Tip: (Better) Shrinking of Ext(2/3) partitions

Discussions about using Linux guests in VirtualBox.

Tip: (Better) Shrinking of Ext(2/3) partitions

Postby the_camel » 11. Aug 2007, 13:09


Well I had the problem that I needed to shrink some VDIs, but the partition they were on had insufficient space to use the "dd -i /dev/null" trick.

I went to look for a solution and found this page Keeping filesystem images sparse whose author had the same problem, but solved it in a way which works equally well and does not blow up the partition to its maximum size:

He wrote a small programm zerofree.c which uses the ext2fs library to get a list of all the blocks, checks if the current block is free but non-zero (i.e. has been used at some point) and fills it with zeroes again. This allows the vboxmanage tool to compact the VDI again.

I used one little patch on the code:
Code: Select all   Expand viewCollapse view
--- zerofree.c.orig·    2007-08-11 12:47:50.000000000 +0200
+++ zerofree.c· 2007-07-22 16:41:23.000000000 +0200
@@ -58,8 +58,8 @@ int main(int argc, char **argv)
 ·      ·       return 1 ;
 ·      }
-·      if ( flags & EXT2_MF_MOUNTED ) {
-·      ·       fprintf(stderr, "%s: filesystem %s is mounted\n",
+·      if ( ( flags & EXT2_MF_MOUNTED ) && ( ! (flags & EXT2_MF_READONLY) ) ) {
+·      ·       fprintf(stderr, "%s: filesystem %s is mounted rw\n",
 ·      ·       ·       ·       ·       argv[0], argv[optind]) ;
 ·      ·       return 1 ;
 ·      }

This patch allows you to shrink a filesystem while it is mounted, but only if it is mounted read only. (Which helps, since you can keep the zerofree binary on the to-be-compacted partition, and do not have to move it around.)

Another Tip: You might want to copy your VDI as a back-up :-)

To use it (in your VM)
1) Compile the (patched or unpachted) zerofree.c
2) Go to single user mode (i.e. "init 1")
3) Remount all your partitions read only (i.e. "mount -n -o remount,ro -t ext2 /dev/XXX /MOUNTPOINT")
3.1)The "-n" is so that mount does not try to update "/etc/mtab" which might be on the now read only disk
3.2)The "-t ext2" is because you really do not need journaling for that :-)
4)Make sure the filesystem is ok now (i.e. "fsck.ext2 -f /dev/XXX")
5)Call zerofree (i.e. "zerofree /dev/XXX")
6)Check if we did not break anything ("fsck.ext2 -f /dev/XXX")
7)Remount all your partitions back to normal (i.e. "mount -t ext3 -o remount,rw /dev/XXX /MOUNTPOINT")
8)Shutdown the VM

And last but not least, call vboxmanage to compact the VDI

Enjoy ;-)

- Camel

P.S. Dear Virtualbox Admins:
I tried to register once before, but the mailaccount I created was not ready by the time the confirmation mail was sent. I tried to contact you to activate the account, but never received an answer. Please delete the account "camel", since it is (as you can see) not longer needed.
Posts: 4
Joined: 11. Aug 2007, 12:36

New Version of Zerofree

Postby the_camel » 12. Aug 2007, 12:19

Ron Yorston, the author of zerofree posted a new version on his website, including the above patch and a makefile :-)

It can be found at: http://intgat.tigress.co.uk/rmy/uml/zerofree-1.0.1.tgz
Posts: 4
Joined: 11. Aug 2007, 12:36

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