how can i find that which user is absorbing more bandwidth? how can i generate the report of user’s bandwidth usage?
How do you distinguish users?
You didn’t provide any information on your network how users are connected to your NAS? other info?
To make this task easier, you should use any RADIUS billing system which keeps track of users usage for daily/monthly/yearly in a nice format. Without any radius, the job will be somewhat very typical and you have to do lot of manual work as well. 