openstack/openstack-manuals
Code Issues Proposed changes

Explain how image download works to compute nodes

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This patch adds a new section to the cloud admin guide that explains
the various features that affect how an image is downloaded to a compute
node.

Change-Id: I2bc3bdacf58815584a538de1663fd1bdb2944b5c
Closes-Bug: 1202891
This commit is contained in:
Tom Fifield 2014-07-17 21:10:14 -07:00 committed by Andreas Jaeger
parent 10106b24fc
commit bd4a5fcfcb
2 changed files with 47 additions and 0 deletions
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1
doc/admin-guide-cloud/compute/section_compute-images-instances.xml
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@ -99,6 +99,7 @@
</figure>
<xi:include href="section_compute-image-mgt.xml"/>
<xi:include href="../image/section_glance-property-protection.xml"/>
<xi:include href="../image/section_glance-nova-image-download.xml"/>
<xi:include href="section_compute-instance-building-blocks.xml"/>
<xi:include href="section_compute-instance-mgt-tools.xml"/>
<section xml:id="section_instance-scheduling-constraints">

46
doc/admin-guide-cloud/image/section_glance-nova-image-download.xml Normal file
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@ -0,0 +1,46 @@
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE section [
<!ENTITY % openstack SYSTEM "../../common/entities/openstack.ent">
%openstack;
]>
<section xmlns="http://docbook.org/ns/docbook"
xmlns:xi="http://www.w3.org/2001/XInclude"
xmlns:xlink="http://www.w3.org/1999/xlink"
version="5.0"
xml:id="glance-nova-image-download">
<title>Image download: how it works</title>
<para>
Prior to starting a virtual machine, the virtual machine image
used must be transferred to the compute node from the Image
Service. How this works can change depending on the settings
chosen for the compute node and the Image Service.
</para>
<para>
Typically, the Compute service will use the image identifier
passed to it by the scheduler service and request the image from the
Image API. Though images are not stored in glance&mdash;rather in a
back end, which could be Object Storage, a filesystem or any other
supported method&mdash;the connection is made from the compute node
to the Image Service and the image is transferred over this
connection. The Image Service streams the image from the back end to the
compute node.
</para>
<para>
Certain back ends support a more direct method, where on request
the Image Service will return a URL that can be used to
download the image directly from the back-end store. Currently the
only store to support the direct download approach is the
filesystem store. It can be configured using the
<option>filesystems</option> option in the
<literal>image_file_url</literal> section of the
<filename>nova.conf</filename> file on compute nodes.
</para>
<para>
Compute nodes also implement caching of images, meaning that if an
image has been used before it won't necessarily be downloaded
every time. Information on the configuration options for caching
on compute nodes can be found in the <link
xlink:href="http://docs.openstack.org/trunk/config-reference/content/"><citetitle>Configuration
Reference</citetitle></link>.
</para>
</section>