zerofree(8) - man - phpman

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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 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 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 modi‐
fied, 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.