zerofree man page
zerofree — zero free blocks from ext2/3 file-systems
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:
it is slow;
it makes the disk image (temporarily) grow to its maximal extent;
it (temporarily) uses all free space on the disk, so other concurrent write actions may fail.
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.
Perform a dry run (do not modify the file-system);
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.