1ZEROFREE(8) System Manager's Manual ZEROFREE(8)
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6 zerofree — zero free blocks from ext2, ext3 and ext4 file-systems
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9 zerofree [-n] [-v] [-f fillval] filesystem
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12 zerofree finds the unallocated, blocks with non-zero value content in
13 an ext2, ext3 or ext4 filesystem (e.g. /dev/hda1) and fills them with
14 zeroes (or another octet of your choice).
15 Filling unused areas with zeroes is useful if the device on which this
19 file-system resides is a disk image. In this case, depending on the
20 type of disk image, a secondary utility may be able to reduce the size
21 of the disk image after zerofree has been run.
22 Filling unused areas may also be useful with solid-state drives (SSDs).
25 On some SSDs, filling blocks with ones (0xFF) is reported to trigger
26 Flash block erasure by the firmware, possibly giving a write perfor‐
27 mance increase.
28 The usual way to achieve the same result (zeroing the unallocated
31 blocks) is to run dd (1) to create a file full of zeroes that takes up
32 the entire free space on the drive, and then delete this file. This has
33 many disadvantages, which zerofree alleviates:
34 • it is slow;
36 • it makes the disk image (temporarily) grow to its maximal extent;
38 • it (temporarily) uses all free space on the disk, so other con‐
40 current write actions may fail.
41 filesystem has to be unmounted or mounted read-only for zerofree to
44 work. It will exit with an error message if the filesystem is mounted
45 writable. To remount the root file-system readonly, you can first
46 switch to single user runlevel (telinit 1) then use mount -o remount,ro
47 filesystem.
48 zerofree has been written to be run from GNU/Linux systems installed as
51 guest OSes inside a virtual machine. In this case, it is typically run
52 from within the guest system, and a utility is then run from the host
53 system to shrink disk image (VBoxManage modifyhd --compact, provided
54 with virtualbox, is able to do that for some disk image formats).
55 It may however be useful in other situations: for instance it can be
58 used to make it more difficult to retrieve deleted data. Beware that
59 securely deleting sensitive data is not in general an easy task and
60 usually requires writing several times on the deleted blocks.
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64 -n Perform a dry run (do not modify the file-system);
65 -v Be verbose: show the number of blocks modified by zerofree
67 (or that would be modified, in case the -n is used), the num‐
68 ber of free blocks and the total number of blocks on the
69 filesystem;
70 -f value Specify the octet value to fill empty blocks with (defaults
72 to 0). Argument must be within the range 0 to 255.
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75 dd (1).
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78 This manual page was written by Thibaut Paumard <paumard@users.source‐
79 forge.net> for the Debian system (but may be used by others). Permis‐
80 sion is granted to copy, distribute and/or modify this document under
81 the terms of the GNU General Public License, Version 2 or any later
82 version published by the Free Software Foundation.
83 On Debian systems, the complete text of the GNU General Public License
86 can be found in /usr/share/common-licenses/GPL-2.
87 ZEROFREE(8)