= Virtual Hard Disk (VHD) image format
Analysis of the Virtual PC VHD format

:toc:
:toclevels: 4

:numbered!:
[abstract]
== Summary

A commonly image format used in Microsoft virtualization products is the Virtual
Hard Disk (VHD) Image format. It is both used the store hard disk images and
snapshots. This specification is based on available documentation
and was enhanced by analyzing test data.

This document is intended as a working document of the data format specification
for the libvhdi project.

[preface]
== Document information

[cols="1,5"]
|===
| Author(s): | Joachim Metz <joachim.metz@gmail.com>
| Abstract: | This document contains information about the Virtual Hard Disk (VHD) Image format
| Classification: | Public
| Keywords: | Virtual Hard Disk, VHD, Virtual PC, Virtual Server, Hyper-V Server
|===

[preface]
== License

....
Copyright (C) 2012-2020, Joachim Metz <joachim.metz@gmail.com>.
Permission is granted to copy, distribute and/or modify this document under the
terms of the GNU Free Documentation License, Version 1.3 or any later version
published by the Free Software Foundation; with no Invariant Sections, no
Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included
in the section entitled "GNU Free Documentation License".
....

[preface]
== Revision history

[cols="1,1,1,5",options="header"]
|===
| Version | Author | Date | Comments
| 0.0.1 | J.B. Metz | September 2012 | Initial version
| 0.0.2 | J.B. Metz | September 2012 | Additional information and changes.
| 0.0.3 | J.B. Metz | May 2013 | Small changes.
| 0.0.4 | J.B. Metz | May 2013 | Additional information about undo disks.
| 0.0.5 | J.B. Metz | December 2013 | Additional information about parent name.
| 0.0.6 | J.B. Metz | October 2014 | Switched to asccidoc format.
| 0.0.7 | J.B. Metz | November 2016 | Textual changes.
| 0.0.8 | J.B. Metz | October 2020 | Changes to formatting.
|===

:numbered:
== Overview

A commonly image format used in Microsoft vitualization products is the Virtual
Hard Disk (VHD) Image format. It is both used the store hard disk images and
and snapshots. The format is the main format used by e.g. Virtual PC,
Virtual Server and Hyper-V Server.

Types of images:

* Fixed-size hard disk image
* Dynamic-size (or sparse) hard disk image
* Differential (or differencing) hard disk image

[cols="1,5",options="header"]
|===
| Characteristics | Description
| Byte order | big-endian
| Date and time values | Number of seconds since January 1, 2000 00:00:00 UTC
| Character strings | Unicode strings are stored in UTF-16 big-endian without the byte order mark (BOM).
|===

=== Test versions

The following versions of programs were used to generate test data to that was
used for the information in this document:

* Diskpart on Windows 10 (2004)
* Virtual PC

=== Fixed-size hard disk image

The fixed-size hard disk image consists of:

* data
* footer

=== Dynamic-size (or sparse) hard disk image

The dynamic-size (or sparse) hard disk image consists of:

* copy of footer
* dynamic disk header
* block allocation table
* data in blocks
* footer

=== Differential hard disk image

The differential (or differencing) hard disk image consists of:

* copy of footer
* dynamic disk header
* block allocation table
* data in blocks
* footer

=== Undo disk image

Virtual PC has a feature to create "Undo Disks". This undo disk feature stores
a differential hard disk image in files named something similar like:

....
VirtualPCUndo_<name>_0_0_hhmmssMMDDYYYY.vud
....

Where the date and time seems to be stored in UTC and <name> represents the
name of the parent image.

== Footer

The footer is 512 bytes of size and consists of:

[cols="1,1,1,5",options="header"]
|===
| Offset | Size | Value | Description
| 0 | 8 | "conectix" | Signature (Cookie)
| 8 | 4 | | Features
| 12 | 4 | 0x00010000 | Format version +
Where the upper 16-bit are the major version and the lower 16-bit the minor version
| 16 | 8 | | Next offset +
Offset to the next (metadata) structure. The offset is relative from the start of the file. It should only be set in dynamic and differential disk images. In fixed disk images it should be set to 0xffffffffffffffff (-1).
| 24 | 4 | | Modification time +
Contains the number of seconds since January 1, 2000 00:00:00 UTC
| 28 | 4 | | Creator application
| 32 | 4 | | Creator version +
Where the upper 16-bit are the major version and the lower 16-bit the minor version
| 36 | 4 | | Creator (host) operating system
| 40 | 8 | | Disk size +
Contains the size of the disk in bytes
| 48 | 8 | | Data size +
Contains the size of the data in bytes
| 56 | 4 | | Disk geometry
| 60 | 4 | | Disk type
| 64 | 4 | | Checksum
| 68 | 16 | | Identifier +
Contains a big-endian GUID
| 84 | 1 | | Saved state +
Flag to indicate the image is in saved state.
| 85 | 427 | 0 | Reserved (empty values)
|===

[yellow-background]*Checksum. It is just a one’s complement of the sum of all
the bytes in the footer without the checksum field.*

=== Features

[cols="1,1,5",options="header"]
|===
| Value | Identifier | Description
| 0x00000000 | | No features enabled
| 0x00000001 | | Is temporary disk +
[yellow-background]*Indicates that this disk is a candidate for deletion on shutdown*
| 0x00000002 | | Reserved +
Must always be set to 1
|===

All other bits are also reserved and should be set to 0.

