[Apr 29,  2018] Clear unused space with zeros (ext3, ext4)

Apr 29,  2018 | unix.stackexchange.com

Grzegorz Wierzowiecki,  Jul 29,  2012 at 10:02

How to clear unused space with zeros ? (ext3, ext4)

I'm looking for something smarter than

cat /dev/zero > /mnt/X/big_zero ; sync; rm /mnt/X/big_zero

Like FSArchiver is looking for "used space" and ignores unused,  but opposite site.

Purpose: I'd like to compress partition images,  so filling unused space with zeros is highly recommended.

Btw. For btrfs : Clear unused space with zeros (btrfs)

Mat,  Jul 29,  2012 at 10:18

Check this out: superuser.com/questions/19326/Mat Jul 29 '12 at 10:18

Totor,  Jan 5,  2014 at 2:57

Two different kind of answer are possible. What are you trying to achieve? Either 1) security,  by forbidding someone to read those data,  or 2) optimizing compression of the whole partition or [SSD performance]( en.wikipedia.org/wiki/Trim_(computing) ? – Totor Jan 5 '14 at 2:57

enzotib,  Jul 29,  2012 at 11:45

Such an utility is zerofree .

From its description:

Zerofree finds the unallocated,  non-zeroed blocks in an ext2 or ext3 file-system and fills them with zeroes. This 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. Zerofree requires the file-system to be unmounted or mounted read-only.

The usual way to achieve the same result (zeroing the unused blocks) is to run "dd" do 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.

Zerofree has been written to be run from GNU/Linux systems installed as guest OSes inside a virtual machine. If this is not your case,  you almost certainly don't need this package.


The description of the .deb package contains the following paragraph now which would imply this will work fine with ext4 too.

Description: zero free blocks from ext2,  ext3 and ext4 file-systems Zerofree finds the unallocated blocks with non-zero value content in an ext2,  ext3 or ext4 file-system and fills them with zeroes...

Grzegorz Wierzowiecki,  Jul 29,  2012 at 14:08

Is it official page of the tool intgat.tigress.co.uk/rmy/uml/index.html ? Do you think it's safe to use with ext4 ? – Grzegorz Wierzowiecki Jul 29 '12 at 14:08

enzotib,  Jul 29,  2012 at 14:12

@GrzegorzWierzowiecki: yes,  that is the page,  but for debian and friends it is already in the repos. I used on a ext4 partition on a virtual disk to successively shrink the disk file image,  and had no problem. – enzotib Jul 29 '12 at 14:12

jlh,  Mar 4,  2016 at 10:10

This isn't equivalent to the crude dd method in the original question,  since it doesn't work on mounted file systems. – jlh Mar 4 '16 at 10:10

endolith,  Oct 14,  2016 at 16:33

zerofree page talks about a patch that lets you do "filesystem is mounted with the zerofree option" so that it always zeros out deleted files continuously. does this require recompiling the kernel then? is there an easier way to accomplish the same thing? – endolith Oct 14 '16 at 16:33

Hubbitus,  Nov 23,  2016 at 22:20

Be careful - I lost ext4 filesystem using zerofree on Astralinux (Debian based)Hubbitus Nov 23 '16 at 22:20

Anon,  Dec 27,  2015 at 17:53

Summary of the methods (as mentioned in this question and elsewhere) to clear unused space on ext2/ext3/ext4: Zeroing unused space File system is not mounted File system is mounted

Having the filesystem unmounted will give better results than having it mounted. Discarding tends to be the fastest method when a lot of previously used space needs to be zeroed but using zerofree after the discard process can sometimes zero a little bit extra (depending on how discard is implemented on the "disk").

Making the image file smaller Image is in a dedicated VM format

You will need to use an appropriate disk image tool (such as qemu-img convert src_image dst_image ) to enable the zeroed space to be reclaimed and to allow the file representing the image to become smaller.

Image is a raw file

One of the following techniques can be used to make the file sparse (so runs of zero stop taking up space):

These days it might easier to use a tool like virt-sparsify to do these steps and more in one go.


cas,  Jul 29,  2012 at 11:45

sfill from secure-delete can do this and several other related jobs.


sfill -l -l -z /mnt/X

There is a source tree that appears to be used by the ArchLinux project on github that contains the source for sfill which is a tool included in the package Secure-Delete.

Also a copy of sfill 's man page is here:

cas,  Jul 29,  2012 at 12:04

that URL is obsolete. no idea where its home page is now (or even if it still has one),  but it's packaged for debian and ubuntu. probably other distros too. if you need source code,  that can be found in the debian archives if you can't find it anywhere else. – cas Jul 29 '12 at 12:04

mwfearnley,  Jul 31,  2017 at 13:04

The obsolete manpage URL is fixed now. Looks like "Digipedia" is no longer a thing. – mwfearnley Jul 31 '17 at 13:04

psusi,  Apr 2,  2014 at 15:27

If you have e2fsprogs 1.42.9,  then you can use e2image to create the partition image without the free space in the first place,  so you can skip the zeroing step.

mwfearnley,  Mar 3,  2017 at 13:36

I couldn't (easily) find any info online about these parameters,  but they are indeed given in the 1.42.9 release notes: e2fsprogs.sf.net/e2fsprogs-release.html#1.42.9mwfearnley Mar 3 '17 at 13:36

user64219,  Apr 2,  2014 at 14:39

You can use sfill . It's a better solution for thin volumes.

Anthon,  Apr 2,  2014 at 15:01

If you want to comment on cas answer,  wait until you have enough reputation to do so. – Anthon Apr 2 '14 at 15:01

derobert,  Apr 2,  2014 at 17:01

I think the answer is referring to manpages.ubuntu.com/manpages/lucid/man1/sfill.1.html ... which is at least an attempt at answering. ("online" in this case meaning "with the filesystem mounted",  not "on the web"). – derobert Apr 2 '14 at 17:01