How to compact VirtualBox's VDI file size?

I've a VirtualBox VM which configured a very large hard disk size (bigger than host). By my mistake, a program on the VM generated lots of log files and the VDI file size keeps growing until there is no space on the host.

Now I've deleted the log files but the VDI file size are not getting smaller after using VBoxManage.exe modifyhd "C:\Virts\mybox-i386.vdi" compact

Is there a way to really compact the VDI file size? Thanks!

Answer

You have to do the following steps:

1. Run defrag in the guest (Windows only)


2. 
   

   Nullify free space:

   
   

   With a Linux Guest run this:

   
   

   dd if=/dev/zero of=/var/tmp/bigemptyfile bs=4096k ; rm /var/tmp/bigemptyfile
   

   
   

   Or:

   
   

   telinit 1
   mount -o remount,ro /dev/sda1
   zerofree -v /dev/sda1
   

   
   

   With a Windows Guest, download SDelete from Sysinternals and run this:

   
   

   sdelete.exe c: -z
   

   
   

   (replace C: with the drive letter of the VDI)

   
   


3. 
   

   Shutdown the guest VM

   
   


4. 
   

   Now run VBoxManage's modifymedium command with the --compact option:

   
   

   With a Linux Host run this:

   
   

   vboxmanage modifymedium --compact /path/to/thedisk.vdi
   

   
   

   With a Windows Host run this:

   
   

   VBoxManage.exe modifymedium --compact c:\path\to\thedisk.vdi
   

   
   

   With a Mac Host run this:

   
   

   VBoxManage modifymedium --compact /path/to/thedisk.vdi
   

This reduces the vdi size.