zerofree [-n] [-v] filesystem
zerofree finds the unallocated, non-zeroed blocks in an ext2 or ext3 filesystem (e.g. /dev/hda1) and fills them with zeroes. This is useful if the device on which this file-system resides is a disk image. In this case, depending on the type of disk image, a secondary utility may be able to reduce the size of the disk image after zerofree has been run.
The usual way to achieve the same result (zeroing the unallocated blocks) is to run dd (1) to create a file full of zeroes that takes up the entire free space on the drive, and then delete this file. This has many disadvantages, which zerofree alleviates:
filesystem has to be unmounted or mounted read-only for zerofree to work. It will exit with an error message if the filesystem is mounted writable. To remount the root file-system readonly, you can first switch to single user runlevel (telinit 1) then use mount -o remount,ro filesystem.
zerofree has been written to be run from GNU/Linux systems installed as guest OSes inside a virtual machine. It may however be useful in other situations.
This manual page was written by Thibaut Paumard <email@example.com> for
the Debian system (but may be used by others). Permission is
granted to copy, distribute and/or modify this document under
the terms of the GNU General Public License, Version 2 or any
later version published by the Free Software Foundation.
On Debian systems, the complete text of the GNU General Public
License can be found in /usr/share/common-licenses/GPL-2.