VHD, which stands for Virtual Hard Disk, is a file extension used for virtual hard disk images. VHD files are typically used with virtualization software to emulate a hard disk drive. The VHD file format allows for the creation of a virtual hard disk that can be connected to a virtual machine as if it were a physical disk drive.
Quick Answer
The main programs that use the VHD file extension are:
- Microsoft Virtual PC
- Microsoft Hyper-V
- Oracle VM VirtualBox
- Citrix XenServer
- VMware Workstation
- KVM
- QEMU
These programs allow you to create and mount VHD files to virtual machines as virtual hard drives. The VHD format is supported by most major virtualization platforms.
What is a VHD File?
A VHD or Virtual Hard Disk file is a storage file format used for virtual hard disk images. VHD files emulate physical hard drives for use in virtual machines. The VHD format encapsulates an entire hard drive into a single file for easy portability between virtual environments.
VHD files contain a virtual hard disk partition formatted with a filesystem such as NTFS or FAT32. They also include virtual representations of hardware like cylinders, heads, sectors, and clusters. This allows the VHD file to simulate a physical disk drive when connected to a virtual machine.
Types of VHD Files
There are three main types of VHD files:
- Fixed VHD: This type has a fixed size that does not change. The VHD file is preallocated to the maximum specified size at creation.
- Dynamic VHD: With this type, the VHD file starts small and grows bigger as more data is written to it, up to a maximum specified size.
- Differencing VHD: This acts as an overlay on top of a parent VHD file. Any writes go to the differencing VHD while reads fall back to the parent VHD.
VHD File Structure
The VHD file format organizes data into three main components:
- Header: This contains metadata like the type, maximum size, and identifier.
- Hard disk metadata: Includes details like cylinder, head, sector, and cluster information.
- Data area: Contains the actual data written to the emulated disk partitions in the VHD file.
By structuring data in this format, VHD files can provide an accurate emulation of a mechanical hard disk drive.
Benefits of VHD Files
Some key benefits of using VHD files include:
- Portability – VHDs can be easily moved between different systems.
- Performance – VHDs provide faster I/O than physical drives.
- File management – VHDs integrate nicely into file management systems.
- Hardware independence – VHDs abstract away physical disk details.
Limitations of VHD Files
There are some limitations to note with VHD files:
- Booting from VHD can be difficult depending on OS and virtualization platform.
- Limited support for resizing compared to physical disks.
- VHD format lacks advanced features like native cryptographic support.
- VHD provides emulation, not virtualization, so requires hypervisor support.
What Programs Use or Support VHD Files?
There are a number of common programs and platforms that support creating, mounting, and using VHD files:
Microsoft Virtual PC
Microsoft Virtual PC allows you to create and run virtual machines on Windows. It uses VHD files to represent virtual hard drives. Virtual PC supports creating fixed and dynamic VHD files up to 2TB in size.
Microsoft Hyper-V
Hyper-V is Microsoft’s enterprise-grade virtualization solution included with some Windows Server editions. It can generate VHD and the newer VHDX format to provide virtual storage for guest operating systems.
Oracle VM VirtualBox
VirtualBox is a popular open source virtualization platform for Windows, Linux, macOS, and Solaris. It leverages VHD files for its virtual disk images. VirtualBox supports VHDs up to 64TB and up to 16,000 virtual disks per VM.
Citrix XenServer
XenServer from Citrix utilizes VHD format disks with its Xen hypervisor technology. XenServer supports all types of VHD files and conversion to other format like VMDK.
VMware Workstation
VMware Workstation is a Type 2 hypervisor that runs on desktop OSes. It can run VHD files by attaching them as virtual disks in VMware virtual machines. An included utility can convert VHD to VMware’s native VMDK format.
KVM
KVM (Kernel-based Virtual Machine) is a popular open source virtualization solution for Linux. The QEMU component in KVM provides support for generating and booting to VHD format disk images in Linux virtual machines.
QEMU
QEMU is an open source machine emulator and virtualizer. It supports emulating hard drives in VHD format. QCOW2 format is also supported which has some advantages over VHD like copy-on-write.
VirtualBox
VirtualBox allows you to create and run virtual machine environments on Windows, Linux, Mac, and Solaris systems. It can use VDI, VMDK, and VHD as its virtual machine disk formats. VHD provides an easy way to migrate disks to other VM platforms.
