What's New in Virtual Hard Disks
Updated: October 15, 2010
Applies To: Windows 7, Windows Server 2008 R2
The Microsoft virtual hard disk (VHD) file format specifies a virtual hard disk, which is encapsulated in a single file and is capable of hosting native file systems and supporting standard disk operations. This topic contains an overview of the new functionality that is offered in this version of Windows.
What's new in VHDs?
In Windows 7 and Windows Server 2008 R2, a VHD can be used as the running operating system on designated hardware without any other parent operating system, virtual machine, or hypervisor. You can use the disk management tools (the DiskPart command-line tool and the Disk Management MMC snap-in) to create a VHD file. You can then deploy a Windows 7 image (in .wim format) to the VHD and configure the boot manager for a native or physical boot of the Windows image, which is contained in the VHD. Additionally, specific to Windows Server 2008 R2, you can connect the VHD file to a virtual machine for use with the Hyper-V™ role. Native VHD boot is not designed or intended to replace full-image deployment on all client or server systems. Previous versions of Windows do not support a native VHD boot, and they require a hypervisor and virtual machine to boot from a VHD.
For more information, see the following:
Who will want to use VHDs?
Enterprise environments that already manage and use VHDs for virtual machine deployment will find the most benefit from the new features in this release. Although enterprise environments are moving an increasing number of applications to virtual machines, they still use physical computers to operate a significant part of the data center. For this reason, IT administrators have to maintain two sets of images: one set based on the .wim format for physical computers, and another set based on the VHD format for virtual machines. The VHD format supports physical computers and virtual machines, and it provides flexibility in image deployment and simplifies image management.
An image format that runs on both physical computers and virtual machines also benefits developers and testers. This is because they use virtual machines to test new system and application software, but sometimes they need to run tests on physical computers to access a specific hardware device, like the graphics card, or to get accurate performance profiling. Native VHD boot also enables developers and testers to boot into a Windows 7 image without creating a separate partition on the physical computer for installing Windows.
What are the benefits of the new and changed features?
Native support for VHDs simplifies image management and reduces the number of images that you must catalog and maintain. To create a VHD on Windows Server 2008, you install the Hyper-V server role, create a VHD file, and then start the virtual machine to install Windows from the CD or DVD onto a partition in the VHD.
In Windows 7, native VHD boot allows you to create and modify VHD files without installing the Hyper-V server role. You can attach VHD files by using the disk management tools, and you can service the Windows image inside the VHD. You can use the deployment tools in the Windows Automated Installation Kit (Windows AIK) to apply a Windows image to the VHD and to apply updates to the image in the VHD file.
What are the dependencies?
The steps for deploying a Windows 7 or Windows Server 2008 R2 image to a VHD file depend on the Windows deployment tools. For example, ImageX is one of the tools in the Windows AIK. You use ImageX to capture a Windows operating system partition into a Windows image (.wim) file, and then to apply the .wim file to a file system partition (which may reside inside a VHD file). You must install the latest version of the Windows AIK at Windows Automated Installation Kit for Windows 7 (http://go.microsoft.com/fwlink/?LinkId=136976). The Windows AIK download is an ISO image that you burn to a DVD and then install on your system. After installing the Windows AIK, ImageX is located in the Windows AIK\PE Tools directory.
Native VHD boot also requires the Windows 7 boot environment. The Windows 7 boot environment is initialized during a full operating system installation, and it includes the Windows Boot Manager, Boot Configuration Data (BCD), and other supporting files. For more information about the tools you use, see Appendix: Tools and APIs for Virtual Hard Disks.