How to implement replication features in a backend driver.
For backend devices that offer replication features, Cinder provides a common mechanism for exposing that functionality on a volume per volume basis while still trying to allow flexibility for the varying implementation and requirements of all the different backend devices.
Most of the configuration is done via the cinder.conf file under the driver section and through the use of volume types.
NOTE: This implementation is intended to solve a specific use case. It’s critical that you read the Use Cases section of the spec here: https://specs.openstack.org/openstack/cinder-specs/specs/mitaka/cheesecake.html
The cinder.conf file is used to specify replication config info for a specific driver. There is no concept of managed vs unmanaged, ALL replication configurations are expected to work by using the same driver. In other words, rather than trying to perform any magic by changing host entries in the DB for a Volume etc, all replication targets are considered “unmanged” BUT if a failover is issued, it’s the drivers responsiblity to access replication volumes on the replicated backend device.
This results in no changes for the end-user. For example, He/She can still issue an attach call to a replicated volume that has been failed over, and the driver will still receive the call BUT the driver will need to figure out if it needs to redirect the call to the a different backend than the default or not.
Information regarding if the backend is in a failed over state should be stored in the driver, and in the case of a restart, the service entry in the DB will have the replication status info and pass it in during init to allow the driver to be set in the correct state.
In the case of a failover event, and a volume was NOT of type replicated, that volume will now be UNAVAILABLE and any calls to access that volume should return a VolumeNotFound exception.
replication_device
Is a multi-dict opt, that should be specified for each replication target device the admin would like to configure.
NOTE:
There is one standaredized and REQUIRED key in the config entry, all others are vendor-unique:
An example driver config for a device with multiple replication targets is show below:
.....
[driver-biz]
volume_driver=xxxx
volume_backend_name=biz
[driver-baz]
volume_driver=xxxx
volume_backend_name=baz
[driver-foo]
volume_driver=xxxx
volume_backend_name=foo
replication_device = backend_id:vendor-id-1,unique_key:val....
replication_device = backend_id:vendor-id-2,unique_key:val....
In this example the result is self.configuration.get(‘replication_device) with the list:
[{backend_id: vendor-id-1, unique_key: val1},
{backend_id: vendor-id-2, unique_key: val1}]
In order for a user to specify they’d like a replicated volume, there needs to be a corresponding Volume Type created by the Cloud Administrator.
There’s a good deal of flexibility by using volume types. The scheduler can send the create request to a backend that provides replication by simply providing the replication=enabled key to the extra-specs of the volume type.
For example, if the type was set to simply create the volume on any (or if you only had one) backend that supports replication, the extra-specs entry would be:
{replication: enabled}
It’s up to the driver to parse the volume type info on create and set things up as requested. While the scoping key can be anything, it’s strongly recommended that all backends utilize the same key (replication) for consistency and to make things easier for the Cloud Administrator.
Additionally it’s expected that if a backend is configured with 3 replciation targets, that if a volume of type replication=enabled is issued against that backend then it will replicate to ALL THREE of the configured targets.
The following entries are expected to be added to the stats/capabilities update for replication configured devices:
stats["replication_enabled"] = True|False
stats["replication_targets"] = [<backend-id_1, <backend-id_2>...]
NOTICE, we report configured replication targets via volume stats_update This information is added to the get_capabilities admin call.
The number of API methods associated with replication is intentionally very limited,
Admin only methods.
Additionally we have freeze/thaw methods that will act on the scheduler but may or may not require something from the driver:
freeze_backend(self, context)
thaw_backend(self, context)
replication_failover
Used to instruct the backend to fail over to the secondary/target device. If not secondary is specified (via backend_id argument) it’s up to the driver to choose which device to failover to. In the case of only a single replication target this argument should be ignored.
Note that ideally drivers will know how to update the volume reference properly so that Cinder is now pointing to the secondary. Also, while it’s not required, at this time; ideally the command would act as a toggle, allowing to switch back and forth between primary and secondary and back to primary.
Keep in mind the use case is that the backend has died a horrible death and is no longer valid. Any volumes that were on the primary and NOT of replication type should now be unavailable.
NOTE: We do not expect things like create requests to go to the driver and magically create volumes on the replication target. The concept is that the backend is lost, and we’re just providing a DR mechanism to preserve user data for volumes that were speicfied as such via type settings.
freeze_backend Puts a backend host/service into a R/O state for the control plane. For example if a failover is issued, it is likely desireable that while data access to existing volumes is maintained, it likely would not be wise to continue doing things like creates, deletes, extends etc.
thaw_backend Clear frozen control plane on a backend.