By default, the ext3 and ext4 filesystems reserve 5% space for use by the root user. This reserved space counts as Non DFS Used.

To view the reserved space use the tune2fs command:

# tune2fs -l /dev/sde1 | egrep "Block size:|Reserved block count"
Reserved block count:  36628312
Block size:            4096

The Reserved block count is the number of ext3/ext4 filesystem blocks that are reserved. The block size is the size in bytes. In this example, 150 GB (139.72 Gigabytes) are reserved on this filesystem.

Cloudera recommends reducing the root user block reservation from 5% to 1% for the DataNode volumes. To set reserved space to 1% with the tune2fs command:

# tune2fs -m 1 /dev/sde1