If you have deleted lots of files and or in subdirectories but the space has not been freed up when you run a  df.
This may be due to a loaded shared library holding locks onto an inode that you have deleted.
When a shared library with read-other permission is loaded, it remains loaded (and available) even if no processes are currently using it. This allows faster execution, but may result in a message like "Cannot open or remove a file containing a running program" on installs or rebuilds of application libraries.

The slibclean command unloads all object files with load and use counts of 0. It can also be used to remove object files that are no longer used
from both the shared library region and in the shared library and kernel text regions by removing object files that are no longer required.