VHD (file format)

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Virtual Hard Disk
File name extension	.vhd
Internet media type	?
Magic number	conectix
Developed by	Connectix / Microsoft
Type of format	Disk image
Container for	Virtual machine disk images

In computing, Virtual Hard Disk (VHD) is a technology used in Virtual PC 2004 (originally created by Connectix, then bought by Microsoft) and Microsoft's Virtual Server 2005 R2. Since June 2005, Microsoft has made the VHD Image Format Specification available to third parties under the Microsoft Open Specification Promise. For example the format was adopted by XenSource for what is now the Citrix XenServer hypervisor.

The VHD format captures the entire virtual machine operating system and the application stack in a single file.^[1]

The VHD format will be used by a future version of Microsoft Windows Server 2008 that includes a hypervisor-based virtualization technology called Hyper-V. Hyper-V will feature offline VHD manipulation — providing administrators with the ability to securely access files within a VHD without having to instantiate a virtual machine. This provides administrators with granular access to VHDs and the ability to perform some management tasks offline.^[2]

The VHD format is also used by Windows Vista's Complete PC Backup feature found in the Business, Enterprise and Ultimate editions.

[edit] Supported formats

VHDs are implemented as files that reside on the native host file system. The following types of VHD formats are supported by Microsoft Virtual PC and Virtual Server:

• Fixed hard disk image - a file that is allocated to the size of the virtual disk.
• Dynamic hard disk image - a file that at any given time is as large as the actual data written to it, plus the size of the header and footer.
• Differencing hard disk image - a set of modified blocks in comparison to a parent image.

[edit] References