=== Creator application

[cols="1,1,5",options="header"]
|===
| Value | Identifier | Description
| "d2v\x00" | | Disk2vhd
| "qemu" | | Qemu
| "vpc\x20" | | Virtual PC
| "vs\x20\x20" | | Virtual Server
| "win\x20" | | Windows (Disk Management)
|===

=== Creator host operating system

[cols="1,1,5",options="header"]
|===
| Value | Identifier | Description
| "Mac\x20" | | Macintosh
| "Wi2k" | | Windows
|===

=== Disk geometry

The disk geometry is 4 bytes of size and consists of:

[cols="1,1,1,5",options="header"]
|===
| Offset | Size | Value | Description
| 0 | 2 | | Number of cylinders
| 2 | 1 | | Number of heads
| 3 | 1 | | Number of sectors per track (cylinder)
|===

[yellow-background]*When a hard disk is configured as an ATA hard disk, the CHS
values (that is, Cylinder, Heads, Sectors per track) are used by the ATA
controller to determine the size of the disk. When the user creates a hard disk
of a certain size, the size of the hard disk image in the virtual machine is
smaller than that created by the user. This is because CHS value calculated
from the hard disk size is rounded down. The pseudo-code for the algorithm used
to determine the CHS values can be found in the appendix of `[TECHNET]`.*

=== Disk type

[cols="1,1,5",options="header"]
|===
| Value | Identifier | Description
| 0 | | None
| 1 | | [yellow-background]*Reserved (deprecated)*
| 2 | | Fixed hard disk
| 3 | | Dynamic hard disk
| 4 | | Differential hard disk
| 5 | | [yellow-background]*Reserved (deprecated)*
| 6 | | [yellow-background]*Reserved (deprecated)*
|===

== Dynamic disk header

The dynamic disk header is 1024 bytes of size and consists of:

[cols="1,1,1,5",options="header"]
|===
| Offset | Size | Value | Description
| 0 | 8 | "cxsparse" | Signature (Cookie)
| 8 | 8 | | Next offset +
Offset to the next (metadata) structure. The offset is relative from the start of the file. Currently this is unused and should be set to 0xffffffffffffffff (-1).
| 16 | 8 | | Block table offset +
Offset to the block allocation table structure. The offset is relative from the start of the file.
| 24 | 4 | 0x00010000 | Format version +
Where the upper 16-bit are the major version and the lower 16-bit the minor version
| 28 | 4 | | Number of blocks +
Maximum number of block allocation table entries
| 32 | 4 | | Block size +
[yellow-background]*The block size must be a power-of-two multitude of the sector size. The default block size is 4096 x 512-byte sectors (2 MB).* +
[yellow-background]*Does not include the size of the data block bitmap.*
| 36 | 4 | | Checksum
| 40 | 16 | | Parent identifier +
The identifier of the parent. +
Contains a big-endian GUID +
Only used by differential hard disk images.
| 56 | 4 | | Parent modification time +
Contains the number of seconds since January 1, 2000 00:00:00 UTC +
Only used by differential hard disk images.
| 60 | 4 | | Reserved (empty values)
| 64 | 512 | | Parent name +
Contains an UTF-16 big-endian string +
Only used by differential hard disk images.
| 576 | 8 x 24 = 192 | | Array of parent locator entries +
Only used by differential hard disk images.
| 768 | 256 | | Reserved (empty values)
|===

The maximum number of block allocation table entries should match the maximum
possible number of blocks in the disk.

[NOTE]
The parent name can also contain a full path, e.g. in .avhd files. The part
segments are separated by the \ character.

