Deprecate CentOS 6 images

Change-Id: I579865795a409bc4710c0bfbaf4d0aa07bc63cff

specs/pike/deprecate-centos6-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
+
+==============================
+Deprecation of CentOS 6 images
+==============================
+
+https://blueprints.launchpad.net/sahara/+spec/deprecate-centos6-images
+
+Starting from the Newton release, the images based on CentOS 6
+are available also with CentOS 7, sometimes even with more choices for
+CentOS 7 version (CDH).
+This spec propose to deprecate and remove the support for CentOS 6
+images.
+
+
+Problem description
+===================
+
+Keeping the support for CentOS 6 is increasinglly difficult.
+
+The cloud image provided by the CentOS team can not be used as it is
+(lack of resize) so a special image should be prepared. This is
+`documented <http://git.openstack.org/cgit/openstack/sahara-image-elements/tree/diskimage-create/Create_CentOS_cloud_image.rst?h=6.0.1>`_
+but the default image, which should be manually regenerated, is hosted on
+sahara-files.mirantis.com, which will be discontinued.
+
+Also, diskimage-builder's support for CentOS 6 is not so effective
+as it should be, as most of the focus is (rightfully) on CentOS 7.
+
+Example of issues which requires a workaround:
+
+* https://bugs.launchpad.net/diskimage-builder/+bug/1534387
+* https://bugs.launchpad.net/diskimage-builder/+bug/1477179
+
+A blocker bug right now is:
+
+* https://bugs.launchpad.net/diskimage-builder/+bug/1698551
+
+The (non-blocking, even if they should be blocking) gate jobs for
+sahara-image-builder fails due to the latter.
+
+
+Proposed change
+===============
+
+The support for CentOS 6 images should be deprecated starting from Pike and
+removed as soon as the compliance with the follows-standard-deprecation
+allows us to do it.
+
+The change mainly affects sahara-image-elements. The CentOS 6 would not be
+built anymore by default while building all the images for a certain plugin
+and a warning message would be printed if one of them is selected.
+
+The code path which checks for CentOS 6 in Sahara services should be kept
+as they are and not changed as long as the features is available even if
+deprecated; after the removal the code can be restructured, if needed,
+to not consider the CentOS 6 use case.
+
+Alternatives
+------------
+
+Keep CentOS 6 support until it is retired officially (November 30, 2020)
+or until diskimage-builder removes the support, but make sure that the
+current issues are fixed. A change is needed anyway in the
+sahara-image-elements jobs, as the building fails right now.
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Other end user impact
+---------------------
+
+Users won't be able to use CentOS 6 as base.
+
+Deployer impact
+---------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+Sahara-image-elements impact
+----------------------------
+
+Most of the described changes are in sahara-image-elements (see above).
+
+Sahara-dashboard / Horizon impact
+---------------------------------
+
+Minor: remove the reference to CentOS 6 and the default cloud image
+from the image registration panel when the feature is removed.
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  ltoscano
+
+
+Work Items
+----------
+
+- do not build CentOS 6 image by default for a certain plugin
+- add a warning message if one of them is requested
+- inform the operators (openstack-operators@) about the change to evaluate
+  the time for the removal
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+If the change is implemented, the existing jobs for sahara-image-elements
+will only test the supported images and won't fail.
+
+
+Documentation Impact
+====================
+
+Add or change the list of supported base images.
+
+
+References
+==========
+
+None