zerofree - Man Page

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:      

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it is slow;

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it makes the disk image (temporarily) grow to its maximal  extent;

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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.  

-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.

dd (1).

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.