I recently encountered this on the AIX system

df command showed usage is 100% i.e 1.5 GB while du command showed usage is only 500MB

Why are the 2 commands showing different output

This command shows usage is 1.5 GB

nlxdsm29:deqadm 24> df -k .
/usr/sap/DEQ (/dev/vgdeq_1/lv_sap ) : 1560576 total allocated Kb
0 free allocated Kb
1560576 used allocated Kb
100 % allocation used
while this command shows usage is just 500MB

nlxdsm29:deqadm 28> *du -sk |sort -nr|head -10
457613 data
24285 log
689 work
51 sec

Also a process was using space on this directory

/usr/sap/DEQ: 12349co(deqadm) 12374co(deqadm) 12358co(deqadm) 12348co(deqadm)


My senior told me that may be the process was writing to a file on this directory . The file must have been deleted for freeing up the space but as the file handle was open the space could not be freed up as reflected in df command

I could nt understand it .

So can anybody explain this different behaviour of commands

Your "senior" is right. A deleted file isn't really as long as a process or more are still using it.

You need to wait for these processes to die or even kill them if that doesn't hurt to see the free space made available again.

See in Windows you cant delete file which is being accessed

I dont know about Unix

As a novice am thinking the file is deleted hence the space is freed up and should be reflected in df command

this "process" concept am really not getting

can u plz explain as if u r explaining to a Unix newbie

A process is a running program.

In windows, you indeed can't delete a file being accessed, which is a major design issue in my opinion. This is one of the reasons why you need to reboot windows that much after administrative actions with which Unix would have stayed up.

Under Unix, you can delete a file being accessed. This file disappears from the directory so its content become unreachable. However, the programs that have it open at the deletion time can still read and write data to it until they die and the space is actually freed.