NameNode safemode

Safemode is a NameNode state, in which it doesn’t replicate blocks and doesn’t allow any modifications to the file system. The operations that don’t imply any changes to data blocks or the system, like read operations, are still available.

The NameNode enters this state automatically during start up to make time for synchronizing with the DataNodes. After it loads the file system state from the fsimage and the edits log file, it waits for DataNodes block reports. If the number of safely replicated blocks is at the acceptable threshold, the NameNode exits the safemode state.

The NameNode front page shows whether safemode is on or off. For more information on the NameNode UI, see HDFS UI overview.

You can place the NameNode in the safemode state using the dfs admin -safemode command.

To turn the safemode on, run:

The ouput will look like this:

If you turned on the NameNode safemode state manually, it has to be manually turned off.

To turn the safemode off, run:

The ouput will look like this:

In some cases, the NameNode will remain in safemode. You can forcefully turn it off, but it might mean losing some metadata.

To forcefully turn off the safemode, run: