User Guide

Using Virtual Disk Wizard

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warn you if the dynamically expanding drive starts to approach the

limits of available space left on the host volume.

Fixed-size disk image

—The simplest type of hard drive image is a

ﬁxed-size disk image that represents the entire virtual hard drive in a

single disk image ﬁle. All of the space is pre-allocated, so these ﬁles

are typically quite large. For example, if you create a ﬁxed-size hard

disk image that represents a 1-GB hard drive, the resulting ﬁle would

be 1 GB in size. Although conceptually simple, ﬁxed-size hard disk

images are not generally recommended because of their large

resource requirements. They do offer a slight performance advantage

over other types of hard disk images, but this difference is small.

Differencing disk image

—A differencing disk image is used in con-

junction with one of the other types of disk images. The disk image

associated with it is known as its

parent

disk image. The differencing

ﬁle itself is similar to a dynamically expanding disk image ﬁle in that

it starts small and grows to accommodate new data. However, data is

only added to the differencing drive image when drive data is modi-

ﬁed. In other words, contents of a differencing drive represents only

the changes from the original disk image. There are several scenarios

when this is useful:

Several Virtual PC users want to share the same base disk image

located on a network ﬁle server. In this case, each user creates a

differencing disk image on his or her local hard drive. Any

modiﬁcations made to the parent disk image are written to the

local differencing disk image, leaving the parent disk image in a

“pristine” state.

You want to conﬁgure a single guest OS in multiple ways. You

can duplicate the original hard disk image, but this requires

substantially more hard drive storage space. Instead, you can

create two differencing disk images, both with the same parent.

NOTE

Regardless of the scenario, Connectix strongly recommends that

you write-protect or lock the parent disk image. If a parent disk image is

modiﬁed, all differencing disk images related to it become invalid, and

any data written to them is effectively lost.