Convert Image Formats

Overview

Images obtained from VMware, VirtualBox, Hyper-V, or other virtualization platforms use formats that differ from what Polystack Compute expects. The qemu-img tool converts between all major disk image formats quickly and without data loss. QCOW2 is the recommended format for Polystack — it provides compression, snapshot support, and efficient sparse storage.

Prerequisites

A Linux workstation with qemu-utils installed
The source image file in its original format
Free disk space: RAW conversions require space equal to the full virtual disk size

Supported Formats

Format	Extension	Description	Use Case
QCOW2	`.qcow2`	Compressed, sparse, copy-on-write	Recommended for Polystack — best balance of size and performance
RAW	`.img`, `.raw`	Uncompressed, full disk image	Maximum I/O performance; large file size
VMDK	`.vmdk`	VMware native format	Import from VMware ESXi or Workstation
VHD / VHDX	`.vhd`, `.vhdx`	Hyper-V / Azure native format	Import from Hyper-V or Microsoft Azure
VDI	`.vdi`	VirtualBox native format	Import from Oracle VirtualBox

Converting to RAW format creates a file equal to the full virtual disk size, not just the used space. A 50 GB virtual disk produces a 50 GB RAW file even if only 5 GB is used. Ensure sufficient disk space before converting.

Install qemu-img

Ubuntu / Debian
CentOS / AlmaLinux / Rocky Linux

Install qemu-utils

apt-get update && apt-get install -y qemu-utils

Install qemu-img

dnf install -y qemu-img

Check Image Information

Before converting, inspect the source image to confirm its format, virtual size, and backing chain:

Inspect image metadata

qemu-img info source-image.vmdk

Expected output:

Example qemu-img info output

image: source-image.vmdk
file format: vmdk
virtual size: 20 GiB (21474836480 bytes)
disk size: 4.2 GiB
cluster_size: 65536

The disk size shows actual data written; virtual size shows the size instances see. QCOW2 preserves this sparse allocation — RAW expands to the full virtual size.

Common Conversions

VMDK to QCOW2
VHD to QCOW2
RAW to QCOW2
QCOW2 to RAW

Convert a VMware disk image to QCOW2:

Convert the image

Convert VMDK to QCOW2

qemu-img convert \
  -f vmdk \
  -O qcow2 \
  source-image.vmdk \
  output-image.qcow2

The -f flag specifies the source format; -O specifies the output format.

Verify the output

Verify converted image

qemu-img info output-image.qcow2

Output shows file format: qcow2 and virtual size matches the source.

Convert a Hyper-V or Azure VHD image to QCOW2:

Convert the image

Convert VHD to QCOW2

qemu-img convert \
  -f vpc \
  -O qcow2 \
  source-image.vhd \
  output-image.qcow2

VHD uses the vpc format identifier in qemu-img. VHDX uses vhdx.

Verify the output

Verify converted image

qemu-img info output-image.qcow2

Output shows file format: qcow2.

Compress a RAW image into QCOW2 to reduce storage footprint:

Convert with compression

Convert RAW to QCOW2 with compression

qemu-img convert \
  -f raw \
  -O qcow2 \
  -c \
  source-image.raw \
  output-image.qcow2

The -c flag enables compression. Compressed QCOW2 images can be significantly smaller than their RAW equivalents.

Compare sizes

Compare source and output sizes

ls -lh source-image.raw output-image.qcow2

QCOW2 output is typically 30–70% smaller than the RAW source for OS images.

Convert to RAW for maximum I/O performance or compatibility testing:

Check available disk space first

Check free space vs. virtual image size

df -h .
qemu-img info source-image.qcow2 | grep "virtual size"

Ensure free space exceeds the virtual size of the image.

Convert to RAW

Convert QCOW2 to RAW

qemu-img convert \
  -f qcow2 \
  -O raw \
  source-image.qcow2 \
  output-image.raw

Verify the output

Verify the RAW image

qemu-img info output-image.raw

Output shows file format: raw and file size equals the virtual disk size.

Validation

After conversion, perform a consistency check before uploading to Polystack:

Check image integrity

qemu-img check output-image.qcow2

Expected output:

Clean image output

No errors were found on the image.

If qemu-img check reports errors, the conversion may have encountered a corrupt source image. Re-obtain the source and retry the conversion. Do not upload a corrupted image — instances launched from it will fail unpredictably.

Next Steps

Upload an Image

Upload the converted QCOW2 image to the Polystack Image Service.

Image Requirements

Verify the image meets all Polystack Compute compatibility requirements.

Modify Images

Customize images with additional packages and configuration before uploading.

Image Formats

Compare all supported image formats and choose the right one for your workload.

Core Services

Other Services

Overview

Supported Formats

Install qemu-img

Check Image Information

Common Conversions

Convert the image

Verify the output

Convert the image

Verify the output

Convert with compression

Compare sizes

Check available disk space first

Convert to RAW

Verify the output

Validation

Next Steps

Upload an Image

Image Requirements

Modify Images

Image Formats

​Overview

​Supported Formats

​Install qemu-img

​Check Image Information

​Common Conversions

Convert the image

Verify the output

Convert the image

Verify the output

Convert with compression

Compare sizes

Check available disk space first

Convert to RAW

Verify the output

​Validation

​Next Steps

Upload an Image

Image Requirements

Modify Images

Image Formats

Overview

Supported Formats

Install qemu-img

Check Image Information

Common Conversions

Validation

Next Steps