Retrieve Supported Volume Type Extra Specs

Provide an interface to obtain a volume driver’s capabilities.

Problem description


  • Volume Type: A group of volume policies.

  • Extra Specs: The definition of a volume type. This is a group of policies e.g. provision type, QOS that will be used to define a volume at creation time.

  • Capabilities: What the current deployed backend in Cinder is able to do. These correspond to extra specs.

The current implementation of volume type extra specs management process in Horizon and the cinder client are error-prone. Operators manage extra specs without having guidance on what capabilities are possible with a backend. Without knowing the capabilities, the operator doesn’t know what the possible extra spec key/values are.

Today operators have to make sure they’re reading the right documentation that corresponds to the version of their storage backend. Documentation can become out of date with their volume driver.

It would be more convenient, and a better user experience if the operator was able to ask Cinder for the capabilities of the current deployed storage backends, instead of having to guess.

Use Cases

As an operator, I want to get a list of the capabilities for my deployed storage backends in Cinder.

With these capabilities, I have keys and possible values that can be used within a volume type’s extra specs.

Potentially with the API endpoint, Horizon could use it to improve the GUI for managing extra specs.

Proposed change

Cinder volume drivers will implement a new method get_capabilities, which will return a dictionary of supported capabilities from the storage backend.

The dictionary will include some brief information about the backend, and capabilities that correspond to extra spec keys and values.

Features like create volume, create snapshots, etc are considered minimum features [1]. Unlike well defined keys [2], minimum features are required to implement. With well defined keys, drivers just need to report if the capability is not supported. If the backend has some vendor unique capabilities, the backend driver can also define vendor unique keys for supported capabilities.


Operators could continue to guess what extra specs they should be setting in volume types. Operators could attempt to find documentation for volume type extra specs. As a last resort, operators could dig through different Cinder volume driver code to figure out the possible extra types. These aren’t reliable solutions.

Data model impact


REST API impact

New endpoint GET /v2/tenant/get_capabilities/ubuntu@lvm1_pool:

  "namespace": "OS::Storage::Capabilities::ubuntu@lvm1_pool",
  "volume_backend_name": "lvm",
  "pool_name": "pool",
  "driver_version": "2.0.0",
  "storage_protocol": "iSCSI",
  "display_name": "Capabilities of Cinder LVM driver",
   "These are volume type options provided by Cinder LVM driver, blah, blah.",
  "visibility": "public",
  "properties": {
   "thin_provisioning": {
      "title": "Thin Provisioning",
      "description": "Sets thin provisioning.",
      "type": "boolean"
    "compression": {
      "title": "Compression",
      "description": "Enables compression.",
      "type": "boolean"
    "vendor:compression_type": {
      "title": "Compression type",
      "description": "Specifies compression type.",
      "type": "string",
      "enum": [
        "lossy", "lossless", "special"
    "replication": {
      "title": "Replication",
      "description": "Enables replication.",
      "type": "boolean"
    "qos": {
      "title": "QoS",
      "description": "Enables QoS.",
      "type": "boolean"
    "vendor:minIOPS": {
      "title": "Minimum IOPS QoS",
      "description": "Sets minimum IOPS if QoS is enabled.",
      "type": "integer"
    "vendor:maxIOPS": {
      "title": "Maximum IOPS QoS",
      "description": "Sets maximum IOPS if QoS is enabled.",
      "type": "integer"
    "vendor:minIOPS": {
      "title": "Burst IOPS QoS",
      "description": "Sets burst IOPS if QoS is enabled.",
      "type": "integer"
    "vendor:persona": {
      "title": "Persona",
      "description": "I am something..." ,
      "default": "Generic",
      "enum": [

The well defined keys are indicated without a prefix like the “qos”. These are fairly standard base keys for Cinder backends. We expect most devices to report at least a boolean True/False for these keys.

