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: 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 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, 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.

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