130 lines
4.6 KiB
ReStructuredText
130 lines
4.6 KiB
ReStructuredText
============
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Introduction
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============
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.. toctree::
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:maxdepth: 1
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image-formats.rst
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image-metadata.rst
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An OpenStack Compute cloud is not very useful unless you have virtual
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machine images (which some people call "virtual appliances").
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This guide describes how to obtain, create, and modify virtual machine
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images that are compatible with OpenStack.
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To keep things brief, we will sometimes use the term ``image``
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instead of virtual machine image.
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**What is a virtual machine image?**
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A virtual machine image is a single file which contains a virtual disk
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that has a bootable operating system installed on it.
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Virtual machine images come in different formats, some of which are
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described below.
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AKI/AMI/ARI
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The `AKI/AMI/ARI
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<http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/AMIs.html>`_
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format was the initial image format supported by Amazon EC2.
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The image consists of three files:
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AKI (Amazon Kernel Image)
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A kernel file that the hypervisor will load initially to boot the image.
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For a Linux machine, this would be a ``vmlinuz`` file.
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AMI (Amazon Machine Image)
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This is a virtual machine image in raw format, as described above.
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ARI (Amazon Ramdisk Image)
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An optional ramdisk file mounted at boot time.
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For a Linux machine, this would be an ``initrd`` file.
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ISO
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The `ISO
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<http://www.ecma-international.org/publications/standards/Ecma-119.htm>`_
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format is a disk image formatted with the read-only ISO 9660 (also known
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as ECMA-119) filesystem commonly used for CDs and DVDs.
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While we do not normally think of ISO as a virtual machine image format,
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since ISOs contain bootable filesystems with an installed operating system,
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you can treat them the same as you treat other virtual machine image files.
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OVF
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`OVF <http://dmtf.org/sites/default/files/OVF_Overview_Document_2010.pdf>`_
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(Open Virtualization Format) is a packaging format for virtual machines,
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defined by the Distributed Management Task Force (DMTF) standards group.
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An OVF package contains one or more image files, a ``.ovf`` XML metadata file
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that contains information about the virtual machine, and possibly other
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files as well.
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An OVF package can be distributed in different ways. For example,
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it could be distributed as a set of discrete files, or as a tar archive
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file with an ``.ova`` (open virtual appliance/application) extension.
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OpenStack Compute does not currently have support for OVF packages,
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so you will need to extract the image file(s) from an OVF package
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if you wish to use it with OpenStack.
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QCOW2
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The `QCOW2 <http://en.wikibooks.org/wiki/QEMU/Images>`_
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(QEMU copy-on-write version 2) format is commonly used with the
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KVM hypervisor.
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It has some additional features over the raw format, such as:
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* Using sparse representation, so the image size is smaller.
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* Support for snapshots.
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Because qcow2 is sparse, qcow2 images are typically smaller than
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raw images. Smaller images mean faster uploads, so it is often
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faster to convert a raw image to qcow2 for uploading instead of
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uploading the raw file directly.
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.. note::
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Because raw images do not support snapshots, OpenStack Compute
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will automatically convert raw image files to qcow2 as needed.
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Raw
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The ``raw`` image format is the simplest one, and is natively
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supported by both KVM and Xen hypervisors.
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You can think of a raw image as being the bit-equivalent of
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a block device file, created as if somebody had copied, say,
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``/dev/sda`` to a file using the :command:`dd` command.
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.. note::
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We do not recommend creating raw images by dd'ing block device
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files, we discuss how to create raw images later.
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UEC tarball
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A UEC (Ubuntu Enterprise Cloud) tarball is a gzipped tarfile that
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contains an AMI file, AKI file, and ARI file.
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.. note::
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Ubuntu Enterprise Cloud refers to a discontinued Eucalyptus-based
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Ubuntu cloud solution that has been replaced by the OpenStack-based
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Ubuntu Cloud Infrastructure.
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VDI
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VirtualBox uses the
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`VDI <https://forums.virtualbox.org/viewtopic.php?t=8046>`_
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(Virtual Disk Image) format for image files. None of the OpenStack Compute
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hypervisors support VDI directly, so you will need to convert these files
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to a different format to use them with OpenStack.
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VHD
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Microsoft Hyper-V uses the VHD (Virtual Hard Disk) format for images.
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VHDX
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The version of Hyper-V that ships with Microsoft Server 2012 uses the newer
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`VHDX <http://technet.microsoft.com/en-us/library/hh831446.aspx>`_ format,
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which has some additional features over VHD such as support for larger disk
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sizes and protection against data corruption during power failures.
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VMDK
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VMware ESXi hypervisor uses the
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`VMDK <https://developercenter.vmware.com/web/sdk/60/vddk>`_
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(Virtual Machine Disk) format for images.
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