The vendor unique keys are optional and are indicated with a prefix of vendor name + colon(:). (ex. abcd:minIOPS) Vendor driver can use anything for the vendor unique keys, but the vendor name prefix shouldn’t contain colon because of the separator and it will be automatically replaced by underscore(_). (abc:d -> abc_d)

Let’s look at compression here:

This is a well defined key, we expect devices to report True or False regarding whether they support it or not. In the case where not only does a device support it, but it can be configured, the option keys are listed under the “options” portion. This is simply the <key-name> of the option, and a list of valid values that can be specified for it. NOTE, if the options key is empty ({}) that means there are NO options that can be set on that capability key.

The vendor:fireproof capability:

This is a vendor unique key, and is indicated by being prefixed with “vendor name” + “:”. Also, note that the default is True and that there are NO options. The example indicates that this device is ALWAYS fireproof, you can’t change that, it just is, what it is.

The thin_provisioning capability:

This is a well defined key which is not supported by this particular vendor. As a result, it defaults to False, and provides no options.

The qos capability describes some corner cases for us:

This key is a well defined key, that’s very customizable via options. Well defined portion is whether the capability is supported or not (again True/False), again, some devices may allow deploying volumes with or without QoS on the same device, so you can specify that with <capability-key-name>=true|false.

If a device doesn’t support this (ie always true), then this entry is omitted.

The other key piece is vendor unique keys. For those that allow additional special keys to set QoS those key names are provided in list format as valid keys that can be specified and set as related to Quality of Service.

The vendor:persona key is another good example of a vendor unique capability:

This is very much like QoS, and again, note that we’re just providing what the valid values are.

You’ll notice that the data-structure follows the settings you would put in your extra-specs. This particular case doesn’t have any options other than the base key itself, so the structure is rather simple.

Security impact

The endpoint mentioned in the API Impact will only be available through the admin_api policy. Operators or other OpenStack services will have the ability to query this information.

Notifications impact


Other end user impact

Cinder Client Example:

The operator wants to define new volume types for their OpenStack cloud. The operator would fetch a list of capabilities for a particular backend’s pool:

First get list of services:

$ cinder service-list

With one of the listed pools, pass that to capabilities-list:

$ cinder capabilities-list block1@lvm#pool

host_name: block1
volume_backend_name: lvm
pool_name: pool
driver_version: 2.0.0
storage_protocol: iSCSI


    default: true
      compression: true, false
      compression_type: lossy, lossless, special

    default: false

    default: false

    default: true,
      qos: true, false

    default: true
    options: {}

    default: Generic


Horizon will be updated to include the displaying of the supported capabilities so operators can select and set the values while creating or editing the volume type extra specs.

If the volume type does not have any volume backend name associated with it, Horizon will not have any extra specs keys to display. Administrators can still enter in key/value pairs of their own. This is the same behavior as the current process.

If a driver does not publish the extra specs dictionary, which will be the case for any drivers that do not get updated, then no client-side filtering will be performed, and the behavior will basically revert to the current situation where the administrator in horizon will need to know and enter the key/value pairs without any additional guidance.

Performance Impact


Other deployer impact


Developer impact

Driver maintainers would need to implement a method get_capabilities. This should fetch from the storage backend a list of capabilities and translate it to the dictionary structure:

  'driver_version:' '2.0.0',
  'storage_protocol:' iSCSI,
  'capabilities:' {
    'compression': {
      'default': True,
      'options': {
        'compression_type': ['lossy', 'lossless', 'special'],
        'compression': [True, False]
    'thin_provisioning': {
      'default': True,
      options: {
        'thin_provisioning': [True, False]
    'qos': {
      'default': True,
      options: {
        'qos': [True, False],
   'replication': {
     'default': True,
     options: {
       replication: [True, False]

There’s nothing keeping a vendor reporting fewer or more keys, but the following are strictly enforced:

  • The data structure

  • The information in the capabilities

  • The well defined capabilities.

  • Driver version

  • Storage protocol



Primary assignee:
Other contributors:

Work Items

  • Add new endpoint to Cinder API.

  • Add RPC call.

  • Add new volume manager method for get_capabilities.

  • Update LVM reference implementation with get_capabilities method.


The decision on what the well defined capabilities will be [2].


  • Unit tests

  • Eventually, tempest tests once all drivers are supporting it.

Documentation Impact

Update the Cinder developer documentation for driver maintainers to reference how to push capabilities from their volume driver.