Add image cache to nova-powervm

This is a blue print add for the Ocata release.  It outlines the
inclusion of the image cache to the PowerVM nova driver.

blueprint: image-cache-powervm
Change-Id: Icb2786bb54280f6a864adaf9e8e2071f2f61c0c6

specs/ocata/image_cache.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
+
+========================================
+Image Cache Support for localdisk driver
+========================================
+
+https://blueprints.launchpad.net/nova-powervm/+spec/image-cache-powervm
+
+The image cache allows for a nova driver to pull an image from glance once,
+then use a local copy of that image for future VM creation.  This saves
+bandwidth between the compute host and glance.  It also improves VM
+deployment speed and reduces the stress on the overall infrastructure.
+
+
+Problem description
+===================
+
+Deploy times on PowerVM can be high when using the localdisk driver.  This is
+partially due to not having linked clones.  The image cache offers a way to
+reduce those deploy times by transferring the image to the host once, and then
+subsequent deploys will reuse that image rather than streaming from glance.
+
+There are complexities with this of course.  The cached images take up disk space,
+but the overall image cache from core Nova takes that into account.  The value
+of using the nova image cache design is that it has hooks in the code to help solve
+these problems.
+
+
+Use Cases
+---------
+
+ - As an end user, subsequent deploys of the same image should go faster
+
+
+Proposed change
+===============
+
+Create a subclass of nova.virt.imagecache.ImageManager in the nova-powervm
+project. It should implement the necessary methods of the cache:
+ - _scan_base_images
+ - _age_and_verify_cached_images
+ - _get_base
+ - update
+
+The nova-powervm driver will need to be updated to utilize the cache.  This
+includes:
+ - Implementing the manage_image_cache method
+ - Adding the has_imagecache capability
+
+The localdisk driver within nova-powervm will be updated to have the
+following logic.  It will check the volume group backing the instance.  If the
+volume group has a disk with the name 'i_<partial uuid of image>', it will
+simply copy that disk into a new disk named after the UUID of the instance.
+Otherwise, it will create a disk with the name 'i_<partial uuid of image>'
+that contains the image.
+
+The image cache manager's purpose is simply to clean out old images that are
+not needed by any instances anymore.
+
+Further extension, not part of this blueprint, can be done to manage overall
+disk space in the volume group to make sure that the image cache is not
+overwhelming the backing disks.
+
+Alternatives
+------------
+
+ - Leave as is, all deploys potentially slow
+ - Implement support for linked clones.  This is an eventual goal, but
+   the image cache is still needed in this case as it will also manage the
+   root disk image.
+
+
+Security impact
+---------------
+
+None
+
+
+End user impact
+---------------
+
+None
+
+
+Performance Impact
+------------------
+
+Performance of subsequent deploys of the same image should be faster.
+The deploys will have improved image copy times and reduced network
+bandwith requirements.
+
+Performance of single deploys using different images will be slower.
+
+
+Deployer impact
+---------------
+
+This change will take effect without any deployer impact immediately after
+merging.  The deployer will not need to take any specific upgrade actions to
+make use of it; however the deployer may need to tune the image cache to make
+sure it is not using too much disk space.
+
+A conf option may be added to force the image cache off if deemed necessary.
+This will be based off of operator feedback in the event that we need a way
+to reduce disk usage.
+
+
+Developer impact
+----------------
+
+None
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  tjakobs
+
+Other contributors:
+  None
+
+Work Items
+----------
+
+* Implement the image cache code for the PowerVM driver
+
+* Include support for the image cache in the PowerVM driver.  Tolerate it
+  for other disk drivers, such as SSP.
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+* Unit tests for all code
+
+* Deployment tests in local environments to verify speed increases
+
+
+Documentation Impact
+====================
+
+The deployer docs will be updated to reflect this.
+
+
+References
+==========
+
+None
+
+
+History
+=======
+
+Optional section intended to be used each time the spec is updated to describe
+new design.
+
+.. list-table:: Revisions
+   :header-rows: 1
+
+   * - Release Name
+     - Description
+   * - Newton
+     - Introduced