.VHD

Introduction

VHD is a file that acts as the hard disk of a virtual machine. It was created by Connectix for Virtual PC, which Microsoft bought in 2003, and Microsoft later published the specification. VHD files are used by Hyper-V and Azure, and Windows can attach one as an ordinary drive.

A VHD can be at most about 2 TB. Its successor, VHDX, lifts that limit and is more resilient to power failures, but VHD remains widely supported.

Software

• Hyper-V, built into Windows and Windows Server
• Windows Disk Management, which attaches VHD files as drives
• VirtualBox (Windows, macOS, Linux)
• QEMU and qemu-img, which run and convert VHD files (Windows, macOS, Linux)
• 7-Zip, which opens VHD files like an archive (Windows)

Structure

Every VHD ends with a 512-byte footer that starts with the cookie conectix. Its fields are big-endian and include the creator application, the size of the disk, its geometry, a checksum, a unique ID and the disk type: 2 for a fixed disk, 3 for a dynamic disk and 4 for a differencing disk.

A fixed disk is simple: the raw contents of the disk, sector by sector, followed by the footer. A dynamic disk only stores the parts that have been written. It starts with a copy of the footer, then a 1024-byte dynamic disk header beginning cxsparse, then a block allocation table that maps each block of the disk, 2 MB by default, to its place in the file. Each stored block begins with a bitmap of which sectors in it hold data. A differencing disk has the same layout and records only the changes from a parent disk.