Xen
The Xen open source hypervisor supports VHD and VHDX virtual disk image formats. Xen can boot VMs directly from VHD files and supports attaching both fixed and dynamic VHD disks to VMs as additional drives.
Virtuozzo
Virtuozzo includes a Type 1 hypervisor that runs on Windows and Linux. It can use VHD files directly as primary or secondary VM storage. SAS, SATA, SCSI, and IDE virtual disk types are available.
Parallels
Parallels offers virtualization products for desktop and servers. The Parallels virtualization solutions support VHD files for both Windows and Linux guest operating systems. VHD provides cross-platform portability.
VirtualBox
VirtualBox is an open source virtualization product from Oracle that runs on Windows, Linux, Mac, and Solaris. It supports the VDI format plus direct VMDK and VHD support. VirtualBox provides tools to convert formats.
VMware Fusion
VMware Fusion allows running Windows and other VMs on macOS. VMDK is used by default but VMware Fusion can also mount and boot from VHD files. Useful for migrating Windows VMs to Fusion.
Hyper-V
Microsoft’s Hyper-V virtualization platform uses VHD and VHDX as its primary virtual disk formats. Hyper-V provides direct native support for creating, booting, and running VMs using VHD files.
What are the Differences Between VHD and Other Virtual Disk Formats?
The main alternatives to VHD for virtual disk images include VMDK, VDI, QCOW2, and VHDX:
VMDK (Virtual Machine Disk)
- Developed by VMware for use in their products.
- Default disk format for VMware virtual machines.
- Supports advanced features like snapshots and encryption.
- Restricted for use only by VMware virtualization products.
VDI (Virtual Disk Image)
- Used primarily by VirtualBox virtual machines.
- Similar to VMDK format but open specification.
- Supports snapshotting and dynamic resizing.
- Less compatibility outside of VirtualBox.
QCOW2 (QEMU Copy-On-Write)
- Developed by QEMU for improved performance over VHD.
- Uses a copy-on-write optimization to delay allocation.
- Mainly used on Linux platforms like KVM hypervisor.
- Lacks compatibility with Hyper-V and other Windows platforms.
VHDX (Windows Virtual Hard Disk)
- Proprietary improved version of VHD from Microsoft.
- Supports larger virtual disk sizes up to 64TB.
- Provides protection against data corruption.
- Designed for Windows platforms so more limited compatibility.
Overall, VHD offers a good balance of platform support and features for broad compatibility across most major virtualization solutions.
How to Create, Mount, and Use VHD Files
There are a few different steps involved in working with VHD files:
Creating VHDs
Virtual hard disks can be created a few different ways depending on the platform:
- Hyper-V Manager or Disk Management on Windows
- qemu-img or virt-manager tools on Linux
- New-VHD cmdlet on Windows PowerShell
- VirtualBox, VMware, Parallels, or other virtualization tools
Dynamic or fixed VHD files can be created specifying a maximum capacity. Set the appropriate VHD type for your needs.
Mounting and Attaching VHDs
Mounting connects the VHD file to make it accessible to the operating system:
- In Disk Management, attach VHDs to mount them on Windows.
- Use virsh attach-disk to mount VHDs on Linux VMs.
- Hyper-V Manager can directly attach VHDs to a Hyper-V VM.
- In VMware Workstation, add a Hard Disk with the VHD file selected.
Working with VHDs
Once mounted, the VHD will appear as an additional disk drive on the VM. A drive letter will be assigned that can be used to access the content of the VHD file. Data can be read/written to the VHD just like a physical disk.
Some things to keep in mind when working with VHDs:
- Make sure to eject/detach VHDs before shutting down the VM.
- Run chkdsk within the VHD’s partitions to fix any detected filesystem errors.
- Back up the VHD periodically and store the backup in a different location.
- Use proper procedures to rename, move, copy, merge, compact, or expand the VHDs.
Conclusion
VHD files provide a handy way to emulate hard drives for virtual machines. The format is supported by Microsoft and various third-party virtualization platforms. VHDs allow for portability and management benefits vs physical disks.
To work with VHD files, you typically create them in a virtualization platform, then mount/attach the VHD as a disk on a virtual machine. Once attached, the VHD appears as an ordinary disk volume that can be accessed and used like any physical drive.
With robust adoption and standard tools for managing them, VHDs remain a key component for running disk-based workloads in VMs across many virtualization solutions.
