After performed the Terasort benchmark. I cleared the input and output directory. But still i can see the HDFS (Disk) usage same . No difference. Does it the files are still in HDFS ?
Like Michael mentions, files are not deleted immediately and rather moved to trash folder if you did not use "-skiptrash" option when deleting the folder. You can call the "hadoop fs -expunge" explicitly to empty trash. Even better, the folder you are trying to delete from has a subfolder called ".Trash". Just clear that up using "rm" command you'll reclaim the space.
hdfs dfs -rm /path/to/trash/folder ///just like any other path. See how Trash works for better understanding: