Shared backend config stanza

https://blueprints.launchpad.net/cinder/+spec/shared-backend-config

Add a new config section for shared backend configuration options.

Problem description

Right now there isn’t a great way to share config options between backends in a multi-backend cinder.conf. You can put things in DEFAULT, but drivers self.configuration won’t be able to see it if they were defined in their own stanza. You can just look directly at the DEFAULT section, but you then can’t easily override it in the driver backend section if you did want to have something special. As an example consider the following cinder.conf:

[DEFAULT]
enabled_backends = backend1, backend2, backend3
...
my_driver_param = foo
...

[backend1]
...
my_driver_param = bar

[backend2]
..
# not specifying the param

[backend3]
...
(also not specifying, maybe even a different volume driver, doesn't matter)

What we end up with is backend1 gets ‘bar’ and backend2/3 get whatever the default was for ‘my_driver_param’. To get them to use ‘foo’ we need something like:

[DEFAULT]
enabled_backends = backend1, backend2, backend3
...

[backend1]
...
my_driver_param = bar

[backend2]
..
my_driver_param = foo

[backend3]
...
my_driver_param = foo

Part of the confusion is that if you don’t use the multi-backend format you actually do want the options set in DEFAULT.

Use Cases

The main use-case here is for shared configs relevent to multiple backends in cinder.conf. Also to help lessen some of the confusion about shared config options in DEFAULT.

Proposed change

The new config section called ‘backend_defaults’ will contain config options that each backend has access to. If any options are also in the backend stanza that backend specific value will take precedence. It will be safe to assume that config options in shared will be shared and available for all backends. None of the guesswork from DEFAULT.

A new config then might look something like:

[DEFAULT]
enabled_backends = backend1, backend2, backend3
...

[backend_defaults]
my_driver_param = foo

[backend1]
...
my_driver_param = bar

[backend2]
..
# not defining the value, but it gets picked up

[backend3]
...
# same here..

This is the same as the example above where all backends define the value. This simplistic example doesn’t really show a ton of the benefits, so a slightly more realistic example might be:

[DEFAULT]
enabled_backends = pure1, pure2, pure3
...

[backend_defaults]
volume_driver = cinder.volume.drivers.pure.PureISCSIDriver
driver_ssl_verify = true
driver_ssl_cert = /etc/blah/my_cert
use_multipath_for_image_xfer = true
use_chap_auth = true
image_volume_cache_enabled = true
image_volume_cache_max_count = 15
image_volume_cache_max_size_gb = 200
max_over_subscription_ratio = 10
pure_eradicate_on_delete = false

[pure1]
san_ip = 1.2.3.4
pure_api_token = abcdef123456
max_over_subscription_ratio = 50

[pure2]
san_ip = 1.2.3.5
pure_api_token = abcdef1234567
image_volume_cache_max_count = 25

[pure3]
san_ip = 1.2.3.6
pure_api_token = abcdef12345678

So here we have a ton of shared config options, leaving only a few specific to each backend.

In addition to the shared config section we will deprecate setting backends via the DEFAULT section. We will only allow multi-backend style configs using the “enabled_backends” config option.

Alternatives

We could change the behavior so that things in DEFAULT do this, but it not be backwards compatible with older cinder.confs. Behavior would probably not be what deployers would expect when upgrading.

Data model impact

None

REST API impact

None

Security impact

None

Notifications impact

None

Other end user impact

None

Performance Impact

None

Other deployer impact

Would have a new config mechanism to learn, but older style configs would still work the same way. They will eventually need to migrate to using the “enabled_backends” config option, this process will follow the standard deprecation process.

Developer impact

Nothing other than needing to be aware of how drivers can get their config values.

Implementation

Assignee(s)

Primary assignee:
patrick-east

Work Items

  • Add a warning when initializing c-vol without “enabled_backends”
  • Modify the driver config utility to look for this section first, and override

values with backend specific ones if they are defined. * Add some unit tests * Incorporate into multi-backend tempest test configuration

Dependencies

None

Testing

  • Unit tests
  • Tempest test config with devstack (reach goal, depends on timing)

Documentation Impact

  • Updated driver config references
  • Driver dev documentation to explain how this works