SUMMARY
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.
MORE INFORMATION
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.