ZEROFREE(8) System Manager's Manual ZEROFREE(8)
NAME
zerofree — zero free blocks from ext2, ext3 and ext4 file-systems
SYNOPSIS
zerofree [-n] [-v] [-f fillval] filesystem
DESCRIPTION
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 been run.
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 perfor-
mance 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 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. 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 formats).
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.
OPTIONS
-n Perform a dry run (do not modify the file-system);
-v Be verbose: show the number of blocks modified by zerofree
(or that would be modified, in case the -n is used), the num-
ber 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.
SEE ALSO
dd (1).
AUTHOR
This manual page was written by Thibaut Paumard <paumard@users.source-
forge.net> for the Debian system (but may be used by others). Permis-
sion 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.
ZEROFREE(8)
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