Merge "Add spec for Ceph PG Autotuning"

2019-08-12 14:30:10 +00:00 · 2019-08-12 14:30:10 +00:00 · 9e914fc328
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+..
+  Copyright <YEARS> <HOLDER>  <--UPDATE THESE
+
+  This work is licensed under a Creative Commons Attribution 3.0
+  Unported License.
+  http://creativecommons.org/licenses/by/3.0/legalcode
+
+..
+  This template should be in ReSTructured text. Please do not delete
+  any of the sections in this template.  If you have nothing to say
+  for a whole section, just write: "None". For help with syntax, see
+  http://sphinx-doc.org/rest.html To test out your formatting, see
+  http://www.tele3.cz/jbar/rest/rest.html
+
+===============================
+Ceph PG Auto Tuning
+===============================
+
+In the Nautilus release, Ceph has introduced placement group (PG) autotuning.
+The charms should support this module allowing a Ceph cluster to grow or shrink
+naturally.
+
+Problem Description
+===================
+
+If an existing cluster is incorrectly sized at deploy time, it is very
+difficult for an operator to resolve the imbalance. PG Autotuning was
+designed to remove the black-magic of PG tuning.
+
+Proposed Change
+===============
+
+A new configuration option (pg-autotune) will be introduced to enable the new
+pg_autoscaler module. This config option will have no effect for Ceph releases
+before Nautilus and will have no default.
+
+In a new Ceph deployment with a release greater than Nautilus, a null value for
+'pg-autotune' will cause the autoscaler module to be enabled and configured on
+all pools, with the global default additionally configured as ON. In an upgrade
+to Nautilus, a null value will not cause the autoscaler module to be loaded,
+nor will pool metadata change.
+
+For an existing deployment to enable the autoscaler, an administrator should
+change the config value to 'True', after which the charm will process the new
+option, as well as updating the existing pools as necessary.
+
+The ceph-mon charms already handle sizing the default placement groups via the
+expected ratios so the required data is already present.
+
+Alternatives
+------------
+
+The current solution is a viable alternative, as long as the expectations about
+pool sizing is correct at deploy time. Other alternatives include manual
+operator intervention to resolve an incorrectly sized pool.
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  chris.macnaughton
+
+Gerrit Topic
+------------
+
+Use Gerrit topic "pg-autotune" for all patches related to this spec.
+
+.. code-block:: bash
+
+    git-review -t pg-autotune
+
+Work Items
+----------
+
+- Enable the pg_autoscaler manager module if the Ceph version is
+  Nautilus or higher
+- Disable the autoscaler globally, by default
+- Enable the autoscaler and configure pools when toggled via configuration
+
+Repositories
+------------
+
+No new repositories will need to be created.
+
+The charms.ceph and charm-ceph-mon repositories will be the primary focus of
+this development.
+
+Documentation
+-------------
+
+There will be a new configuration option added to toggle on the
+autoscaler for the Ceph cluster. This configuration option should additionally
+be called out in the README for the ceph-mon charm.
+
+Security
+--------
+
+There are no expected security risks associated with this change.
+
+Testing
+-------
+
+An additional test-case will be added enabling this config option to ensure
+that the functionality works as expected. Additionally, the functionality will
+be validated via inspection of Ceph's suggestions for auto-tuning.
+
+Dependencies
+============
+
+There are no new dependencies.