Booting From Virtual Disks

Virtualization has become one of the biggest and most useful technologies in

the tech industry today. Most big companies are investing heavily in this area

and giving newer and better features. Microsoft has a number of products in this

as well — from the consumer level Virtual PC product (with a new version for

Windows 7) to the enterprise class Hyper-V product range. Windows 7 brings in

one more very useful feature out of the box in this space.

The Boot-to-VHD feature is the ability of Windows 7 to directly boot into an

operating system that is not sitting on your hard disk — but inside a file on

your hard disk. Think of this as a dual or multi-boot scenario without requiring

you to re-partition hard disks. By simply installing the OS and apps of your

choice inside a file, you not only get the ability to boot your machine into

that OS, but also allow for extreme portability and backup. All you need to do

is copy the file over to a new machine and you're up and running with the new OS

in seconds!

This feature is built-in to all the “business” editions of Windows 7, namely

Professional, Enterprise and Ultimate. The file format is the standard virtual

hard disk format - VHD — used by Virtual PC and Hyper-V. The Windows 7 Disk

Management UI also recognizes the VHD file as a native hard disk. You can create

a new VHD or attach an existing VHD into the system. As soon as you do, the disk

is recognized as a normal volume on your system and you can use it like any

other drive.

One of the most useful features of Windows 7 is that it lets you create or mount a VHD like a regular hard disk.	The system uses the native hardware even when virtualized, which allows for a bare-metal performance.

But the great part comes when you can install an OS into it and boot off it

natively. For this, make sure you have enough space on the drive where you are

going to create the VH file. Remember that the VHD has to hold the complete OS +

any apps you want to install. Once you have determined the size you wish for

your VHD, open up Command Prompt as Administrator on Windows 7 and run DISKPART.

In the DiskPart prompt issue these commands:

CREATE VDISK FILE=D:\VPC\Win2008R2.vhd



MAXIMUM=25000


TYPE=EXPANDABLE


SELECT VDISK FILE= D:\VPC\Win2008R2.vhd


ATTACH VDISK


CREATE PARTITION PRIMARY


ASSIGN LETTER=V


FORMAT QUICK FS=NTFS LABEL=VHD

The Device Manager in the VHD shows the virtual disk driver, but everything else (including the video card) is run natively. .	You can 'mount' virtual hard disks using the Attach command in Disk Management on Windows 7.

They create a new VHD file in D:\VPC with a size of 25 GB. It also formats

the virtual drive with NTFS and attaches it to the system as drive V:. You can

now exit DiskPart and open and explore the drive as with any other drive.

Currently the operating systems that know of booting natively from VHD are

Windows 7 and Windows 2008 R2 only. But it's only a matter of time before you

should be able to boot any OS — such as Linux or Windows XP if you want to.

There are multiple ways of getting the OS onto this drive. The first is to

boot your machine with the Win7/2008R2 DVD and then supply the install option of

VHD with path during install. The easier way is to do it from within an

installed Windows 7 instance. You need to download a small PowerShell script

called Install-WindowsImage.ps1 from http:// code.msdn. microsoft.com/

InstallWindowsImage. Copy the script to the location where the VHD file is

located and mount or drop in the OS DVD. You need to first locate the install

source which is a .WIM file on the DVD. The file is usually named Install.WIM.

Run PowerShell as administrator and issue the following commands (assuming

that the DVD is in D:).

set-ExecutionPolicy RemoteSigned



.\Install-WindowsImage.ps1


-WIM D:\sources\install.wim


The second command will list all the different editions of Windows 7/2008R2 that
are available to install:



Index Image Name


<1> Windows Server 2008 R2 Standard (Full Installation)


<2> Windows Server 2008 R2 Standard (Server Core Installation)


<3> Windows Server 2008 R2 Enterprise (Full Installation)


<4> Windows Server 2008 R2 Enterprise (Server Core Installation)


<5> Windows Server 2008 R2 Datacenter (Full Installation)


<6> Windows Server 2008 R2 Datacenter (Server Core Installation)


<7> Windows Web Server 2008 R2 (Full Installation)


<8> Windows Web Server 2008 R2 (Server Core Installation)

Note the number of editions you wish to install. For instance if you wish to

install Win2008R2 Enterprise, full version, the number is “3”. Now issue the

following command in PowerShell:

.\Install-WindowsImage.ps1 -WIM D:\sources\install.wim -Apply -Index 3

-Destination V:

This will show you a message and then start the install. Expect it to take

about 15 minutes to complete. Once done, you have a VHD file with the OS

installed. But you still need to be able to configure the system to boot with

this new VHD file. To do this, open up Command Prompt as an administrator and

run the following command:

bcdedit /copy {default} /d “Windows 2008 R2 Enterprise VHD”

This command will return a GUID that you will need to use in the following

commands, wherever (GUID) is mentioned:

bcdedit /set {GUID} device vhd=\VPC\Win2008R2.vhd



bcdedit /set {GUID} osdevice vhd=\VPC\Win2008R2.vhd

This will create a new boot entry in boot manager that allows it to boot from

the VHD file. Reboot your machine and select the new entry. You will be taken to

the other OS and will have to finish some final setup steps before entering your

desktop. The great part about booting from a VHD is that you get the full power

of your system and only the hard disk is virtualized. You only lose 3-5% of

performance rather than the 50% or more when running it under VPC. The VHD also

allows you to install 64-bit editions — something that VPC does not allow. And

finally, all the native hardware is recognized and drivers installed for it,

which allows bare-metal performance. The VHD boot also recognizes the “real”

partitions and loads them — so that you still have access to your files in the

real machine. All these features together make the Boot-to-VHD a great addition

in the virtualization family in Windows 7.

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