From 19ca38d61ba46955e05026fb45489fc59bce23aa Mon Sep 17 00:00:00 2001 From: KATO Tomoyuki Date: Mon, 13 Feb 2017 11:20:09 +0900 Subject: [PATCH] [config-ref] update 'raw' to 'flat' image back end backport: newton Change-Id: Ie1ed3c02dfb67074bcc15a69200f075daeba3f7a Closes-Bug: #1583253 --- .../source/compute/hypervisor-kvm.rst | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) diff --git a/doc/config-reference/source/compute/hypervisor-kvm.rst b/doc/config-reference/source/compute/hypervisor-kvm.rst index 9dfae742cc..3828162744 100644 --- a/doc/config-reference/source/compute/hypervisor-kvm.rst +++ b/doc/config-reference/source/compute/hypervisor-kvm.rst @@ -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.