openstack/openstack-manuals
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Merge "[config-ref] update 'raw' to 'flat' image back end"

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Jenkins 2017-02-15 11:34:09 +00:00 committed by Gerrit Code Review
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doc/config-reference/source/compute/hypervisor-kvm.rst
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@ -26,7 +26,7 @@ To enable KVM explicitly, add the following configuration options to the
The KVM hypervisor supports the following virtual machine image formats:
* Raw
* QEMU Copy-on-write (qcow2)
* QEMU Copy-on-write (QCOW2)
* QED Qemu Enhanced Disk
* VMware virtual machine disk format (vmdk)
@ -231,15 +231,24 @@ Backing Storage is the storage used to provide the expanded operating system
image, and any ephemeral storage. Inside the virtual machine, this is normally
presented as two virtual hard disks (for example, ``/dev/vda`` and ``/dev/vdb``
respectively). However, inside OpenStack, this can be derived from one of
these methods: ``lvm``, ``qcow``, ``rbd`` or ``raw``, chosen using the
these methods: ``lvm``, ``qcow``, ``rbd`` or ``flat``, chosen using the
``images_type`` option in ``nova.conf`` on the compute node.
.. note::
The option ``raw`` is acceptable but deprecated in favor of ``flat``.
The Flat back end uses either raw or QCOW2 storage. It never uses
a backing store, so when using QCOW2 it copies an image rather than
creating an overlay. By default, it creates raw files but will use
QCOW2 when creating a disk from a QCOW2 if ``force_raw_images`` is
not set in configuration.
QCOW is the default backing store. It uses a copy-on-write philosophy to delay
allocation of storage until it is actually needed. This means that the space
required for the backing of an image can be significantly less on the real disk
than what seems available in the virtual machine operating system.
RAW creates files without any sort of file formatting, effectively creating
Flat creates files without any sort of file formatting, effectively creating
files with the plain binary one would normally see on a real disk. This can
increase performance, but means that the entire size of the virtual disk is
reserved on the physical disk.