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.
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 |
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".
| 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. |
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
| 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). |
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
The dynamic-size (or sparse) hard disk image consists of:
-
copy of footer
-
dynamic disk header
-
block allocation table
-
data in blocks
-
footer
The differential (or differencing) hard disk image consists of:
-
copy of footer
-
dynamic disk header
-
block allocation table
-
data in blocks
-
footer
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.
The footer is 512 bytes of size and consists of:
| Offset | Size | Value | Description |
|---|---|---|---|
0 |
8 |
"conectix" |
Signature (Cookie) |
8 |
4 |
Features |
|
12 |
4 |
0x00010000 |
Format version |
16 |
8 |
Next offset |
|
24 |
4 |
Modification time |
|
28 |
4 |
Creator application |
|
32 |
4 |
Creator version |
|
36 |
4 |
Creator (host) operating system |
|
40 |
8 |
Disk size |
|
48 |
8 |
Data size |
|
56 |
4 |
Disk geometry |
|
60 |
4 |
Disk type |
|
64 |
4 |
Checksum |
|
68 |
16 |
Identifier |
|
84 |
1 |
Saved state |
|
85 |
427 |
0 |
Reserved (empty values) |
Checksum. It is just a one’s complement of the sum of all the bytes in the footer without the checksum field.
| Value | Identifier | Description |
|---|---|---|
0x00000000 |
No features enabled |
|
0x00000001 |
Is temporary disk |
|
0x00000002 |
Reserved |
All other bits are also reserved and should be set to 0.
| Value | Identifier | Description |
|---|---|---|
"d2v\x00" |
Disk2vhd |
|
"qemu" |
Qemu |
|
"vpc\x20" |
Virtual PC |
|
"vs\x20\x20" |
Virtual Server |
|
"win\x20" |
Windows (Disk Management) |
The disk geometry is 4 bytes of size and consists of:
| Offset | Size | Value | Description |
|---|---|---|---|
0 |
2 |
Number of cylinders |
|
2 |
1 |
Number of heads |
|
3 |
1 |
Number of sectors per track (cylinder) |
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].
The dynamic disk header is 1024 bytes of size and consists of:
| Offset | Size | Value | Description |
|---|---|---|---|
0 |
8 |
"cxsparse" |
Signature (Cookie) |
8 |
8 |
Next offset |
|
16 |
8 |
Block table offset |
|
24 |
4 |
0x00010000 |
Format version |
28 |
4 |
Number of blocks |
|
32 |
4 |
Block size |
|
36 |
4 |
Checksum |
|
40 |
16 |
Parent identifier |
|
56 |
4 |
Parent modification time |
|
60 |
4 |
Reserved (empty values) |
|
64 |
512 |
Parent name |
|
576 |
8 x 24 = 192 |
Array of parent locator entries |
|
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. |
Checksum. It is just a one’s complement of the sum of all the bytes in the footer without the checksum field.
The parent locator entry is 24 bytes of size and consists of:
| Offset | Size | Value | Description |
|---|---|---|---|
0 |
4 |
Locator platform code |
|
4 |
4 |
Platform data space |
|
8 |
4 |
Locator data size |
|
12 |
4 |
0 |
Reserved (empty values) |
16 |
8 |
Locator data offset |
| 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" |
(deprecated) |
|
"Wi2r" |
Wi2r |
(deprecated) |
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 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.
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.
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 |................| *
[TECHNET]
| 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 |
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