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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)

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Check this out: superuser.com/questions/19326/… – Mat Jul 29 '12 at 10:18
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
up vote 42 down vote accepted

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...

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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
@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
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 at 10:10

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:

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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

Summary of the methods (as mentioned in this question and elsewhere) to clear unused space on ext2/ext3/ext4:

  • If the "disk" your filesystem is on is thin provisioned (e.g. a modern SSD supporting TRIM, a VM file whose format supports sparseness etc.) and your kernel says the block device understands it, you can use e2fsck -E discard src_fs to efficiently zero unused space (requires e2fsprogs 1.42.2 or higher).
  • Using zerofree to explicitly write zeros over unused blocks.
  • Using e2image -rap src_fs dest_fs to only copy blocks in use (new filesystem should be on an otherwise zero'd "disk", requires e2fsprogs 1.42.9 or higher).
  • Using cat /dev/zero > /mnt/fs/zeros; sync; rm /mnt/fs/zeros (sfill from secure-delete uses this technique). This method is inefficient, not recommended by Ted Ts'o (author of ext4), may not zero certain things and can slow down future fsck's.

Once one of the above has been done a disk image can be made sparse by using one of

  • cp --sparse=always src_image dst_image
  • fallocate -d src_image (requires util-linux v2.25 or higher).


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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.

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

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If you want to comment on cas answer, wait until you have enough reputation to do so. – Anthon Apr 2 '14 at 15:01
This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post - you can always comment on your own posts, and once you have sufficient reputation you will be able to comment on any post. – Anthon Apr 2 '14 at 15:01
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

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