Generic image-volume cache

Generic image-volume cache

This introduces a new feature for backends to be able to use cached
glance images when creating volumes from images.

If enabled, and the cinder internal tenant is configured, it will
create a minimal sized clone of a volume the first time it is
downloaded from glance. A new db table ‘image_volume_cache’ tracks
these volumes, which are owned by the Cinder internal tenant. The
original will then be extended to full size. Any subsequent requests to
create a volume from an image will be able to do a volume clone from
the image-volume cache entry instead of downloading the image data from
glance again.

In the future we could create an entry upon creating an image from a
volume right before uploading the volume to glance. This version
however does not aim to do more than add the initial framework and
help with subsequent image downloads. There are certainly more
improvements that can be done over time on top of this.

These image-volumes are host specific, so each backend may end up with
its very own image-volume to do clones from.

The cache is limited in size by number of entries allowed and size in
gb. When creating a new entry if space is needed the last used entries
will be evicted to make room.

In the current implementation the image-volumes will be viewable by a
cloud admin in the volume list, and show up as owned by the Cinder
internal tenant. They can safely be deleted manually by an admin, this
will cause the entry to be removed from the cache. These volumes will
use quota for the internal tenant.

Cache actions will post notifications to Ceilometer. There are
notifications like ‘image_volume_cache.miss’, ‘image_volume_cache.hit’,
and ‘image_volume_cache.evict’. A change will be required to the
event_definitions.yaml to see them nicely. Until then you only need to
add a new event type 'image_volume_cache.*' and look for the ‘image_id’
and ‘host’ fields.

DocImpact: We probably want some instructions on restrictions of the
cache and how to use it. The three new config options should also be
documented somewhere: ‘image_volume_cache_enabled’,
’image_volume_cache_max_size_gb’, ‘image_volume_cache_max_size_count’

Co-Authored-By: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Implements: blueprint image-volume-cache
Change-Id: If22bbaff89251e4e82a715170a48b4040f95c09f