844efef3cd
The administrator guide includes many links to the install guides, configuration guide, and so forth. As the administrator guide is the current version all links should link to current (Juno) vs. Icehouse or trunk (Kilo) versions of the docs. Closes-Bug: #1410565 Change-Id: I8fa3bc6d0af2ee90f436fe065b8897eefefa2be9
54 lines
2.4 KiB
XML
54 lines
2.4 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE section [
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<!ENTITY % openstack SYSTEM "../../common/entities/openstack.ent">
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%openstack;
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]>
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<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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version="5.0"
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xml:id="glance-nova-image-download">
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<title>Image download: how it works</title>
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<para>
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Prior to starting a virtual machine, the virtual machine image
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used must be transferred to the compute node from the Image
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Service. How this works can change depending on the settings
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chosen for the compute node and the Image Service.
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</para>
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<para>
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Typically, the Compute service will use the image identifier
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passed to it by the scheduler service and request the image from the
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Image API. Though images are not stored in glance—rather in a
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back end, which could be Object Storage, a filesystem or any other
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supported method—the connection is made from the compute node
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to the Image Service and the image is transferred over this
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connection. The Image Service streams the image from the back end to the
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compute node.
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</para>
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<para>
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It is possible to set up the Object Storage node on a separate network,
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and still allow image traffic to flow between the Compute and Object
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Storage nodes. Configure the <literal>my_block_storage_ip</literal>
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option in the storage node configuration to allow block storage traffic
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to reach the Compute node.
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</para>
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<para>
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Certain back ends support a more direct method, where on request
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the Image Service will return a URL that can be used to
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download the image directly from the back-end store. Currently the
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only store to support the direct download approach is the
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filesystem store. It can be configured using the
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<option>filesystems</option> option in the
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<literal>image_file_url</literal> section of the
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<filename>nova.conf</filename> file on compute nodes.
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</para>
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<para>
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Compute nodes also implement caching of images, meaning that if an
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image has been used before it won't necessarily be downloaded
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every time. Information on the configuration options for caching
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on compute nodes can be found in the <link
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xlink:href="http://docs.openstack.org/juno/config-reference/content/"><citetitle>Configuration
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Reference</citetitle></link>.
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</para>
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</section>
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