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  sec-
       ondary  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 allevi-
       ates:

          •  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 ac-
             tions 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 (VBoxMan-
       age 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 number of free blocks and the to-
                 tal 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.sourceforge.net> for
       the Debian system (but may be used by others).  Permission is granted to  copy,  dis-
       tribute  and/or  modify  this  document under the terms of the GNU General Public Li-
       cense, 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)

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