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I am using Linux as guest OS in VirtualBox. I deleted huge number of files from its filesystem. Now i want to shrink the filesystem image file (vdi). The shrinking works by compressing filesystem image wherever it has "null" value in disk.

It seems an application called zerofree can write "null" into free space of filesystem in such a way that it becomes sparse. But the instructions say it works only on ext2/ext3. I have ext4 on my guest OS.

  1. Why won't it work on ext 4 (reason cited is "extents", but can someone shed more light on it) ?

  2. Will it work, If i mount the ext 4 as ext 3 and then remount as ext 4 ?

  3. Any other tools that can do similiar thing as zerofree on ext ?

rajaganesh87
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2 Answers2

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The page you reference (http://intgat.tigress.co.uk/rmy/uml/index.html) states:

The utility also works on ext3 or ext4 filesystems.

So I'm not sure where you're getting that it doesn't work on ext4 filesystems.

Note that the zerofree utility is different from the zerofree kernel patch that is mentioned on the same page (which indeed does not seem to have a version for ext4).

Update: At least in the case of VirtualBox, I don't think you need this utility at all. In my testing, on a stock Ubuntu 10.04 install on ext4, you can just zero out the filesystem like so:

$ dd if=/dev/zero of=test.file

...wait for the virtual disk to fill, then

$ rm test.file

and shut the VM down. Then on your VirtualBox host do:

$ VBoxManage modifyhd --compact yourImage.vdi

and you'll recover all the unused space.

pdo
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  • Thanks for your reply. The manpage of zerofree did not specify ext4, thats why i needed an alternative. – rajaganesh87 Apr 14 '11 at 08:37
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    The alternative proposed (dd if=/dev/zero of=test.file) worked for me! Much simpler than other solutions. – Stefano Nov 05 '11 at 14:58
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    This should be the accepted answer. Works beautifully. – Chris Pratt Jun 06 '12 at 18:19
  • Do a sync after the dd. – ignis Jan 28 '13 at 09:47
  • dd idea worked for me too. Nicely done! – Stefan Apr 15 '14 at 14:53
  • Would you do this for ZFS? It seems to keep going forever. – CMCDragonkai Jul 07 '14 at 05:39
  • @CMCDragonkai If you've got compression or de-duplication turned on in ZFS filling the disk up with zeroes will likely never complete. Otherwise, it should work.

    If you've got compression turned on, you'll have to use incompressible data to fill the disk, change if=/dev/zero to if=/dev/random (or other randomness device as appropriate for your system). I'm not sure this will work for the purposes of shrinking the disk image afterwards though. I'll have to test it and get back to you.

     – pdo Jul 07 '14 at 20:28
  • That makes sense using /dev/random. But I wonder if it does reclaim the disk space from deleted stuff. – CMCDragonkai Jul 07 '14 at 22:53
  • @pdo wouldn't make more sense to just add "1" to the file instead of calling random all the time? Would this be faster? – CMCDragonkai Jul 07 '14 at 22:56
  • @CMCDragonkai you could always just turn compression off, fill the disk with zeroes, delete and shrink, then turn compression back on :) – pdo Jul 08 '14 at 13:22
  • Yea I was thinking about that. I wonder if it will work? I'll try and report back. – CMCDragonkai Jul 09 '14 at 07:30
  • I tried this on my Lubuntu virtual machine which has 2 partitions. It only filled the sda partition with zeros (although that worked great). How do I modify the if= parameter to use a different partition? – Organic Marble Apr 28 '15 at 02:18
  • @OrganicMarble you need to change the of= parameter, not the if=. The example of=test.file just means put the output in a file called "test.file" in the current working directory. To zero out a different partition, you'll need to specify the output file in that different partition, for example: of=/path/to/other/partition/test.file – pdo May 06 '15 at 17:50
  • I test this solution for ext4 and xfs partition, and works well. Thanks. – Nabi K.A.Z. Nov 07 '17 at 23:43
  • One advantage of using zerofree over dd is that it only writes zeroes over the non-zero unused sectors of your disk - making it much faster and meaning it won't grow your virtual disk to its maximum size before it can be compacted. – Jacob Hume Jan 24 '20 at 22:17
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You could use e2fsprogs, there is a version that is usable with ext4.

Make yourself superuser:

# su

Unmount your drive:

# umount /dev/sda1

Remove the journal of your drive:

# tune2fs -O ^has_journal /dev/sda1

And then resize your file system:

# resize2fs /dev/sda1 30G