Working with Images

A Metacloud Compute cloud is not very useful unless you have virtual machine images, sometimes referred to as “virtual appliances”. A virtual machine image is a file that contains a virtual disk with a bootable operating system. These images are used to create virtual machine instances within the cloud.

Consult your Cloud Administrator to obtain the appropriate privileges for uploading and managing images in a project.

You can use the Dashboard, the CLI, or the Image service to manage images. See Uploading and Managing Images with Dashboard and Managing Images using the CLI.

Virtual machine images come in different formats. The following matrix shows image file formats supported in Metacloud:

Image Format Local Storage NFS-backed Storage Ceph-backed Storage
VDI (VirtualBox)
VHD (Hyper-V)

Amazon Machine Image (AMI)

The AKI/AMI/ARI format was the initial image format supported by Amazon EC2.

  • AMI (Amazon Machine Image)—This is a virtual machine image in raw format, as described above.
  • AKI (Amazon Kernel Image)—A kernel file that the hypervisor loads initially to boot the image. For a Linux machine, this would be a vmlinuz file.
  • 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. Even though ISO files contain bootable filesystems with an installed operating system, treat ISO files the same as you treat other virtual machine image files.


The 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, an 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.

Metacloud Compute does not support OVF packages, so you must extract the image file(s) from an OVF package.


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, Metacloud Compute automatically converts raw image files to qcow2 as needed.


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

Do not create raw images by dd’ing block device files.

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 must convert these files to a different format to use them with OpenStack.


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


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