Where does my space gone in AIX/Linux filesystem ?

One of my friend got a situation where in she is seeing 9 GB allocated to one of the filesystems which is 100%utilized  but actual usage is 4GB  when verified with "du" command

#df -k  /mytest
Filesystem 1024-blocks Free %Used Iused %Iused Mounted on
/dev/mytestlv  9216 9216 100% 48804 12% /mytest 

She was wondering where does the other 5GB gone . 

Reason: 

This situation happens  when a  process is opening a file and dumping data into it and the file is removed while said process still has file open.So called process still holds that file space even file deleted.

How to Rectify ?

At first you need to check  what are all the  processes using  a particular filesystem using "fuser" command.

# fuser -c /mytest
/mytest:  2567 4006c 6548c 8657

You need to kill the above process if you want to free up the space.

Note: You need to inform the respective application owner/support team and take the application down time if this file-system is used by any-application.

How to Kill the proceses ?

# fuser -kc /mytest 

This  will kill  all the processes and  space will be freed up.

Check the space now 

#df -k  /mytest
Filesystem 1024-blocks Free %Used Iused %Iused Mounted on
/dev/mytestlv  9216 4096 45% 48804 12% /mytest