A dynamically growing Virtual Box virtual hard drive file is capable of growing on demand of the virtual machine up to the maximum size we defined in setting up this file. It does not however free the space of files we deleted in the guest OS.
In case the disk had grown too much we can compact it again, provided the unused space is filled with zeros, and the drive is in VDI format.
For an ext2 to ext 4 filesystem this can be done from the Ubuntu guest with the command line utility zerofree * .
This needs the drive to not be in use, and to be unmounted. We therefore may have to bind it temporarily to another VM we had created for this purpose, or we need to boot an Ubuntu live environment on this VM.
/dev/sdxX with the ext2, ext3 or ext4 formatted partition in question.
On more than one partition we my have to repeat
zerofree for each partition. Keep in mind that by filling with zeros the virtual file will temporarily grow up to it's maximum size.
Once all unused drive space is filled with zeros we then shrink the drive with
VBoxManage modifyhd <name>.vdi --compact
This will considerably decrease the file size of our VDI file.
* In a Windows guest we can replace unused space with the utility