zerofree — zero free blocks from ext2, ext3 and ext4 file-systems
] [-f fillval
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 performance increase.
The usual way to achieve the same result (zeroing the unallocated blocks) is to
(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
- it (temporarily) uses all free space on the disk, so other
concurrent write actions may fail.
has to be unmounted or mounted read-only for zerofree
to work. It will exit with an error message if the filesystem
mounted writable. To remount the root file-system readonly, you can first
switch to single user runlevel ( telinit 1
) then use mount -o
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.
- 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
- -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
<firstname.lastname@example.org> 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.