So you're considering implementing Hyper-V and are just that little bit befuddled by the .vhdx file format - well search no more. These slides should help you learn the past, the present and the possible future of the format.

1. VHDX details and file format
Mahesh Mankar
www.ryussi.com
.vhdx

2. Hyper-V VM
 Hyper-V VM consists following files
 Configuration file
○ VMGUID.xml
 Running memory file
○ VMGUID.bin
 Virtual disk file(s)
○ VMNAME.vhdx

3. VM files
 VMGUID.xml
 Consists all configuration of VM in xml format
...
<name type="string">Win8</name>
<notes type="string"></notes>
<type_id type="string">Virtual Machines</type_id>
...
<MacAddress type="string">00-15-5D-01-08-76</MacAddress>
...
<controller0>
<drive0>
<pathname type="string">D:Win8.vhdx</pathname>
...
<processors>
<count type="integer">1</count>
...
 VMGUID.bin
 Created when machine is running, consiste memory map.
 Used for save/pause the VM.

4. What is vhdx
 vhdx is a file format used by Hyper-V ( Server
2012)
(Earlier with server 2008, it was .vhd)
This is the file formate used by hypervisor (Windows Hyper-V) and Windows
Azure to create virtual disk drive for virtual machines.

5. History
 The .vhd format was created by Connectix for
Connectix Virtual PC product
○ Acquired by Microsoft in 2003
 Supported by
○ Microsoft Virtual PC
○ Microsoft Virtual Server
○ Hyper-V
○ Oracle VirtualBox
○ Sun xVM
○ VMware ESX
○ VMware Workstation
○ Citrix XenServer
 .vhdx over .vhd
○ From Windows 8 & Server 2012 Microsoft modified vhd format to vhdx

6. Types
 Fixed size
 This type of disk are created with the specified size. Space is allocated
at the time of creation
 This gives higher performance
500 GB -
actual
FixedDisk.vhdx (500 GB)

7. Types
 Dynamic expanding
 Dynamically growing. Space is allocated as required
 This gives better space management but slower in performance
500 GB - logical
DynamicDisk.vhdx (500 GB)
50 GB -actual

8. Types
 Differencing
 It freezes the parent disk and creates delta disk
 Used for snapshot purpose
FixedDisk.vhdx (100 GB)
100 GB
DiffDiskA.avhdx (10 GB)
10 GB
DiffDiskAA.avhdx (5 GB)
5 GB
DiffDiskB.vhdx (20 GB)
20 GB
Parent
Parent
Child
Child
Child

9. Features
 Maximum size supported is 64 TB.
 With VHD it is 2 TB
 Block size is 4k
 Improved performance
 Larger logical sector sizes for a virtual disk up to 4 KB
 Large block sizes for a virtual disk up to 256 MB that enables tuning block
size to match the IO
 Updated metadata structures
 To prevent data loss during power failures

10. .vhdx file layout
.vhdx file logical layout

11. .vhdx file layout
.vhdx file layout

12. .vhdx file layout
 Header
 1MB -This is first object of file contains following sections
○ File type identifier
 Contains a short, fixed signature to identify the file as a VHDX
○ Two headers
 Acts as a root of the VHDX data structure tree, providing version
information, the location and size of the log, and some basic file metadata
 Only one header is active at a time
VHDX_HEADER
Signature(UINT32) Checksum(UINT32) SequenceNumber(UINT64)
FileWriteGuid(GUID) DataWriteGuid(GUID) LogGuid(GUID)
LogVersion(UINT16) Version(UINT16) LogLength(UINT32)
LogOffset(UINT64) Reserved[4016](UINT8)
○ Two region table
 Lists regions, which are virtually contiguous, variable-size, MB aligned
pieces of data within the file
 Header Layout
text
File
Identifier
Header
1
Header
2
Region
Table 1
0
KB
64
KB
128
KB
192
KB
256
KB
1MB
Reserved
320
KB
Region
Table 2

13. .vhdx file layout
 Log
 The log is a single circular buffer stored contiguously at a location that is
specified in the VHDX header
 Consists of variable-sized log entries to be written after unexpected
system failure event
 Blocks
 Payload blocks contain virtual disk payload data.
 Sector bitmap blocks contain parts of the sector bitmap
 BAT
 BAT is a region consisting of a single array of 64-bit values, with an entry
for each block that determines the state and file offset of that block
 Metadata Region
 MetaData table - Mapping to MetaData items
 MetaData items - User metadata and system metadata

14. API for vhdx
 C++ API is available to create update .vhdx file.
 Virtdisk.h, Virtdisk.dll, Virtdisk.lib
 Supported from windows 7 onward
 Available calls for .vhdx
 AddVirtualDiskParent AttachVirtualDisk
 BreakMirrorVirtualDisk CompactVirtualDisk
 CreateVirtualDisk DeleteVirtualDiskMetadata
 DetachVirtualDisk EnumerateVirtualDiskMetadata
 ExpandVirtualDisk GetStorageDependencyInformation
 GetVirtualDiskInformation GetVirtualDiskMetadata
 GetVirtualDiskOperationProgress GetVirtualDiskPhysicalPath
 MergeVirtualDisk MirrorVirtualDisk
 OpenVirtualDisk ResizeVirtualDisk
 SetVirtualDiskInformation SetVirtualDiskMetadata

15. VDS - Virtual Disk Service
 Manages a wide range of storage configurations
 From single disk to external storage arrays
 Exposes an API (Windows Storage Management API)

16. Usability
 Create new disk
 Delete disk
 Expand disk
 Compact disk
 Mirror disk
 Attach disk

17. Comparison
 Comparison with other virtual disk format
VIRTUAL DISK TYPE VHD VHDX VMDK VDI
BUILT BY Microsoft Microsoft VMware Sun Microsystems
MAX SIZE 2TB 64TB 62TB
(2TB before Sep-
2013)
2TB
TYPE - FIXED Yes Yes Yes Yes
TYPE - DYNAMIC Yes Yes Yes Yes
TYPE - DIFFERENCING/DELTA Yes Yes Yes No
MULTIPART No No Yes No

18.  Get vhdx file information (GetVirtualDiskInformation)
 Expand size (ResizeVirtualDisk)
Example

19. Example

20. Thanks
 Have Questions ? Feel free to reach out to us
at info@ryussi.com !