Changes

* '''Differencing''' - Differencing is a concept which involves a parent disk and a child disk. The parent disk contains a baseline hard disk image with the operating system and perhaps a range of applications and data installed. Once this baseline parent disk is configured, a differencing disk is assigned as a child. When the virtual machine to which the disk is assigned runs, any changes made the baseline parent image are actually stored on the child differencing disk, leaving the parent disk unaltered. Multiple child disks may be assigned to a parent disk allowing multiple virtual machines to share the same parent disk. Differencing disks may also be chained together. The changes contained in a differencing disk may be merged into the parent disk. Alternatively, a new virtual hard disk may be created containing the merger of the parent disk and the child disk, thereby leaving the parent disk unaltered. When creating a differencing disk, the parent disk will need to be specified. The size of the differencing disk increases as space is required. Differencing disks may also be compacted to free up unused space (see the section below entitled Modifying Virtual Hard Disks).