODE要這兩個文件，當NAME NODE收到請求時，就會韋一個新的EditLog來記錄，這時SECOND NAME NODE就會將取得的這兩個文件合并，成一個新的FsImage，再發給NAME NODE，NAME NODE收到后，就會以這個為準，舊的就會歸檔不用。
SECOND NAME NODE還有一個用途就是當NAME NODE DOWN了的時候，可以改SECOND NAME NODE的IP為NAME NODE所用的IP，當NAME NODE用。
secondary namenoded 配置很容易被忽視，如果jps檢查都正常，大家通常不會太關心，除非namenode發生問題的時候，才會想起還有個secondary namenode，它的配置共兩步：
- 集群配置文件conf/master中添加secondarynamenode的機器
- 修改/添加 hdfs-site.xml中如下屬性：
<property>
<name>dfs.http.address</name>
<value>{your_namenode_ip}:50070</value>
<description>
The address and the base port where the dfs namenode web ui will listen on.
If the port is 0 then the server will start on a free port.
</description>
</property>
這兩項配置OK后，啟動集群。進入secondary namenode 機器，檢查fs.checkpoint.dir（core-site.xml文件，默認為${hadoop.tmp.dir}/dfs/namesecondary）目錄同步狀態是否和namenode一致的。
如果不配置第二項則，secondary namenode同步文件夾永遠為空，這時查看secondary namenode的log顯示錯誤為：
2011-06-09 11:06:41,430 INFO org.apache.hadoop.hdfs.server.common.Storage: Recovering storage directory /tmp/hadoop-hadoop/dfs/namesecondary from failed checkpoint.
2011-06-09 11:06:41,433 ERROR org.apache.hadoop.hdfs.server.namenode.SecondaryNameNode: Exception in doCheckpoint:
2011-06-09 11:06:41,434 ERROR org.apache.hadoop.hdfs.server.namenode.SecondaryNameNode: java.net.ConnectException: Connection refused
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:351)
at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:211)
at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:200)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:366)
at java.net.Socket.connect(Socket.java:529)
at java.net.Socket.connect(Socket.java:478)
at sun.net.NetworkClient.doConnect(NetworkClient.java:163)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:394)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:529)
at sun.net.www.http.HttpClient.<init>(HttpClient.java:233)
at sun.net.www.http.HttpClient.New(HttpClient.java:306)
at sun.net.www.http.HttpClient.New(HttpClient.java:323)
at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:970)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:911)
at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:836)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1172)
at org.apache.hadoop.hdfs.server.namenode.TransferFsImage.getFileClient(TransferFsImage.java:151)
at org.apache.hadoop.hdfs.server.namenode.SecondaryNameNode.downloadCheckpointFiles(SecondaryNameNode.java:256)
at org.apache.hadoop.hdfs.server.namenode.SecondaryNameNode.doCheckpoint(SecondaryNameNode.java:313)
at org.apache.hadoop.hdfs.server.namenode.SecondaryNameNode.run(SecondaryNameNode.java:225)
at java.lang.Thread.run(Thread.java:662)
可能用到的core-site.xml文件相關屬性：
<property>
<name>fs.checkpoint.period</name>
<value>300</value>
<description>The number of seconds between two periodic checkpoints.
</description>
</property>
<property>
<name>fs.checkpoint.dir</name>
<value>${hadoop.tmp.dir}/dfs/namesecondary</value>
<description>Determines where on the local filesystem the DFS secondary
name node should store the temporary images to merge.
If this is a comma-delimited list of directories then the image is
replicated in all of the directories for redundancy.
</description>
</property>