What is a virtual disk file?

A virtual disk file is a file that functions as a virtual hard disk drive. Virtual disk files allow you to create and run virtual machines on your computer without having to dedicate an entire physical hard drive to each virtual machine.

Table of Contents

What are the benefits of using virtual disk files?

There are several key benefits to using virtual disk files rather than physical hard drives for virtual machines:

Portability – Virtual disk files can easily be moved, copied, and backed up like any other file on your system.

Flexibility – You can dynamically resize virtual disk files as needed to give more storage space to your virtual machines.
Isolation – Changes made inside a virtual machine stay contained within its virtual disk file and don’t affect the host or other VMs.
Cost – Virtual disk files eliminate the need to purchase additional physical hard drives.

How do virtual disk files work?

Virtual disk files work by emulating a real hard drive for the guest operating system running inside a virtual machine. The virtualization software handles intercepting disk read/write operations from the guest VM and mapping them to the appropriate locations within the virtual disk file on the host machine.

This process allows the guest VM to act as though it is reading and writing to a real disk drive when in reality the data is being stored in a file. The virtual disk file contains all the contents that would normally be stored on a physical hard drive, including the guest operating system, applications, and user files.

What types of virtual disk files are there?

There are a few common virtual disk file formats used in virtualization:

VMDK – Virtual Machine Disk format used by VMware products. VMDK allows creating virtual disks as files on the host machine.
VHD/VHDX – Virtual Hard Disk format used by Microsoft and Hyper-V. Like VMDK, it stores virtual disk contents in a host file.
QCOW2 – QEMU Copy On Write version 2 format. Used by QEMU and KVM hypervisors.

VDI – Virtual Disk Image format used by Oracle VM VirtualBox.

The most common formats are VMDK and VHD. Each hypervisor has its preferred format, but some formats like VMDK can be used across hypervisors.

How are virtual disk files created?

Virtual disk files are created in a few different ways depending on the hypervisor and format:

In the hypervisor management software, such as vSphere for VMDK files. You specify the storage location, size, and other details.
By converting an existing physical hard drive into a virtual disk file through a process called physical-to-virtual (P2V) conversion.
By attaching an ISO installation image of an OS like Linux or Windows to a VM and installing onto a blank virtual disk file.

The process creates a file at the specified location that can be attached to a virtual machine. The VM can then treat this virtual disk file exactly like a real storage drive.

Where are virtual disk files stored?

Virtual disk files are typically stored in a few common locations on the host machine:

Local folder – Stored in a local directory or folder on the host computer running the hypervisor.

External drive – Saved to an external HDD or SSD connected to the host.
Network storage – Saved to a network attached storage (NAS) device or storage area network (SAN).
Cloud storage – Stored in a cloud storage bucket like S3, Azure Storage, or GCP Storage.

Storing virtual disk files on external or networked storage provides more flexibility. It allows moving and accessing them from different hosts. Local storage is faster while cloud storage offers unlimited capacity.

What is the typical file extension for a virtual disk file?

The most common virtual disk file extensions include:

.vmdk – Used for VMware VMDK files.

.vhd/.vhdx – Used for Microsoft Hyper-V VHD/VHDX formats.
.qcow2 – Used for QEMU QCow2 files.
.vdi – Used for VirtualBox VDI files.

.img – A common generic image file extension used by many emulators.

The extension helps identify what type of virtual disk format the file contains. This allows the correct hypervisor to recognize and work with the file.

How does a virtual machine access a virtual disk file?

When a virtual disk file is attached to a virtual machine, the hypervisor handles mapping disk read/write operations from the VM to the virtual disk file. This process happens behind the scenes:

The VM requests to read a block of data from what it believes is a physical hard drive.
The hypervisor intercepts the request and determines the location of that block in the virtual disk file.
The hypervisor reads the requested block of data from the file.

The hypervisor returns the data to the VM.

This on the fly translation allows the VM to interact with the virtual disk just like a physical one. The hypervisor abstracts away the fact it is actually a file on the host system.

Can you access and edit virtual disk files directly?

In most cases, you should not directly access or edit the contents of a virtual disk file from the host OS while it is attached to a running VM. Doing so can corrupt the file system and cause crashes.