[yellow-background]*Checksum. It is just a one’s complement of the sum of all
the bytes in the footer without the checksum field.*

=== Parent locator entry

The parent locator entry is 24 bytes of size and consists of:

[cols="1,1,1,5",options="header"]
|===
| Offset | Size | Value | Description
| 0 | 4 | | Locator platform code
| 4 | 4 | | [yellow-background]*Platform data space* +
[yellow-background]*This field stores the number of 512-byte sectors needed to store the parent hard disk locator.*
| 8 | 4 | | Locator data size
| 12 | 4 | 0 | Reserved (empty values)
| 16 | 8 | | Locator data offset +
Offset to the locator data. The offset is relative from the start of the file.
|===

==== Locator platform code

[cols="1,1,5",options="header"]
|===
| Value | Identifier | Description
| 0 | | None
| | |
| "Max\x20" | | Mac OS alias stored as a blob
| "MacX" | | A file URL with UTF-8 encoding conforming to RFC 2396.
| | |
| "W2ku" | | Absolute Unicode (UTF-16) pathname on Windows.
| "W2ru" | W2ru | Unicode path (UTF-16) on Windows relative to the differential disk path.
| "Wi2k" | | [yellow-background]*(deprecated)*
| "Wi2r" | Wi2r | [yellow-background]*(deprecated)*
|===

== Block allocation table

The block allocation table is only used in dynamic and differential disk images.

The block allocation table consists of 32-bit entries. The entries represent
the sector number where the data block starts or unused when set to
0xffffffff (-1).

....
if block allocation table entry == 0xffffffff (-1):
	block is sparse or stored in parent
else:
	file offset = ( block allocation table entry x 512 ) + sector bitmap size
....

Unused block in a dynamic disk are sparse and should be filled with zero byte
values. In a differential disk the sparse block is defined in the parent disk
image.

== Data blocks

Data blocks are only used in dynamic and differential disk images.

A data block consists of:

* sector bitmap
* sector data

....
size of bitmap (in bytes) = block size / ( 512 * 8 )
....

The size of the bitmap is rounded up to the next multitude of the sector size.

=== Sector bitmap

In dynamic disk images the sector bitmap indicates which sectors contain data
(bit set to 1) or are sparse (bit set to 0).

In differential disk images the sector bitmap indicates which sectors are stored
within the image (bit set to 1) or in the parent (bit set to 0).

The bitmap is padded to a 512-byte sector boundary.

The bitmap is stored on a per-byte basis with the MSB represents the first bit
in the bitmap.

== Notes

Virtual PC fixed disk image, repeated data after initialization:

....
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
0007fe80  00 00 00 00 00 00 00 00  cc 68 3c e9 11 44 09 00  |.........h<..D..|
0007fe90  e0 99 21 00 00 00 00 00  e0 ef 83 03 00 00 00 00  |..!.............|
0007fea0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000ffe80  00 00 00 00 00 00 00 00  cc 68 3c e9 11 44 09 00  |.........h<..D..|
000ffe90  e0 99 21 00 00 00 00 00  e0 ef 83 03 00 00 00 00  |..!.............|
000ffea0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
....

:numbered!:
[appendix]
== References

`[TECHNET]`

[cols="1,5",options="header"]
|===
| Title: | Virtual Hard Disk Image Format Specification
| Author(s): | Microsoft
| Version: | 1.0
| Date: | October 11, 2006
| URL: | https://www.microsoft.com/en-us/download/details.aspx?id=23850
|===

[appendix]
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violation prior to 30 days after your receipt of the notice.

Termination of your rights under this section does not terminate the licenses
of parties who have received copies or rights from you under this License. If
your rights have been terminated and not permanently reinstated, receipt of a
copy of some or all of the same material does not give you any rights to use it.

=== 10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of the GNU Free
Documentation License from time to time. Such new versions will be similar in
spirit to the present version, but may differ in detail to address new problems
or concerns. See http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number. If the
Document specifies that a particular numbered version of this License "or any
later version" applies to it, you have the option of following the terms and
conditions either of that specified version or of any later version that has
been published (not as a draft) by the Free Software Foundation. If the
Document does not specify a version number of this License, you may choose any
version ever published (not as a draft) by the Free Software Foundation. If the
Document specifies that a proxy can decide which future versions of this
License can be used, that proxy's public statement of acceptance of a version
permanently authorizes you to choose that version for the Document.

=== 11. RELICENSING

"Massive Multiauthor Collaboration Site" (or "MMC Site") means any World Wide
Web server that publishes copyrightable works and also provides prominent
facilities for anybody to edit those works. A public wiki that anybody can edit
is an example of such a server. A "Massive Multiauthor Collaboration" (or
"MMC") contained in the site means any set of copyrightable works thus
published on the MMC site.

"CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0 license
published by Creative Commons Corporation, a not-for-profit corporation with a
principal place of business in San Francisco, California, as well as future
copyleft versions of that license published by that same organization.

"Incorporate" means to publish or republish a Document, in whole or in part, as
part of another Document.

An MMC is "eligible for relicensing" if it is licensed under this License, and
if all works that were first published under this License somewhere other than
this MMC, and subsequently incorporated in whole or in part into the MMC, (1)
had no cover texts or invariant sections, and (2) were thus incorporated prior
to November 1, 2008.

The operator of an MMC Site may republish an MMC contained in the site under
CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC
is eligible for relicensing.

