To help reduce excess disk usage, VirtualBox provides a mechanism for compacting dynamically allocated guest images. Below are the steps to follow if your guest operating system is Linux:

1. Start the Linux virtual machine;
2. Clean the free space on the disk of the Linux virtual machine;
3. Shutdown the Linux virtual machine;
4. Use the VirtualBox VBoxManage utility to compact the Linux guest image.

The most effective way to clean free disk space on a Linux drive is to use the Linux dd utility which is a bit-stream duplicator. Open up a terminal window and type the following command:

dd if=/dev/zero of=zerofillfile bs=1M

This command will zero-fill any free disk space on the virtual Linux drive.

• if= specifies the input file;
• /dev/zero indicates a bit-stream of zeros
• of= specifies the output file
• zerofillfile name of the file containing the bit-stream of zeros
• bs= indicates the block size
• 1M indicates that the block size will be 1 megabyte

You can now remove zerofillfile using the Linux rm utility:

rm zerofillfile

Step 3: Shutdown the Linux Virtual Machine

End your session and shutdown the Linux Virtual Machine.

Step 4: Compact the Windows guest image

Assuming a Windows host, use the following command at the DOS prompt:

"C:\Program  Files\Oracle\VirtualBox\VBoxManage.exe modifyhd --compact "[drive]:\[path_to_image_file]\[name_of_image_file].vdi"