VHD (file format)

Infobox file format
name = Virtual Hard Disk
icon =
logo =
extension = .vhd
mime = ?
type code =
uniform type =
magic = conectix
owner = Connectix / Microsoft
genre = Disk image
container for = Virtual machine disk images
contained by =
extended from =
extended to =
standard = In computing, Virtual Hard Disk (VHD) is a file format that holds virtual operating systems and their associated programs in various Virtualization programs.

The format was created for Virtual PC, but since June 2005, Microsoft has made the VHD Image Format Specification available to third parties under the Microsoft Open Specification Promise.

The VHD format captures the entire virtual machine operating system and the application stack in a single file. [cite web
url = http://technet.microsoft.com/en-us/bb738373.aspx
title = Microsoft Virtual Hard Disk Overview
publisher = Microsoft
date = November 6, 2006]

oftware Using the VHD file format

The format was created for Microsoft Virtual PC.

The format was adopted by XenSource for what is now the Citrix XenServer hypervisor.

The VHD format is used by Microsoft Windows Server 2008 which includes a hypervisor-based virtualization technology called Hyper-V. Hyper-V features 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. [cite web
url = http://technet.microsoft.com/en-us/windowsserver/2008/bb414776.aspx
title = Windows Server 2008 Reviewers Guide
publisher = Microsoft
date = February 4, 2008]

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

The VHD format is also supported by VirtualBox 2 and newer, which can read VHD format files on a wide variety of Operating Systems.

upported 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.

Potential uses

The ability to directly modify a virtual machine’s hard disk from a host server supports many applications, including:
* 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

See also

* Fibre Channel
* Storage area network (SAN)
* Network-attached storage (NAS)
* Redundant array of independent disks (RAID)
* iSCSI
* ATA over Ethernet
* Storage Resource Management (SRM)
* Direct-attached storage (DAS)
* File Area Network
* Massive array of idle disks

References