VHD (file format)

VHD (Virtual Hard Disk) and its successor VHDX are file formats representing a virtual hard disk drive (HDD). They may contain what is found on a physical HDD, such as disk partitions and a file system, which in turn can contain files and folders. They are typically used as the hard disk of a virtual machine, are built into modern versions of Windows, and are the native file format for Microsoft's hypervisor (virtual machine system), Hyper-V.

The format was created by Connectix for their Virtual PC product, known as Microsoft Virtual PC since Microsoft acquired Connectix in 2003. VHDX was introduced in Windows 8/Windows Server 2012 to add features and flexibility missing in VHD that had become apparent over time.

Since June 2005, Microsoft has made the VHD and VHDX Image Format Specifications available to third parties under the Microsoft Open Specification Promise.

Features

A Virtual Hard Disk allows multiple operating systems to reside on a single host machine. This method enables developers to test software on different operating systems without the cost or hassle of installing a second hard disk or partitioning a single hard disk into multiple volumes. 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 vice versa)

Life-cycle management and provisioning (re)

VHDX was added in Hyper-V in Windows Server 2012 to add larger storage capacity, data corruption protection, and optimizations to prevent performance degradation on large-sector physical disks.

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.