This specification describes the Microsoft Virtual Hard Disk format, currently used by Microsoft Virtual PC 2004 and Microsoft Virtual Server 2005, including details of how to read and modify the contained data.
Full access to the detailed specification is available by signing the Virtual Hard Disk Image Format Specification License Agreement. For further information on the specification or to request a copy of the licensing agreement, please contact firstname.lastname@example.org.
Since June 2005, Microsoft has made the Virtual Hard Disk (VHD) Image Format Specification available to third parties under a royalty-free license. This specification describes the Microsoft VHD format, currently used by Microsoft Virtual PC 2004 and Microsoft Virtual Server 2005, including details of how to read and modify the contained data. By providing this information, Microsoft seeks to foster innovation by partners and customers and further enhance interoperability, security, and operational efficiency for customers.
About the VHD Format
The Microsoft VHD file format specifies a virtual machine hard disk that can reside on a native host file system encapsulated within a single file. The format is used by Virtual PC 2004 and Virtual Server 2005, and the format will be used by a future version of Microsoft Windows Server that includes hypervisor-based virtualization technology. Beyond that, the VHD format is broadly applicable, because it is agnostic to the virtualization technology, host operating system, or guest operating system with which it is used.
Customers and partners who invest the VHD file format will have a clear path forward to future Windows virtualization technologies. In addition, Microsoft plans to design its systems management tools around the VHD file format for improved patching and manageability.
Potential Uses of the VHD Format
The ability to directly modify a virtual machine’s hard disk from a host server supports many interesting applications that may be of interest to customers. These include:
Moving files between a VHD and the host file system
Backup and recovery
Antivirus and security
Image management and patching
Disk conversion (physical to virtual, and so on)
Life-cycle management and provisioning
VHD Specification Details
Virtual machine hard disks are implemented as files that reside on a native host file system. The document focuses on how to store and modify the data in files on the host file system.
The VHD Image Format Specification does not explain how hard disks interface with a virtual machine, nor does it provide information about ATA (AT Attachment) hard disks or Small Computer System Interface (SCSI) hard disks.
The reader of the VHD Image Format Specification Document should be familiar with virtual machine technology and terminology, such as the terms guest and host as used in the context of virtual machine architectures. The user should also be familiar with hard disk technologies and should understand how data is accessed and laid out on a physical medium.
Microsoft is committed to enhancing and extending the capabilities of this format over time.
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