QCOW2: A Virtual Disk Format Designed for Modern Virtualization

QCOW2: A Virtual Disk Format Designed for Modern Virtualization

by Karine Huang | May 28, 2025 | Blog, QCOW2 | 0 comments

 

In the world of virtualization, it’s not enough to simply create powerful virtual machines. Choosing the right virtual disk format is just as crucial for efficient storage, manipulation, and deployment. 

Among the many available formats, QCOW2 (QEMU Copy-On-Write version 2) is widely used in QEMU/KVM environments and cloud platforms like OpenStack. While not always well-known to the general public, it plays a key role in many virtual infrastructures.

In this article, we’ll cover:

• What is the QCOW2 format
• What can a QCOW2 image contains
• What you can do with it
• Its main competitors and how it compares
• How it differs from its predecessor, QCOW
• Creating and converting Images
• Use cases and best practices
• Real-World Examples of QCOW2 in Action

The QCOW2 Format 

What Is the QCOW2 Format?

 QCOW2 is an evolution of the original QCOW format, designed to meet growing demands for flexibility, storage efficiency, and advanced features in virtualized environments. It enables the creation of dynamic disk images with support for features such as snapshots, compression, encryption, and dynamic space allocation.

What Can a QCOW2 Image Contain? 

A QCOW2 image is a file that simulates a hard disk for a virtual machine. It can contain:

• The operating system of the VM
• User files, applications, configurations
• The disk structure (partitions, tables, etc.)
• Optionally, saved snapshots of the machine’s state

In short, it is a complete virtual disk container, ready to be used in a hypervisor like QEMU.

What Can You Do with it? 

QCOW2 offers advanced features that make it especially useful in virtualized environments:

• Copy-On-Write (COW): VM data changes are written to a separate layer, not the base image, saving disk space.
• Snapshots: Capture and revert the state of a VM at a given time.
• Compression: Store data in a compressed format to save space.
• AES Encryption: Basic built-in encryption, optionally combined with external solutions like LUKS.
• Dynamic Allocation: The file grows as needed, instead of allocating full disk size upfront.
• Backing Files: Create new QCOW2 images based on a reference image, useful for deploying lightweight VM clones.

These features make it a flexible, space-efficient, and modular format for system administrators and developers.

Its Competitors 

Other Common Virtual Disk Formats

 

However, it is not the only player in the game. Here’s a comparison of major formats used in different virtualization environments:

 

Format	Used by	Strengths	Limitations
RAW	QEMU/KVM, generic	Simple, fast	No advanced features
VMDK	VMware	Deep VMware integration	Closed format, limited portability
VHD/VHDX	Microsoft Hyper-V	Resilient, supports large volumes	Tied to Microsoft exosystem
VDI	Virtual Box	Snapshots, compacting	Mostly limited to VirtualBox usage
QCOW2	QEMU/KVM, OpenStack	Snapshots, compression, dynamic	Slightly slower than RAW, can fragment

 

 

QCOW2 vs. the Others 

• Against RAW: it offers more features, but RAW remains faster, especially for intensive I/O (Input/Output) operations.
• Against VMDK: it is open-source and more portable, but less deeply integrated in VMware environments.
• Against VHDX: it is better suited for Linux-based platforms, while VHDX fits Windows-based systems.
• Against VDI: it is more efficient and widely supported, whereas VDI is niche and slower.

QCOW2 finds its strength in feature-richness and open-source compatibility, even if it’s not always the top performer in raw speed.

Its Predecessor: QCOW 

QCOW vs. QCOW2: Key Differences

 

 

Feature	QCOW (v1)	QCOW2 (v2)
Snapshots
Compression
Built-in Encryption		Basic AES
Dynamic Allocation
Tool Compatibility	Obsolete	Compatible with libvirt, KVM
Current Usage	Very rare	Widely used

 

 

Why QCOW has been replaced

 QCOW2 offers better performance, more features, and broad compatibility with modern virtualization tools. It has completely replaced QCOW in most production and development environments.

Creating and Converting QCOW2 Images 

The qemu-img tool allows you to create and convert QCOW2 images:

• Create an image:

qemu-img create -f qcow2 image_name.qcow2 20G

• Convert an existing image to QCOW2:

qemu-img convert -f raw -O qcow2 image.raw image.qcow2

You can also convert formats like VMDK or VHD to QCOW2 by adjusting the input/output flags.

• Convert QCOW2 to an existing image:

qemu-img convert -f qcow2 -O raw disque.qcow2 disque.raw

• -f qcow2 : specifies the source format
• -O raw : specifies the destination format
• disque.qcow2 : source file
• disque.raw : output file

Use Cases and Best Practices 

QCOW2 is especially well-suited to scenarios like:

• Deploying VMs with snapshots: Great for testing environments and rollback capabilities.
• Development environments: Quickly clone VMs from a base image.
• Storage optimization: Thanks to compression and dynamic allocation.
• Data security: Built-in encryption adds a layer of protection.

However, it’s worth noting that QCOW2 may have slightly higher performance overhead than RAW due to its feature set.

Real-World Examples

 While QCOW2 is known for its technical capabilities like snapshots, compression, and encryption, these features become even more relevant when applied to real-life scenarios. Here are a few examples of how QCOW2 is used effectively in the field:

1. Cloud Hosting and Image Deployment 

A cloud hosting provider manages hundreds of virtual machines for clients. By using a single QCOW2 base image and layering individual differences on top (thanks to QCOW2’s backing file feature), they save hundreds of gigabytes in storage and dramatically reduce provisioning times.

2. Software Testing and Development 

A DevOps engineer regularly tests OS updates and kernel patches. Instead of creating fresh VMs every time, they use snapshots within QCOW2 to roll back to a clean state in seconds—speeding up test cycles and improving consistency across environments.

3. Secure Virtual Desktops for Remote Work 

A cybersecurity-focused company needs to provide secure, encrypted virtual desktops to remote workers. Using QCOW2 with AES encryption and centralized disk management ensures that no data is stored locally, minimizing risks in case of device theft or compromise.

4. Efficient Backups and Disaster Recovery  

A systems administrator schedules regular backups of VMs using QCOW2’s compression feature to keep image file sizes manageable. This allows for faster snapshot-based backup routines, and a quicker restore process in case of system failure.

Conclusion 

QCOW2 is one of many available virtual disk formats, primarily used in QEMU/KVM and open-source cloud environments. It supports complete VM storage, along with features like snapshots, compression, and dynamic space allocation.

While not always the fastest or most universal option, it remains a relevant, modular, and flexible format suited for various use cases in modern infrastructure.

In short: It isn’t always the default choice, but it’s a format worth knowing and understanding when working with virtual machines.