Virtual Hard Disk Image Format Specification

Published: Februrary 27, 2009

	Introduction
	About the VHD Format
	Potential Uses of the VHD Format
	VHD Specification Details
	Specification Outline
	VHD Specification Download

Introduction

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 2007 and Microsoft Virtual Server 2005 R2 and Hyper-V, 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.

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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 2007, Virtual Server 2005 R2 and Hyper-V and the format will be used by future versions 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.

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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
  

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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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Specification Outline

The following is the outline of the information contained in the complete VHD Image Format specification:

• Introduction
  
• Overview of Virtual Hard Disk Image Types
  
• Hard Disk Foot Format
  
• Dynamic Disk Header Format
  
• Block Allocation Table and Data Blocks
  
• Implementing a Dynamic Disk
  
• Mapping a Disk Sector to a Sector in the Block
  
• Splitting Hard Disk Images
  
• Implementing a Differencing Hard Disk
  
• CHS Calculation
  

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VHD Specification Download

As of Tuesday, October 17th 2006, Microsoft is providing access to the VHD Image Format Specification Document as a part of the Open Specification Promise (OSP). The OSP provides broad use of Microsoft patented technology necessary to implement a list of covered specifications. The goal of the OSP is to provide our customers and partners with additional options for implementing interoperable solutions. Please reference the OSP Website for complete details.

Register and Download the Specifications

Registration is optional, and enables you to stay up to date with the latest updates to the specifications. The process takes only a few moments and allows Microsoft to provide you with the latest resources relevant to your interests.

Download the Specifications Without Registering

For More Information

For general questions, or if you have licensed access to the VHD Image Format Specification prior to October 6th, 2006 and would like to understand how the OSP applies to you, please contact vhdinfo@microsoft.com

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