The hard disk image types that are available when you use the Virtual Disk Wizard

This article discusses the following hard disk image types:

• Dynamically expanding disk image.
• Fixed-size disk image.
• Differencing disk image.
• Disk image that is linked to a host volume.
• Disk image that is linked to a host physical drive.

Microsoft Virtual PC for Windows supports these hard disk image types. These hard disk image types are available when you use the Virtual Disk Wizard to create a new hard disk image.

Dynamically expanding disk image

The dynamically expanding disk image is the most frequently used type of disk image in Virtual PC. By default, the PC Setup Wizard creates disk images of this type. Dynamically expanding disk images are similar to fixed-size disk images. But unlike fixed-size disk images, the hard disk file does not include free space on the hard disk.

For example, if you create a 1 gigabyte (GB) dynamically expanding disk image, the initial file is only about 3 megabytes (MB). As you write more information to the disk image, the image grows to contain the new data.

Although dynamic growth is convenient, Virtual PC may not be able to expand the drive because of host hard disk size constraints. Virtual PC tries to monitor the available space on the hard disk of your host PC and warns you if the dynamically expanding drive approaches the limits of the available space that remains on the host volume.

Fixed-size disk image

The fixed-size disk image represents the whole virtual hard disk in a single disk image file. Typically, these files are large because all the space is already allocated. For example, if you create a fixed-size disk image that represents a 1 GB hard disk, the file is 1 GB. Although conceptually simple, fixed-size disk images use a lot of resources. They offer a small performance advantage over other types of hard disk images.

Differencing disk image

You can use a differencing disk image with one of the other types of disk images. The disk image that is associated with the differencing disk image is known as its parent disk image. The differencing disk image (child image) contains only the changes that are made to the parent disk image. The child image is unusable without the parent disk image. The differencing file is similar to a dynamically expanding disk image file because it starts small and grows to contain new data. However, data is added only to the differencing drive image when you modify the drive data. Therefore, the contents of a differencing drive represent only the changes from the original parent disk image. There are several scenarios where this is useful:

• Several Virtual PC users want to share the same base disk image that is located on a network file server. In this case, each user creates a differencing disk image on his or her local hard disk. Any modifications that the user makes to the parent disk image are written to the local differencing disk image. As a result, the parent disk image remains unmodified.
• You want to configure a single guest operating system in several ways. You can duplicate the original hard disk image, but this requires substantially more hard disk storage space. Instead, you can create two differencing disk images that each have the same parent.

Warning Microsoft strongly recommends that you write-protect or lock the parent disk image. If a parent disk image is modified after a child image is created and linked to the parent image, the child image becomes unusable, and you cannot recover any part of the data in the child image.

Disk image that is linked to a host volume

You can link a disk image to a volume on the host operating system. In this case, the disk image is a small placeholder that refers to a partition on the host PC. If the associated volume is mounted by the host operating system, Virtual PC can only read from it. If the volume is not mounted by the host operating system, you can permit writing to the volume. With this type of disk image, Virtual PC creates a virtual boot partition and partition map that contains one entry. The virtual boot partition is saved to the disk image file. Therefore, changes to the sector persist. All other read and write requests are forwarded directly to the host volume.

Note This option is not available on Windows 98 Second Edition and Windows Millennium Edition host PCs.

Disk image that is linked to a host physical drive

This type of disk image is similar to an image that is linked to a host PC volume except that it represents the whole host PC's hard disk, potentially with multiple volumes. With this type of disk image, Virtual PC uses the real boot partition and partition map from the host PC's hard disk. All read and write requests are forwarded directly to the host PC's hard disk.

Note This option is not available on Windows 98 Second Edition and Windows Millennium Edition host PCs.