HDFS Commands

You can use HDFS commands to manipulate metadata files and directories.

hdfs namenode

Automatically saves a new checkpoint at NameNode startup. As stated earlier, checkpointing is the process of merging any outstanding edit logs with the latest fsimage, saving the full state to a new fsimage file, and rolling edits. Rolling edits means finalizing the current edits_inprogress and starting a new one.

hdfs dfsadmin -safemode enter

hdfs dfsadmin -saveNamespace

Saves a new checkpoint (similar to restarting NameNode) while the NameNode process remains running. The NameNode must be in safe mode, and all attempted write activity fails while this command runs.

hdfs dfsadmin -rollEdits

Manually rolls edits. Safe mode is not required.

This can be useful if a standby NameNode is lagging behind the active NameNode and you want it to get caught up more quickly. The standby NameNode can only read finalized edit log segments, not the current in progress edits file.

hdfs dfsadmin -fetchImage

Downloads the latest fsimage from the NameNode. This can be helpful for a remote backup type of scenario.