2. Back Up Critical HDFS Metadata

Back up the following critical data before attempting an upgrade.

On the node that hosts the NameNode, open the Hadoop Command Line shortcut (or open a command window in the Hadoop directory). As the hadoop user, go to the HDFS home directory:
```
runas /user:hadoop "cmd /K cd %HDFS_DATA_DIR%"
```
Run the fsck command to fix any file system errors.
```
hdfs fsck / -files -blocks -locations > dfs-old-fsck-1.log 
```
The console output is printed to the dfs-old-fsck-1.log file.
Capture the complete namespace directory tree of the file system:
```
hdfs dfs -ls -R / > dfs-old-lsr-1.log
```

Create a list of DataNodes in the cluster:

hdfs dfsadmin -report > dfs-old-report-1.log

Capture output from the fsck command:
```
hdfs fsck / -blocks -locations -files > fsck-old-report-1.log 
```
Verify that there are no missing or corrupted files/replicas in the fsck command output.
Save the HDFS namespace:
1. Place the NameNode in safe mode, to keep HDFS from accepting any new writes:
```
hdfs dfsadmin -safemode enter 
```
2. Save the namespace.
```
hdfs dfsadmin -saveNamespace 
```
  Warning
  From this point on, HDFS should not accept any new writes. Stay in safe mode!
3. Finalize the namespace:
```
 hdfs namenode -finalize
```
4. On the machine that hosts the NameNode, copy the following checkpoint directories into a backup directory:
```
%HDFS_DATA_DIR%\hdfs\nn\edits\current
%HDFS_DATA_DIR%\hdfs\nn\edits\image 
%%HDFS_DATA_DIR%\hdfs\nn\edits\previous.checkpoint
```

	Warning
From this point on, HDFS should not accept any new writes. Stay in safe mode!