add support for image cache

[Edward Hope-Morley]

If you boot a vm from a bootable volume and that volume, when that volume is created it might be necessary to convert the image to raw format if it is stored as qcow2 in Glance. By default Cinder will not cache downloaded images but doing so can save a lot of time when booting from volume where conversions and/or downloads are necessary.

https://docs.openstack.org/cinder/latest/admin/blockstorage-image-volume-cache.html

[David Ames (thedac)]

Bug hygiene:

Triage:

Adding the following to the template and any other requirements per the doc [0]

[ceph]
image_volume_cache_enabled

1) Add a config option to turn on the cache which should default to off
2) Update pertinent templates i.e. parts/backends
3) * Potentially add config options or calculations for image_volume_cache_max_size_gb and image_volume_cache_max_count.

* Keep in mind cinder instances are most often in a LXD container and may not have an infinite amount of disk space.

This would make an excellent on-boarder task.

[0] https://docs.openstack.org/cinder/latest/admin/blockstorage-image-volume-cache.html

[Eric Chen (eric-chen)]

I will start to do the implementation.
Here is the my initial proposal.

Add 3 configuration in charm-cinder

image-volume-cache-enabled:
   type: boolean
   default: False
   description: Enable the image volume cache

image-volume-cache-max-size-gb:
   type: int
   default: 0
   description: Max size of the image volume cache in GB. 0 => unlimited

image-volume-cache-max-count:
   type: int
   default: 0
   description: Max number of entries allowed in the image volume cache. 0 => unlimited.

When `image-volume-cache-enabled` is enabled, three configuration will be generated in cinder.conf under [DEFAULT]

```
[DEFAULT]
image_volume_cache_enabled = true
image_volume_cache_max_size_gb = 0
image_volume_cache_max_count = 0

```

I propose to put it under [DEFAULT] session to handle the basic situation: there is only 1 backend or all the backends use the same cache settings.

Anyone who are interested in this feature, welcome to leave your comment.

[Aurelien Lourot (aurelien-lourot)]

@Eric this sounds good and is in line with the upstream defaults. [1]

[1] https://docs.openstack.org/cinder/latest/configuration/block-storage/samples/cinder.conf.html

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/charm-cinder/+/808575

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/charm-cinder/+/808576

[OpenStack Infra (hudson-openstack)]

Change abandoned by "<email address hidden> <email address hidden>" on branch: master
Review: https://review.opendev.org/c/openstack/charm-cinder/+/808575

[Aurelien Lourot (aurelien-lourot)]

Release notes: https://review.opendev.org/c/openstack/charm-guide/+/808848

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/charm-cinder/+/808576
Committed: https://opendev.org/openstack/charm-cinder/commit/dd5137c2e77da52a27f7aceb47370f0b371029b3
Submitter: "Zuul (22348)"
Branch: master

commit dd5137c2e77da52a27f7aceb47370f0b371029b3
Author: eric-chen <email address hidden>
Date: Sun Sep 12 15:50:58 2021 +0800

    Add support for configuring image volume cache

    Support new configuration to turn on image volume cache in Cinder
    service. User can control the maximum size and count from
    configuration too.

    Closes-Bug: #1869903
    Change-Id: If96dbb9c0974bfa1f6d67405bb430a7cd251f821