VirtualBox allows you to run virtual machines on your computer that share the computer's hardware resources with the host operating system. VirtualBox also provides the ability to use dynamically growing storage, which allows you to create drives that are (virtually) the same fixed size inside, but the physical image files themselves are only a few megabytes in size when created and grow depending on the amount of data loaded. This is unfortunately not true backwards, so if you delete something from the virtual machine, for example, the size of the physical image file on the virtual storage device will unfortunately not be reduced. In this description, we will look at how to manually shrink the dynamically growing image files of our VirtualBox virtual machines to the size actually used on Linux and Windows systems.

Unfortunately, the more common compression programs work on only one thread, so they can't take advantage of today's modern, multi-core processors. In this description, we will learn about the pigz program, which can be used to run the compression process on multiple threads, thereby taking advantage of all the processor cores.