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 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 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;

       -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,  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)