How to Create a Virtual Disk: VHDX on Windows, qcow2 and Disk Image Files on Linux

Applies to: Windows 10/11 and Windows Server (Disk Management, diskpart, New-VHD) according to Microsoft’s documentation; Linux tested on Ubuntu 26.04 LTS with qemu-img 10.2.1 and e2fsprogs 1.47.2

A virtual disk is a file that behaves like a hard drive. On Windows, open Disk Management and choose Action > Create VHD, or run New-VHD in PowerShell. On Linux, create one with qemu-img create -f qcow2 disk.qcow2 20G for a virtual machine, or with truncate and mkfs for a disk image you mount on the host.

Which format?

• VHDX: Windows and Hyper-V. Microsoft recommends it over the older VHD.
• qcow2: KVM and QEMU.
• VDI: VirtualBox. VMDK: VMware.
• Raw image (.img): a plain byte-for-byte disk that any tool can read.

Each can be fixed (the full size is reserved at once) or dynamic (the file grows as data is written).

Windows: Disk Management

1. Open Disk Management as administrator (search for it in the Start menu).
2. Choose Action > Create VHD.
3. Click Browse, pick a folder (not inside \Windows), enter a file name and set Save as type to Virtual Disk files (*.vhdx).
4. Enter the Virtual hard disk size and choose Dynamically expanding or Fixed size, then click OK.

The new disk appears in the list like a physical disk. To use it, right-click the disk and choose Initialize Disk (GPT is the default partition style), then right-click the Unallocated space and choose New Simple Volume to format it and give it a drive letter. Later, Action > Attach VHD connects an existing file again, and right-click > Detach VHD disconnects it without deleting anything. According to Microsoft, you need to be in the Administrators or Backup Operators group, and a VHD must be at least 3 MB.

Windows: PowerShell or diskpart

With the Hyper-V PowerShell module, Microsoft’s example creates, attaches, initialises and formats a disk in one line:

New-VHD -Path C:\VHDs\Data.vhdx -Dynamic -SizeBytes 20GB | Mount-VHD -Passthru | Initialize-Disk -Passthru | New-Partition -AssignDriveLetter -UseMaximumSize | Format-Volume -FileSystem NTFS -Confirm:$false -Force

The file extension decides the format (.vhdx or .vhd). If New-VHD is not recognised, the Hyper-V module is not installed; use Disk Management or diskpart instead:

diskpart
create vdisk file="C:\VHDs\Data.vhd" maximum=20000 type=expandable
select vdisk file="C:\VHDs\Data.vhd"
attach vdisk

maximum is in megabytes. Without type=expandable, diskpart creates a fixed disk, which takes longer because the whole file is allocated at once. After attach vdisk, initialise and format it as above.

Linux: a disk for a virtual machine (qemu-img)

sudo apt install qemu-utils        # Debian, Ubuntu (qemu-img on AlmaLinux and Rocky Linux)
qemu-img create -f qcow2 vm-disk.qcow2 20G

Formatting 'vm-disk.qcow2', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=21474836480 lazy_refcounts=off refcount_bits=16

The same tool writes the other formats:

qemu-img create -f vhdx data.vhdx 10G                      # Hyper-V
qemu-img create -f vpc -o subformat=dynamic data.vhd 10G   # older VHD
qemu-img create -f vdi disk.vdi 10G                        # VirtualBox

All of them start small: the 20 GB qcow2 file used 196 KB on disk, the 10 GB VHDX 1.3 MB. Check a file with qemu-img info, and convert between formats with qemu-img convert (see convert a VMware disk for KVM). VirtualBox users can also run VBoxManage createmedium disk --filename=disk.vdi --size=10240 (size in MB).

Linux: a disk image file with a file system

To get a file you can format and mount on the host itself, create a raw image:

truncate -s 1G disk.img             # sparse: takes no space until used
fallocate -l 1G disk.img            # or reserve the full 1 GB now
mkfs.ext4 -L data disk.img
blkid disk.img

disk.img: LABEL="data" UUID="45779706-7e03-41e1-b175-09bf00016c92" BLOCK_SIZE="4096" TYPE="ext4"

ls -l shows a sparse file at its full size, while du -h disk.img shows the space really used: 0 right after truncate, and 33M in our test once the ext4 file system was on it. Mount it through a loop device with sudo mkdir -p /mnt/data && sudo mount -o loop disk.img /mnt/data; the details, including partitions inside an image, are in create a file system in a file and mount it as a loop device and partitions inside a loopback device.

Choosing a hypervisor to use these disks with? See KVM, QEMU and VirtualBox compared.

Official documentation: Manage virtual hard disks, New-VHD and qemu-img.