ZEROFREE(8)                 System Manager's Manual                ZEROFREE(8)

       zerofree — zero free blocks from ext2, ext3 and ext4 file-systems

       zerofree [-n]  [-v]  [-f fillval]  filesystem

       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
       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 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.

       -n        Perform a dry run  (do not modify the file-system);

       -v        Be verbose;

       -f value  Specify the octet value to fill empty blocks with (defaults
                 to 0). Argument must be within the range 0 to 255.

       dd (1).

       This manual page was written by Thibaut Paumard
       <> 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

       On Debian systems, the complete text of the GNU General Public License
       can be found in /usr/share/common-licenses/GPL-2.