# Virtual Hard Disk

> file format

**Wikidata**: [Q1326659](https://www.wikidata.org/wiki/Q1326659)  
**Wikipedia**: [English](https://en.wikipedia.org/wiki/VHD_(file_format))  
**Source**: https://4ort.xyz/entity/virtual-hard-disk

## Summary
Virtual Hard Disk (VHD) is a disk image file format that represents a virtual hard disk drive. Developed by Connectix and later Microsoft, it encapsulates the entire contents and structure of a disk volume, including its partitions and file systems, into a single file. This format is a key component in virtualization software, allowing a virtual machine to store its operating system and data.

## Key Facts
- **Classification:** Virtual Hard Disk is a type of disk image file format.
- **Developers:** The format was developed by Connectix and Microsoft.
- **File Extension:** Files using this format typically have the `.vhd` extension.
- **Successor Format:** VHD was followed by the Virtual Hard Disk v2 (VHDX) file format.
- **File Signature:** VHD files can be identified by the hexadecimal value `636F6E6563746978` at the beginning of the file.
- **Media Types:** Associated media types include `application/x-vhd`, `application/x-vhd-disk`, and `application/x-virtualbox-vhd`.
- **Aliases:** The format is also known as VHD, VHD-Format, and .vhd.

## FAQs
### Q: What is a VHD file?
A: A VHD file is a disk image that acts as a virtual hard drive for a virtual machine. It contains all the elements of a physical hard drive, such as partitions, a file system, and data, all stored within a single, portable file.

### Q: Who created the Virtual Hard Disk format?
A: The Virtual Hard Disk format was originally created by the company Connectix. The technology was later acquired and adopted by Microsoft for its virtualization products.

### Q: What is the difference between VHD and VHDX?
A: VHDX, or Virtual Hard Disk v2, is the successor to the VHD file format. It was developed to offer greater capacity, performance, and reliability compared to the original VHD format.

## Why It Matters
The Virtual Hard Disk (VHD) format is significant because it was a foundational technology for the widespread adoption of PC virtualization. By encapsulating an entire hard drive—including the operating system, applications, and user data—into a single portable file, VHD simplified the management, backup, and deployment of virtual machines.

This innovation allowed IT professionals and developers to easily create, copy, and move complete computing environments between physical hosts. It abstracted the software from the underlying hardware, providing unprecedented flexibility. The format became a core component of Microsoft's virtualization strategy, powering products like Virtual PC and Hyper-V. The principles established by the VHD format paved the way for more advanced virtualization and cloud computing technologies, changing how servers are deployed and software is developed and tested.

## Notable For
- **Connectix Origin:** The format's file identification signature (`636F6E6563746978` in hexadecimal) spells "conectix," a direct reference to its original developer, Connectix, before the technology was acquired by Microsoft.
- **Single-File Encapsulation:** VHD is notable for packaging the entire structure and content of a hard disk volume into one file, making virtual machine disks portable and easy to manage.
- **Foundation for Microsoft Virtualization:** The format became an integral part of Microsoft's virtualization products, including Microsoft Virtual PC and Hyper-V, establishing it as a standard in Windows environments.
- **Evolution to VHDX:** The VHD format laid the groundwork for its more robust and feature-rich successor, VHDX (Virtual Hard Disk v2), which addressed the limitations of the original format.

## Body
### Definition and Purpose
Virtual Hard Disk (VHD) is a file format that functions as a disk image. Its primary purpose is to represent a complete, virtualized hard disk drive in a single file. This file contains all the structural components of a physical disk, such as partitions and file systems, along with the data stored on them.

### Development and Ownership
The VHD format was originally created by Connectix. Later, the technology was acquired by Microsoft, which continued its development and integrated it into its own product lines.

### Technical Specifications
- **File Extension:** The standard file extension for this format is `.vhd`.
- **Media Types:** Registered MIME types for identifying VHD files include `application/x-vhd`, `application/x-vhd-disk`, and `application/x-virtualbox-vhd`.
- **File Identification:** A VHD file can be programmatically identified by a unique signature at the very beginning of the file (offset 0). This signature is the hexadecimal value `636F6E6563746978`.

### Evolution
The VHD format was eventually succeeded by an updated version, the Virtual Hard Disk v2 file format, commonly known as VHDX.

## References

1. Freebase Data Dumps. 2013
2. [TrID](https://mark0.net/soft-tridscan-e.html)
3. [Source](https://gitlab.freedesktop.org/xdg/shared-mime-info/-/blob/2.3/data/freedesktop.org.xml.in#L8031-8041)
4. Gary Kessler's File Signature Table