【Hadoop】namenode与secondarynamenode的checkpoint合并元数据

Checkpoint Node(检查点节点)

NameNode persists its namespace using two files: fsimage, which is the latest checkpoint of the namespace and edits, a journal (log) of changes to the namespace since the checkpoint. When a NameNode starts up, it merges the fsimage and edits journal to provide an up-to-date view of the file system metadata. The NameNode then overwrites fsimage with the new HDFS state and begins a new edits journal.

• NameNode使用两个文件来保留其命名空间：fsimage，它是命名空间和编辑的最新检查点，是自检查点以来命名空间更改的日志（日志）。当NameNode启动时，它会合并fsimage和edits
  
  journal以提供文件系统元数据的最新视图。NameNode然后用新的HDFS状态覆盖fsimage并开始一个新的编辑日志。

The Checkpoint node periodically creates checkpoints of the namespace. It downloads fsimage and edits from the active NameNode, merges them locally, and uploads the new image back to the active NameNode. The Checkpoint node usually runs on a different machine than the NameNode since its memory requirements are on the same order as the NameNode. The Checkpoint node is started by bin/hdfs namenode -checkpoint on the node specified in the configuration file.

• Checkpoint节点定期创建命名空间的检查点。它从活动的NameNode下载fsimage和编辑，在本地合并它们，并将新映像上传回活动的NameNode。Checkpoint节点通常在与NameNode不同的机器上运行，因为它的内存要求与NameNode的顺序相同。Checkpoint节点由配置文件中指定的节点上的bin / hdfs namenode -checkpoint启动。

The location of the Checkpoint (or Backup) node and its accompanying web interface are configured via the dfs.namenode.backup.address and dfs.namenode.backup.http-address configuration variables.

• 检查点（或备份）节点及其随附Web界面的位置是通过dfs.namenode.backup.address和dfs.namenode.backup.http-address配置变量配置的。

The start of the checkpoint process on the Checkpoint node is controlled by two configuration parameters.

• Checkpoint节点上检查点进程的开始由两个配置参数控制。

dfs.namenode.checkpoint.period, set to 1 hour by default, specifies the maximum delay between two consecutive checkpoints

dfs.namenode.checkpoint.txns, set to 1 million by default, defines the number of uncheckpointed transactions on the NameNode which will force an urgent checkpoint, even if the checkpoint period has not been reached.

• dfs.namenode.checkpoint.period，默认设置为1小时，指定两个连续检查点之间的最大延迟
• dfs.namenode.checkpoint.txns，默认设置为1百万，定义NameNode上未经检查的事务的数量，这将强制紧急检查点，即使尚未达到检查点周期。

The Checkpoint node stores the latest checkpoint in a directory that is structured the same as the NameNode’s directory. This allows the checkpointed image to be always available for reading by the NameNode if necessary. See Import checkpoint.

• Checkpoint节点将最新的检查点存储在与NameNode的目录结构相同的目录中。这允许检查点图像始终可供NameNode读取（如有必要）。请参阅导入检查点。

Multiple checkpoint nodes may be specified in the cluster configuration file.

• 可以在群集配置文件中指定多个检查点节点。