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 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 trig-
       ger 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:

	  o  it is slow;

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

	  o  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,  pro-
       vided 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).  Permis-
       sion  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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