qemu-img — QEMU Disk Image Utility

qemu-img is a powerful command-line utility included with QEMU (Quick EMUlator), used to create, convert, inspect, resize, and manage disk image files. It supports various formats and is commonly used in virtualization workflows involving KVM, QEMU, libvirt, or cloud image preparation.

Key Features

  • Create and Convert Disk Images: Supports multiple formats like qcow2, raw, vmdk, vdi, vhd, etc.

  • Inspect Disk Image Info: Displays format, virtual size, actual size, backing files, and more.

  • Resize and Modify Images: Extend or shrink image sizes, adjust formats, or preallocate space.

  • Snapshot and Backing File Support: Works with layered images using base files and copy-on-write.

  • Efficient Compression and Sparsification: Reduce image size for storage or distribution.

Basic Syntax

qemu-img <command> [options]

Common Commands

Create a New Disk Image

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

Creates a 20 GB image in qcow2 format.

Convert Between Formats

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

Converts a raw image to qcow2.

Display Image Information

qemu-img info disk.qcow2

Shows details such as format, virtual size, and backing files.

Resize a Disk Image

qemu-img resize disk.qcow2 +5G

Expands the disk image size by 5 GB.

Check Image Integrity

qemu-img check disk.qcow2

Performs a consistency check to detect errors in the image.

Compress a Disk Image

qemu-img convert -O qcow2 -c disk.raw disk-compressed.qcow2

Converts and compresses the image.

Supported Formats

  • qcow2: QEMU Copy-On-Write (default for QEMU/KVM)

  • raw: Unformatted binary image

  • vmdk: VMware disk format

  • vhd / vhdx: Hyper-V

  • vdi: VirtualBox format

  • luks: Encrypted image

  • Others supported via plugins or drivers

Example Use Cases

Cloud Image Preparation

Convert cloud-init images or shrink images for minimal cloud deployments:

qemu-img convert -O qcow2 cloud.img prepared-cloud.qcow2

Snapshot Workflows

Combine with backing_file for snapshot-based workflows:

qemu-img create -f qcow2 -b base.qcow2 snapshot.qcow2

Creates a copy-on-write snapshot from a base image.

Disk Image Shrinking (Zero out unused space first)

qemu-img convert -O qcow2 disk.qcow2 disk-shrink.qcow2

Reduces the size of the image file if it's sparse.

Example Output

qemu-img info disk.qcow2
image: disk.qcow2
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 1.5G
cluster_size: 65536
backing file: base.qcow2

Use Case Scenarios

  • Automation and CI/CD: Automate creation and conversion of disk images in CI pipelines.

  • Virtualization Infrastructure: Prepare VM images for KVM, libvirt, or cloud deployment.

  • Forensics and Testing: Duplicate or analyze disk contents without modifying originals.

  • Snapshot Chains and Rollbacks: Manage base images and snapshots for test environments.

Conclusion

qemu-img is an essential tool for managing virtual disk images in QEMU/KVM environments. With robust support for various image formats, snapshot management, conversion, and resizing features, it is critical in both development and production scenarios involving virtualization, cloud deployment, and image management.

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