# zerofree - zero free blocks from ext2, ext3 and ext4 file-systems - man(8) - [phpMan]

[_ZEROFREE_(8)](https://www.chedong.com/phpMan.php/man/ZEROFREE/8/markdown)                            System Manager's Manual                           [_ZEROFREE_(8)](https://www.chedong.com/phpMan.php/man/ZEROFREE/8/markdown)

## 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 read‐
       only, 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  util‐
       ity  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        

### -v        
                 fied,  in  case  the **-n **is used), the number of free blocks and the total number of
                 blocks on the filesystem;

### -f value  
                 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  **De‐**
       **bian  **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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