reports based on used blocks, du
reports based on actuale file sizes (source: http://www.unix.com/unix-dummies-que...isk-space.html
Further if files are deleted but actually still open in an application, df
will see that they're still in use and du
will not. You can find open files with lsof
To see if the latter is the case, you can reboot your system. As this however seems to relate to mysql, stopping and starting mysql might also do the trick.