Add support for basic image conversion

https://blueprints.launchpad.net/glance/+spec/basic-import-conversion

Change-Id: I67b87a6aba9bdb646a196b28fce04d7809ae833e

specs/kilo/conversion-of-images.rst

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