Convert Image Formats

Convert virtual machine images between QCOW2, RAW, VMDK, VHD, and VDI formats using qemu-img before uploading to NxtSight.

Overview

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

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Supported Formats

Format	Extension	Description	Use Case
QCOW2	.qcow2	Compressed, sparse, copy-on-write	Recommended for NxtSight — 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.

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Install qemu-img

Ubuntu / Debian

Install qemu-utils

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

CentOS / AlmaLinux / Rocky Linux

Install qemu-img

dnf install -y qemu-img

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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.

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Common Conversions

VMDK to QCOW2

Convert a VMware disk image to QCOW2:

1. 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.

2. Verify the output

   Verify converted image

   qemu-img info output-image.qcow2
   

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

VHD to QCOW2

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

1. 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.
2. Verify the output

   Verify converted image

   qemu-img info output-image.qcow2
   

   Output shows file format: qcow2.

RAW to QCOW2

Compress a RAW image into QCOW2 to reduce storage footprint:

1. 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.

2. 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.

QCOW2 to RAW

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

1. 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.

2. Convert to RAW

   Convert QCOW2 to RAW

   qemu-img convert \
     -f qcow2 \
     -O raw \
     source-image.qcow2 \
     output-image.raw
   
3. 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.

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Validation

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

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.

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Next Steps

Upload an Image

Upload the converted QCOW2 image to the NxtSight Image Service.

Image Requirements

Verify the image meets all NxtSight 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.