Hadoop-0.20.2中的SecondaryNameNode只是用来周期性从NameNode下载镜像（fsimage）和日志（edits），将其合并形成新的镜像后上传给NameNode，这个过程被称为CheckPoint。运行SecondaryNameNode节点的内存中没有命名空间和数据块Block与DataNode的映射信息，所以当NameNode宕机时，无法完全恢复元数据，只能恢复上一次做CheckPoint时的元数据。

因此，Hadoop-0.21.0中引入了一个BackupNode，相对于SecondaryNameNode，BackupNode内存中命名空间的与NameNode保持实时同步，这种机制保证了当NameNode宕机时，不丢失元数据或丢失很少的元数据。

-Hadoop-0.20.2 SecondaryNameNode only used periodically from the NameNode download mirror (fsimage) the log (edits) merged to form the new mirror then upload to the NameNode This process is called CheckPoint. Memory running SecondaryNameNode node mapping information namespace and data block Block DataNode and so when NameNode downtime, can not be fully restored metadata, metadata can only restore the last CheckPoint. , Hadoop-0.21.0 introduces a BackupNode, relative to memory SecondaryNameNode, BackupNode, namespace NameNode maintain real-time synchronization mechanism to ensure that the meta-data is not lost when the NameNode downtime or lost little data.