47 lines
2.0 KiB
XML
47 lines
2.0 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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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/trunk/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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