ZEROFREE(8)
NAME
zerofree -- zero free blocks from ext2/3 file-systems
SYNOPSIS
zerofree [-n] [-v] filesystem
DESCRIPTION
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:
- o it is slow;
- o it makes the disk image (temporarily) grow to its maximal extent;
- o it (temporarily) uses all free space on the disk, so other con
current 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.
OPTIONS
-n Perform a dry run (do not modify the file-system);
-v Be verbose.
SEE ALSO
dd (1).
AUTHOR
This manual page was written by Thibaut Paumard <paumard@users.sourceforge.net> 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.