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Drivers
Cinder exposes an API to users to interact with different storage backend solutions. The following are standards across all drivers for Cinder services to properly interact with a driver.
Basic attributes
There are some basic attributes that all drivers classes should have:
- VERSION: Driver version in string format. No naming convention is imposed, although semantic versioning is recommended.
- CI_WIKI_NAME: Must be the exact name of the ThirdPartySystems wiki page. This is used by our tooling system to associate jobs to drivers and track their CI reporting status correctly.
The tooling system will also use the name and docstring of the driver class.
Minimum Features
Minimum features are enforced to avoid having a grid of what features are supported by which drivers and which releases. Cinder Core requires that all drivers implement the following minimum features.
Core Functionality
- Volume Create/Delete
- Volume Attach/Detach
- Snapshot Create/Delete
- Create Volume from Snapshot
- Get Volume Stats
- Copy Image to Volume
- Copy Volume to Image
- Clone Volume
- Extend Volume
Volume Stats
Volume stats are used by the different schedulers for the drivers to provide a report on their current state of the backend. The following should be provided by a driver.
- driver_version
- free_capacity_gb
- storage_protocol
- total_capacity_gb
- vendor_name
- volume_backend_name
NOTE: If the driver is unable to provide a value for free_capacity_gb or total_capacity_gb, keywords can be provided instead. Please use 'unknown' if the backend cannot report the value or 'infinite' if the backend has no upper limit. But, it is recommended to report real values as the Cinder scheduler assigns lowest weight to any storage backend reporting 'unknown' or 'infinite'.
Feature Enforcement
All concrete driver implementations should use the
cinder.interface.volumedriver decorator on the driver
class:
@interface.volumedriver
class LVMVolumeDriver(driver.VolumeDriver):This will register the driver and allow automated compliance tests to run against and verify the compliance of the driver against the required interface to support the Core Functionality listed above.
Running tox -e compliance will verify all registered
drivers comply to this interface. This can be used during development to
perform self checks along the way. Any missing method calls will be
identified by the compliance tests.
The details for the required volume driver interfaces can be found in
the cinder/interface/volume_*_driver.py source.
Driver Development Documentations
The LVM driver is our reference for all new driver implementations. The information below can provide additional documentation for the methods that volume drivers need to implement.
Base Driver Interface
The methods documented below are the minimum required interface for a volume driver to support. All methods from this interface must be implemented in order to be an official Cinder volume driver.
cinder.interface.volume_driver
Snapshot Interface
Another required interface for a volume driver to be fully compatible is the ability to create and manage snapshots. Due to legacy constraints, this interface is not included in the base driver interface above.
Work is being done to address those legacy issues. Once that is complete, this interface will be merged with the base driver interface.
cinder.interface.volume_snapshot_driver
Manage/Unmanage Support
An optional feature a volume backend can support is the ability to manage existing volumes or unmanage volumes - keep the volume on the storage backend but no longer manage it through Cinder.
To support this functionality, volume drivers must implement these methods:
cinder.interface.volume_manageable_driver
Manage/Unmanage Snapshot Support
In addition to the ability to manage and unmanage volumes, Cinder backend drivers may also support managing and unmanaging volume snapshots. These additional methods must be implemented to support these operations.
cinder.interface.volume_snapshotmanagement_driver
Volume Consistency Groups
Some storage backends support the ability to group volumes and create write consistent snapshots across the group. In order to support these operations, the following interface must be implemented by the driver.
cinder.interface.volume_consistencygroup_driver
Generic Volume Groups
The generic volume groups feature provides the ability to manage a group of volumes together. Because this feature is implemented at the manager level, every driver gets this feature by default. If a driver wants to override the default behavior to support additional functionalities such as consistent group snapshot, the following interface must be implemented by the driver. Once every driver supporting volume consistency groups has added the consistent group snapshot capability to generic volume groups, we no longer need the volume consistency groups interface listed above.
cinder.interface.volume_group_driver