However, when powered off, it is possible to mount, access, and edit a virtual disk file from the host if needed. For example, you may want to:

Use host OS tools to repair or restore the VM filesystem.
Manually extract files from the virtual disk.

Modify the files and registry settings of the guest OS.

This requires mounting the disk file as a drive on the host OS using utilities like qemu-nbd (Linux) or Disk Management snap-in (Windows). Caution should still be taken not to access the virtual disk while powered on.

Can you convert a physical computer into a virtual disk file?

Yes, it is possible to perform a physical to virtual (P2V) conversion to turn a physical computer into a virtual machine. There are a few approaches to doing this:

Disk imaging – Take a raw disk image of the physical disk and convert it into a virtual disk file.
Cloning – Use virtualization software to clone the physical disk to a new virtual disk file.
Backup/restore – Take backups of the physical system and restore to a new VM.

This process allows migrating physical systems like servers into virtual infrastructure. It eliminates the need to reinstall and reconfigure the OS.

What are the risks of corrupted virtual disk files?

Some potential risks of virtual disk file corruption include:

VM crashes or fails to start.

Data loss or inaccessible data within the VM.
File system damage preventing OS from booting.
Errors and performance issues until the disk is repaired.

Permanent VM failure if backups are not available.

To reduce the chance of corruption, proper storage and VM backup/replication should be used. But corruption can still occur due to bugs, host failures, or improper virtual disk modifications.

How can you optimize the performance of virtual disk files?

Some tips for optimizing virtual disk performance include:

Use fixed VMDK files instead of dynamically expanding disks.
Place virtual disk files on high performance storage like SSDs.
Avoid using virtual disks over slow networks if possible.

Monitor disk queues and latencies for bottlenecks.
Defragment the file periodically within the guest OS.

Performance tuning the underlying storage is key. Mixing virtual disks with other workloads on the same volume will often cause contention.

Can you password protect and encrypt virtual disk files?

Many hypervisor platforms allow encrypting the contents of virtual disk files for security:

VMware vSphere – Provides encrypted VMDK support.
Hyper-V – VHD files can be encrypted with BitLocker.

KVM – Supports QCOW2 encryption and LUKS encrypted disk images.
VirtualBox – Allows encrypting disk images to protect VM data.

Encrypting virtual disks protects their contents at rest on the host system or storage platform. It prevents unauthorized access to the VM data.

What are the main use cases for virtual disk files?

Some of the most common use cases for virtual disk files include:

Running guest OSes like Linux, Windows, etc on desktop virtualization software like VirtualBox.
Virtualizing enterprise servers, applications, and workloads on platforms like vSphere, Hyper-V, Xen, KVM, etc.

Migrating physical systems to virtual infrastructure through P2V conversion.
Modeling production systems and workloads in development, staging, and testing environments.
Providing isolated, portable virtual environments for developers, testers, etc.

The main benefits revolve around portability, manageability, isolation, and consolidation of workloads into virtual machines.

What are the alternatives to using virtual disk files?

Some alternatives to virtual disk files include:

Physical hard drives – Using dedicated HDD/SSD hardware for each VM.

Logical or raw disk partitions – Creating partitioned logical drives rather than files.
SAN/NAS storage – Block/file based storage area networks rather than local files.
Cloud block storage – Cloud eBS/EFS volumes assigned directly to VMs.

These options remove the file abstraction layer. But they can limit features like portability, snapshots, and thin provisioning.

Can you boot operating systems directly from virtual disk files?

Yes, virtual disk files allow booting and running guest operating systems without the need for a physical disk drive. Each virtual disk file acts as a full virtual hard disk drive for the VM.

The VM boots from the virtual disk using a bootloader like BIOS or EFI, loads the OS kernel, and initializes the filesystem – the same way it would from a physical drive. All of this occurs through the abstraction provided by the hypervisor and virtual disk file.

Bootable virtual disk files are created either from existing OS installations or by attaching an ISO and performing a fresh OS install within a new VM.

Conclusion

Virtual disk files are a key enabling technology for virtualization. They provide portability, management, and flexibility benefits compared to physical storage. By encapsulating VM disks as files, virtual disks allow full VMs to be stored, copied, and migrated easily.

All major hypervisor platforms have adopted virtual disks using default formats like VMDK, VHD, and QCOW2. Continued innovation in file based VM storage is improving performance, security, availability, and integration.