cinder/doc/source/contributor/drivers.rst

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.

Configuration options

Each driver requires different configuration options set in the cinder.conf file to operate, and due to the complexities of the Object Oriented programming mechanisms (inheritance, composition, overwriting, etc.) once your driver defines its parameters in the code Cinder has no automated way of telling which configuration options are relevant to your driver.

In order to assist operators and installation tools we recommend reporting the relevant options:

• For operators: In the documentation under doc/source/configuration/block-storage.
• For operators and installers: Through the get_driver_options static method returning that returns a list of all the Oslo Config parameters.

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

Security Requirements

• Drivers must delete volumes in a way where volumes deleted from the backend will not leak data into new volumes when they are created. Cinder operates in multi-tenant environments and this is critical to ensure data safety.
• Drivers should support secure TLS/SSL communication between the cinder volume service and the backend as configured by the "driver_ssl_cert_verify" and "driver_ssl_cert_path" options in cinder.conf.
• Drivers should use standard Python libraries to handle encryption-related functionality, and not contain custom implementations of encryption code.

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'.

NOTE: By default, Cinder assumes that the driver supports attached volume extending. If it doesn't, it should report 'online_extend_support=False'. Otherwise the scheduler will attempt to perform the operation, and may leave the volume in 'error_extending' state.

Value of storage_protocol is a single string representing the transport protocol used by the storage. Existing protocols are present in cinder.common.constants and should be used by drivers instead of string literals.

Variant values only exist for older drivers that were already reporting those values. New drivers must use non variant versions.

The storage_protocol can be used by operators using the cinder get-pools --detail command, by volume types in their extra specs, and by the filter and goodness functions.

We must not mistake the value of the storage_protocol with the identifier of the os-brick connector, which is returned by the initialize_connection driver method in the driver_volume_type dictionary key. In some cases they may have the same value, but they are different things.

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.

New Driver Review Checklist

There are some common issues caught during the review of new driver patches that can easily be avoided. New driver maintainers should review the new_driver_checklist for some things to watch out for.

new_driver_checklist

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.

Volume ID

Drivers should always get a volume's ID using the name_id attribute instead of the id attribute.

A Cinder volume may have two different UUIDs, a user facing one, and one the driver should use.

When a volume is created these two are the same, but when doing a generic migration (create new volume, then copying data) they will be different if we were unable to rename the new volume in the final migration steps.

So the volume will have been created using the new volume's UUID and the driver will have to look for it using that UUID, but the user on the other hand will keep referencing the volume with the original UUID.

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

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

Revert To Snapshot

Some storage backends support the ability to revert a volume to the last snapshot. To support snapshot revert, the following interface must be implemented by the driver.

cinder.interface.volume_snapshot_revert