zerofree (8) - Linux Man Pages
NAMEzerofree --- zero free blocks from ext2, ext3 and ext4 file-systems
zerofree finds the unallocated,
blocks with non-zero value content in an ext2, ext3 or ext4
filesystem (e.g. /dev/hda1) and
fills them with zeroes (or another octet of your choice).
Filling unused areas with zeroes 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
Filling unused areas may also be useful with solid-state
drives (SSDs). On some SSDs, filling blocks with ones (0xFF)
is reported to trigger Flash block erasure by the firmware,
possibly giving a write performance increase.
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
zerofree has been written to be run
from GNU/Linux systems installed as guest OSes inside a virtual
machine. In this case, it is typically run from within the guest
system, and a utility is then run from the host system to shrink
disk image (VBoxManage modifyhd --compact,
provided with virtualbox, is able to do that for some disk image
It may however be useful in other situations: for instance
it can be used to make it more difficult to retrieve deleted
data. Beware that securely deleting sensitive data is not in
general an easy task and usually requires writing several times
on the deleted blocks.
- Perform a dry run (do not modify the file-system);
- Be verbose: show the number of blocks modified by zerofree (or that would be modified, in case the -n is used), the number of free blocks and the total number of blocks on the filesystem;
- -f value
- Specify the octet value to fill empty blocks with (defaults to 0). Argument must be within the range 0 to 255.
This manual page was written by Thibaut Paumard <paumard [at] 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.