In a recent project, I helped build a selfcontained
virtualized infrastructure
based on a couple of virtual machines
(VMs) with AD, DHCP, DNS, RIS, and WINS
services running on them. The team started
with Microsoft Virtual PC 2007 and installed
the latest Virtual Machine Additions. (To learn
about the often-overlooked task of installing
Virtual Machine Additions, see the Web-exclusive
sidebar “Essential Virtual Machine Additions,
InstantDoc ID 97288.) After placing the
solution in production for a while, we saw that
the VM infrastructure just couldn’t cope with
the load that our project demanded. To solve
the problem, we ultimately installed Microsoft
Virtual Server 2005 SP1, and ported the VMs
over by making the necessary modifications
and attaching the VHDs to SCSI adapters.
Virtual PC is an excellent, inexpensive tool
for building precinct environments, whether
for Help desk support or for testing security
patches before production rollout. But as your
needs grow, you might encounter similar
limitations in performance and scalability. Or
perhaps you’ve already built a core library of
VMs and would like to reuse them in a secured
server-farm environment.
Virtual Server might just be the answer
you’re seeking. Microsoft’s well-designed virtualization
architecture permits an almost seamless
integration and interoperability between
Virtual PC and Virtual Server. After a couple of
free downloads, and a little tinkering, you’ll be
able to tackle any virtual environment.
VM Architecture
For information about how to download both
Virtual Server and Virtual PC for free, see the
Learning Path. Before you start your download,
however, you need to understand the
subtle differences between Virtual PC 2007
and Virtual Server 2005 to help make porting
VMs between the two platforms a little less
painful. For the purpose of this article, all
references are to Virtual Server 2005 R2 SP1
Enterprise Edition and
Virtual PC 2007. Discussions
about Virtual PC
2007 also apply to Virtual
PC 2004 SP1, unless
otherwise stated.
The design philosophies
of Virtual PC and
Virtual Server differ in
their purpose and market
segments. Put simply,
Virtual PC targets
the average desktop user, focusing on tight
host-guest integration, ease of use, and a rich
user experience. Virtual Server, by contrast, is
aimed toward the enterprise-server sector, in
which buzzwords such as manageability, scalability,
and security are paramount.
Even considering this market distinction,
you’ll find that Virtual PC and Virtual Server
share a fundamental set of core features. The
key enabler that facilitates VM porting between
the platforms is the common file architecture:
the virtual machine configuration (VMC) file
and the virtual hard disk (VHD). The VMC
file is an XML configuration file that contains
metadata describing the VM. The actual disk
storage as seen by the VM is enclosed in one
or more VHDs.
The system automatically creates virtual
machine saved state (VSV) files and undo disk
(VUD) files, if enabled, in the same folder in
which the VM is defined. The VSV file stores
the suspended state of a running VM for
restoration at a later time. The system writes
changes made while a VM is in use to the VUD
file, where rollback to a good known state can
be accomplished by discarding the changes
instead of committing them to disk. VSV files
are incompatible between the two platforms.
Hence, to avoid potential problems, modifications
should always be flushed to disk and the
VM properly shut down before moving VMs
between Virtual PC and Virtual Server. The
virtual network configuration (VNC) file is
unique to Virtual Server and isn’t used under
Virtual PC.
The Basics
Moving a Virtual PC–built VM to Virtual Server
is as simple as accessing Virtual Server’s administration
Web site and specifying the fully
qualified path of the VMC file under Virtual
Machines, Add. I do recommend that you first
use the Virtual Disk, Inspect option. Doing so helps to verify the integrity of the VHD, including
any dependency, such as the case when the
VHD is set up as a differencing disk linked to its
parent, as Figure 1 shows.
You might have a need to port a VM in the
reverse direction, from Virtual Server to Virtual
PC—for example, when the target machine
doesn’t have Virtual Server installed or you
want to enable some Virtual PC–specific features.
To do so, open the Virtual PC console
and start the New Virtual Machine Wizard.
Select Add an existing virtual machine and
point to the fully qualified path of the VMC.
As you can see, deploying an existing VM on
either platform is as straightforward as adding
the VM itself. Nevertheless, a few device types
and functionalities will behave differently due
to architectural variation between the products Web Table 1 (InstantDoc
ID 97084) outlines some
potential interoperability
problems.
Networking
Details
Both Virtual PC and Virtual
Server support a
maximum of four virtual
network adapters per VM.
Although the internal
VMC file structure is common,
networking is one
aspect that differs slightly
on the two platforms. Virtual
PC stores network-configuration information
directly in the VMC file with the same
name as the VM, under the ethernet_adapter
section, which Figure 2 shows.
The actual description of host network
adapters available to a user resides in the
options.xml file, which resides at %APPDATA% Microsoft\Virtual PC. Because Virtual
PC has no GUI front end for changing the
network adapter name, you’ll have to modify
<name type="string">Manufacturer Network
Adapter Name</name> in the <virtual_network
id="n"> section. Remember to make a
backup copy of the file before editing it.
Virtual Server stores network settings in
separate XML configuration files with a VNC
extension. You’ll find them under %ALLUSERSPROFILE% Documents\Shared Virtual Networks, and only local administrators have
access to this folder in a standard installation.
By default, the system creates a number of
files that match the physical network adapters
present on the host. Suppose you have two
internal network adapters, one wired and the
other wireless. Virtual Server will create two
files named External Network (manufacturer
name and wired NIC model).vnc and External
Network (manufacturer name and wireless NIC
model).vnc. Additionally, Internal Network
.vnc is also automatically created to facilitate
VM-to-VM communication only.
Virtual Server lets you create an infinite
number of virtual networks, each with a fully
customizable virtual DHCP Server, as you see in
Figure 3. There’s also no limit to the number of
VMs that can connect to each virtual network.
By separating VNC files that describe various
common network settings, you achieve isolation
without tying a physical network adapter
to a specific VM or user.
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