VHD (file format)
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Virtual Hard Disk

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

A Virtual Hard Disk (VHD) is a file format containing the complete contents and structure representing a Hard Disk Drive, and is used to store virtual operating systems and their associated programs in a single file by various virtualization programs or a virtual machine.¹The format was created by Connectix which was later acquired by Microsoft for Virtual PC. Since June 2005 Microsoft has made the VHD Image Format Specification available to third parties under the Microsoft Open Specification Promise.

Uses

Virtual Hard Disks allow multiple operating systems to reside on a single host machine. This enables developers to test software on different operating systems without the cost or hassle of actual hardware. 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

Software using the VHD file format

The format is used for Microsoft Virtual PC and was also 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.²

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.

Windows 7 and Windows Server 2008 R2 will include support for creating, mounting, and booting from VHD files.³

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.

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