Add spec for Cinder Nimblestorage backend

Change-Id: I3776380a744e7621e0388984c25b3f57b60c95ed

specs/yoga/approved/cinder-nimblestorage.rst

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+..
+  Copyright 2022 Canonical Ltd.
+
+  This work is licensed under a Creative Commons Attribution 3.0
+  Unported License.
+  http://creativecommons.org/licenses/by/3.0/legalcode
+
+..
+
+=============================================
+Cinder subordinate charm cinder-nimblestorage
+=============================================
+
+Problem Description
+===================
+
+Cinder has a great number of drivers for different backends. Almost all
+currently available commercial storage arrays and many other software only
+solutions have a driver available, if not already in the upstream cinder
+project, as an installable package that enables cinder with the extra driver.
+
+Because of how many possibilities exist, a model was developed where a separate
+subordinate charm will be deployed to take care of the specifics of the driver
+and pass configuration to the main cinder charm over a relation so that cinder
+can write to its own configuration file.
+
+Proposed Change
+===============
+
+Instead of changing the cinder charm, this proposal aims to create a separate
+subordinate charm specific for the nimblestorage driver. This cinder
+installation would enable access to Nimblestorage devices. The following points
+are of condiderable importance for the development of this subordinate charm:
+
+* The new implementation should not conflict with current implementation. Both
+  should co-exist, even in the same deployment if possible.
+* The new charm should implement all features currently existing in the cinder
+  charm, regarding the nimblestorage functionality.
+
+Note that because the Nimblestorage driver is part of the cinder-common package, no
+additional packages need to be installed for this charm to work.
+
+This charm would be supported in the Ussuri release of Openstack and newer.
+
+Alternatives
+------------
+
+At the time of this writing, there exist no alternatives.
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+    Gustavo Sanchez <gustavo.sanchez@canonical.com> - LP ID: gustavosr98
+
+Secondary assignee:
+    Luciano Lo Giudice <luciano.logiudice@canonical.com> - LP ID: lmlogiudice
+
+Gerrit Topic
+------------
+
+Use Gerrit topic "charm-cinder-nimblestorage" for all patches related to this spec.
+
+.. code-block:: bash
+
+    git-review -t charm-cinder-nimblestorage
+
+Work Items
+----------
+
+The charm, as listed above. Plus, both unit and functional tests will be
+written.
+
+Repositories
+------------
+
+Presently, the following repository hosts the charm code:
+
+https://opendev.org/openstack/charm-cinder-nimblestorage
+
+Documentation
+-------------
+
+Once this charm is completed, charm-guide will be updated to reference it.
+
+Security
+--------
+
+No security aspect is going to change in relation to the current solution.
+
+Testing
+-------
+
+Code changes will be covered by both unit and functional tests.
+Functional tests would require Nimblestorage hardware.
+
+Dependencies
+============
+
+No dependencies in addition to the ones that exist in the current solution.