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image-formats.rst image-metadata.rst

An OpenStack Compute cloud is not very useful unless you have virtual machine images (which some people call "virtual appliances"). This guide describes how to obtain, create, and modify virtual machine images that are compatible with OpenStack.

To keep things brief, we will sometimes use the term image instead of virtual machine image.

What is a virtual machine image?

A virtual machine image is a single file which contains a virtual disk that has a bootable operating system installed on it.

Virtual machine images come in different formats, some of which are described below.


The AKI/AMI/ARI format was the initial image format supported by Amazon EC2. The image consists of three files:

AKI (Amazon Kernel Image)

A kernel file that the hypervisor will load initially to boot the image. For a Linux machine, this would be a vmlinuz file.

AMI (Amazon Machine Image)

This is a virtual machine image in raw format, as described above.

ARI (Amazon Ramdisk Image)

An optional ramdisk file mounted at boot time. For a Linux machine, this would be an initrd file.


The ISO format is a disk image formatted with the read-only ISO 9660 (also known as ECMA-119) filesystem commonly used for CDs and DVDs. While we do not normally think of ISO as a virtual machine image format, since ISOs contain bootable filesystems with an installed operating system, you can treat them the same as you treat other virtual machine image files.


OVF (Open Virtualization Format) is a packaging format for virtual machines, defined by the Distributed Management Task Force (DMTF) standards group. An OVF package contains one or more image files, a .ovf XML metadata file that contains information about the virtual machine, and possibly other files as well.

An OVF package can be distributed in different ways. For example, it could be distributed as a set of discrete files, or as a tar archive file with an .ova (open virtual appliance/application) extension.

OpenStack Compute does not currently have support for OVF packages, so you will need to extract the image file(s) from an OVF package if you wish to use it with OpenStack.


The QCOW2 (QEMU copy-on-write version 2) format is commonly used with the KVM hypervisor. It has some additional features over the raw format, such as:

  • Using sparse representation, so the image size is smaller.
  • Support for snapshots.

Because qcow2 is sparse, qcow2 images are typically smaller than raw images. Smaller images mean faster uploads, so it is often faster to convert a raw image to qcow2 for uploading instead of uploading the raw file directly.


Because raw images do not support snapshots, OpenStack Compute will automatically convert raw image files to qcow2 as needed.


The raw image format is the simplest one, and is natively supported by both KVM and Xen hypervisors. You can think of a raw image as being the bit-equivalent of a block device file, created as if somebody had copied, say, /dev/sda to a file using the dd command.


We do not recommend creating raw images by dd'ing block device files, we discuss how to create raw images later.

UEC tarball

A UEC (Ubuntu Enterprise Cloud) tarball is a gzipped tarfile that contains an AMI file, AKI file, and ARI file.


Ubuntu Enterprise Cloud refers to a discontinued Eucalyptus-based Ubuntu cloud solution that has been replaced by the OpenStack-based Ubuntu Cloud Infrastructure.


VirtualBox uses the VDI (Virtual Disk Image) format for image files. None of the OpenStack Compute hypervisors support VDI directly, so you will need to convert these files to a different format to use them with OpenStack.


Microsoft Hyper-V uses the VHD (Virtual Hard Disk) format for images.


The version of Hyper-V that ships with Microsoft Server 2012 uses the newer VHDX format, which has some additional features over VHD such as support for larger disk sizes and protection against data corruption during power failures.


VMware ESXi hypervisor uses the VMDK (Virtual Machine Disk) format for images.