Linux - disk free command (df)
disk free command (df) is a storage function that can give you the free space.
Filesystem Size Used Avail Use% Mounted on /dev/mapper/VolGroup00-LogVol00 17G 16G 0 100% / /dev/sda1 99M 12M 82M 13% /boot tmpfs 1.5G 0 1.5G 0% /dev/shm
Difference between du and df
du shows the space by file whereas df shows the space by block of file.If there is a big difference in their output, try to see if some deleted file are not held by a process as open.
Example with linux and lsof
lsof /device | grep -i deleted
If this is the case, you need to restart the process to reclaim this buffers.