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173 lines
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This work is licensed under a Creative Commons Attribution 3.0 Unported
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License.
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http://creativecommons.org/licenses/by/3.0/legalcode
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==============================
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Basic conversion of Images
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==============================
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https://blueprints.launchpad.net/glance/+spec/basic-import-conversion
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Some store engines work better when working with specific image
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formats. For example, in the case of ceph, having a raw image improves
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performance and allows ceph to do smarter things with the image
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data. Therefore, this spec proposes adding a basic support to image
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conversion that will give operators full control on the destination
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format. The proposed change is the starting point for more complex
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conversion operations.
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Problem description
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===================
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With the support of asynchronous operations and the new import
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workflow in Glance (see the dependency tree), it will be possible to
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have an image format conversion on the fly while importing an image.
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The conversion will be provided by a plugin of the import
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workflow. This plugin can be activated or not based on the deployer
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configuration. This means that the deployer will need to specify the
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preferred format of images for the deployment.
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This blueprint will handle the conversion of formats supported by
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qemu-img convert: raw, qcow2, vdi, vmdk and vpc.
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Internally, Glance will receive the bits of the image in a XX
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format. These bits will be stored in a temporary location. The plugin
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will be triggered to convert the image to its target format YY and
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moved to its final destination. When the task is finished, the
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temporary location is deleted. This means that the format uploaded
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initially is not kept by Glance.
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Proposed change
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===============
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Using `Glance's Asynchronous Workers`_, we can execute a background
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task that converts images from the source format to a pre-configured
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format.
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.. _Glance's Asynchronous Workers:
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https://blueprints.launchpad.net/glance/+spec/async-glance-workers
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Alternatives
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------------
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Let the user convert images themselves and upload the final, converted, file.
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Data model impact
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-----------------
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None
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REST API impact
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---------------
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None
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Security impact
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---------------
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None
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Notifications impact
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--------------------
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None
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Other end user impact
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---------------------
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None
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Performance Impact
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------------------
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Based on the maximum number of workers and the size that the administrator has
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configured for the conversion workspace, performance could be negatively
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affected if the operator misconfigures their maximum number of workers and
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allocated workspace.
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In addition to the above, it's also important to consider the time
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required to download the image locally, convert it and then upload it
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to the final store. The implementation proposes using ``qemu-image``,
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which executes random accesses to the image data.
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Other deployer impact
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---------------------
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When configuring this feature, operators will need to know the average size of
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the images that they are managing and appropriately configure the
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``max_workers`` setting and an appropriate amount of space for the workers to
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use.
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Developer impact
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----------------
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None
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Implementation
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==============
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Assignee(s)
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-----------
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Primary assignee:
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flaper87
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Other contributors:
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None
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Reviewers
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---------
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Core reviewer(s):
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jokke\_
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nikhil-komawar
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kragniz
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icordasc
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Work Items
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----------
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- Create specific workers for different conversion tasks
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- Create workflows for tasks
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- Update configuration files with additions
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- Update configuration documentation
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Dependencies
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============
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This depends on `Glance's Asynchronous Workers`_.
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Testing
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=======
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Unit testing will be needed for the conversion tasks and for the new task
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flows.
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Documentation Impact
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====================
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This may have an impact on the upgrade and installation parts of the
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documentation. For operators upgrading, they'll need to understand how to
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properly configure a system for image conversion. For new users, they'll
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need to be warned about appropriately allocating space for the workers to use
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and possibly choosing a more conservative maximum worker number than is
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default until they can determine the appropriate number.
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References
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==========
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None
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