zerofree is a utility to set unused filesystem inodes and blocks of an
ext2/3/4 filesystem to zero. This can improve the compressibility and
privacy of an ext2/3/4 filesystem.
This tool was inspired by the ext2fs privacy (i.e. secure deletion)
patch described in a Linux kernel mailing list thread.
WARNING: The filesystem to be processed should be unmounted or mounted
read-only. The tool tries to check this before running, but you
should be careful.