VHD

VHD stands for Virtual Hard Disk. It is a computer file format that represents a virtual disk drive, containing disk sectors, partitions, file systems, and data. It was introduced by Microsoft for its virtualization products, first appearing with Virtual Server 2005 and later used in Hyper-V and Virtual PC.

A VHD can be created as a fixed-size image or a dynamically expanding image that grows as data is written. There is also the differencing disk type used to create child disks that depend on a parent for data. The VHD file is platform-agnostic but optimized for Windows environments.

Limitations: Maximum size 2 TB for the original VHD; VHDX (introduced 2012 with Windows 8/ Server 2012) increases capacity to 64 TB and provides resilience against power loss and better alignment. VHD has some performance and reliability issues with sudden power loss.

Usage: Primary use is virtualization: in Hyper-V, Virtual PC, and Virtual Server. The VHD can host an operating system or data. It can be attached as a drive in Windows by mounting the file, or be converted to other formats such as VMDK (VMware) or VDI (VirtualBox). Tools exist to convert and manage.

Related formats include VHDX, the successor to VHD, and other disk image formats used by virtualization platforms.