Using Windows 7 Virtualization

Windows 7 includes a number of new virtualization features that have not previously been available in the desktop version of Windows-and in fact have only recently been available in the Server editions aimed at large corporate data centers. The two primary features in this are Virtual Hard Disks, used in creating virtual systems, and the new Windows XP virtualization feature, called Windows XP Mode.

Virtual Hard Disks

A VHD is a single file that contains the complete contents and structure that represents a hard disk drive. VHDs are frequently used to store multiple copies of operating systems, their associated programs, and even local copies of data in a single file that can be used by various virtualization programs. A VHD lets you move back and forth between operating systems, depending on the environment in which you want to run.

Virtual hard disks are different from logical hard disks, which you may have run into in the past. A logical hard disk is a single physical hard disk that has been divided into two or more "logical" drives, each of which is assigned its own drive letter. You might do this when you have a single big drive and want to keep the contents of the logical drives separate, such as when you're sharing the contents to a group of users over a network or if you want to keep one logical drive (the C: drive) for the operating system and applications, and another (the D: drive, in this example) for your working files. Properly speaking, you always have a logical drive on your hard disk-usually, the physical drive is configured with just one logical drive (the C: drive) so you don't see it split in two.

A virtual hard disk, in contrast, resides on the same logical drive as any other file. It is, however, a big file that contains an image of the contents of an entire hard disk, including the operating system. Because it is a single file, you can do a number of things easily with the virtual hard disk that would be much more difficult to do with, for instance, a logical hard disk.

Virtual hard disks are an easy way to deploy virtual versions of a system. After you have created a single instance of a VHD, you can copy that file, easily duplicating your virtual computer system. Because the information is still stored in a single file, you can also easily back up the entire system image by copying the entire file to a safe location.

The VHD Image file format is currently supported by a number of virtualization software packages, including the following:

• Microsoft Windows Hyper-V (on Windows Server 2008)
• Sun Microsystems VirtualBox
• VMWare ESX Server
• Citrix XenServer Hypervisor
• Microsoft Windows 7