Flavour and Image defined ephemeral storage encryption


This spec outlines a new approach to ephemeral storage encryption in Nova allowing users to select how their ephemeral storage is encrypted at rest through the use of flavors with specific extra specs or images with specific properties. The aim being to bring the ephemeral storage encryption experience within Nova in line with the block storage encryption implementation provided by Cinder where user selectable encrypted volume types are available.

Note that this spec will only cover the high level changes to the API and compute layers, implementation within specific virt drivers is left for separate specs.

Problem description

At present the only in-tree ephemeral storage encryption support is provided by the libvirt virt driver when using the lvm imagebackend. The current implementation provides basic operator controlled and configured host specific support for ephemeral disk encryption at rest where all instances on a given compute are forced to use encrypted ephemeral storage using the dm-crypt PLAIN encryption format.

This is not ideal and makes ephemeral storage encryption completely opaque to the end user as opposed to the block storage encryption support provided by Cinder where users are able to opt-in to using admin defined encrypted volume types to ensure their storage is encrypted at rest.

Additionally the current implementation uses a single symmetric key to encrypt all ephemeral storage associated with the instance. As the PLAIN encryption format is used there is no way to rotate this key in-place.

Use Cases

  • As a user I want to request that all of my ephemeral storage is encrypted at rest through the selection of a specific flavor or image.

  • As a user I want to be able to pick how my ephemeral storage is encrypted at rest through the selection of a specific flavor or image.

  • As an admin/operator I want to either enforce ephemeral encryption per flavor or per image.

  • As an admin/operator I want to provide sane choices to my end users regarding how their ephemeral storage is encrypted at rest.

  • As a virt driver maintainer/developer I want to indicate that my driver supports ephemeral storage encryption using a specific encryption format.

  • As a virt driver maintainer/developer I want to provide sane default encryption format and options for users looking to encrypt their ephemeral storage at rest. I want these associated with the encrypted storage until it is deleted.

Proposed change

To enable this new flavor extra specs, image properties and host configurables will be introduced. These will control when and how ephemeral storage encryption at rest is enabled for an instance.


The following hw_ephemeral_encryption image properties do not relate to if an image is encrypted at rest within the Glance service. They only relate to how ephemeral storage will be encrypted at rest when used by a provisioned instance within Nova.

Separate image properties have been documented in the Glance image encryption and Cinder image encryption specs to cover how images can be encrypted at rest within Glance.

Allow ephemeral encryption to be configured by flavor, image or config

To enable ephemeral encryption per instance the following boolean based flavor extra spec and image property will be introduced:

  • hw:ephemeral_encryption

  • hw_ephemeral_encryption

The above will enable ephemeral storage encryption for an instance but does not control the encryption format used or the associated options. For this the following flavor extra specs, image properties and configurables will be introduced.

The encryption format used will be controlled by the following flavor extra specs and image properties:

  • hw:ephemeral_encryption_format

  • hw_ephemeral_encryption_format

When neither of the above are provided but ephemeral encryption is still requested an additional host configurable will be used to provide a default format per compute, this will initially default to luks:

  • [ephemeral_storage_encryption]/default_format

This could lead to requests against different clouds resulting in a different ephemeral encryption format being used but as this is transparent to the end user from within the instance it shouldn’t have any real impact.

The format will be provided as a string that maps to a BlockDeviceEncryptionFormatTypeField oslo.versionedobjects field value:

  • plain for the plain dm-crypt format

  • luks for the LUKSv1 format

BlockDeviceMapping changes

The BlockDeviceMapping object will be extended to include the following fields encapsulating some of the above information per ephemeral disk within the instance:


A simple boolean to indicate if the block device is encrypted. This will initially only be populated when ephemeral encryption is used but could easily be used for encrypted volumes as well in the future.


As the name suggests this will contain the UUID of the associated encryption secret for the disk. The type of secret used here will be specific to the encryption format and virt driver used, it should not be assumed that this will always been an symmetric key as is currently the case with all encrypted volumes provided by Cinder. For example, for luks based ephemeral storage this secret will be a passphrase.


A new BlockDeviceEncryptionFormatType enum and associated BlockDeviceEncryptionFormatTypeField field listing the encryption format. The available options being kept in line with the constants currently provided by os-brick and potentially merged in the future if both can share these types and fields somehow.


A simple unversioned dict of strings containing encryption options specific to the virt driver implementation, underlying hypervisor and format being used.

Populate ephemeral encryption BlockDeviceMapping attributes during build

When launching an instance with ephemeral encryption requested via either the image or flavor the BlockDeviceMapping.encrypted attribute will be set to True for each BlockDeviceMapping record with a destination_type value of local. This will happen after the original API BDM dicts have been transformed into objects within the Compute API but before scheduling the instance(s).

The encryption_format attribute will also take its’ value from the image or flavor if provided. Any differences or conflicts between the image and flavor for this will raise a 409 Conflict error being raised by the API.

Introduce new compatibility traits

A new COMPUTE_EPHEMERAL_ENCRYPTION compute compatibility trait will be added to os-traits and reported by virt drivers to indicate overall support for ephemeral storage encryption using this new approach. This trait will always be used by pre-filter outlined in the following section when ephemeral encryption has been requested, regardless of any format being specified in the request, allowing the compute that eventually handles the request to select a format it supports using the [ephemeral_storage_encryption]/default_format configurable.

New COMPUTE_EPHEMERAL_ENCRYPTION_$FORMAT compute compatibility traits will be added to os-traits and reported by virt drivers to indicate support for specific ephemeral storage encryption formats. For example:




These traits will only be used alongside the COMPUTE_EPHEMERAL_ENCRYPTION trait when the hw_ephemeral_encryption_format image property or hw:ephemeral_encryption_format extra spec have been provided in the initial request.

Introduce an ephemeral encryption request pre-filter

A new pre-filter will be introduced that adds the above traits as required to the request spec when the aforementioned image properties or flavor extra specs are provided. As outlined above this will always include the COMPUTE_EPHEMERAL_ENCRYPTION trait when ephemeral encryption has been requested and may optionally include one of the format specific traits if a format is included in the request.

Expose ephemeral encryption attributes via block_device_info

Once the BlockDeviceMapping objects have been updated and the instance scheduled to a compute the objects are transformed once again into a block_device_info dict understood by the virt layer that at present contains the following:


The root device path used by the instance.


A list of DriverEphemeralBlockDevice dict objects detailing the ephemeral disks attached to the instance. Note this does not include the initial image based disk used by the instance that is classified as an ephemeral disk in terms of the ephemeral encryption feature.


A list of DriverVol*BlockDevice dict objects detailing the volume based disks attached to the instance.


An optional DriverSwapBlockDevice dict object detailing the swap device.

For example:

    "root_device_name": "/dev/vda",
    "ephemerals": [
            "guest_format": null,
            "device_name": "/dev/vdb",
            "device_type": "disk",
            "size": 1,
            "disk_bus": "virtio"
    "block_device_mapping": [],
    "swap": {
        "swap_size": 1,
        "device_name": "/dev/vdc",
        "disk_bus": "virtio"

As noted above block_device_info does not provide a complete overview of the storage associated with an instance. In order for it to be useful in the context of ephemeral storage encryption we would need to extend the dict to always include information relating to local image based disks.

As such a new DriverImageBlockDevice dict class will be introduced covering image based block devices and provided to the virt layer via an additional image key within the block_device_info dict when the instance uses such a disk. As with the other Driver*BlockDevice dict classes this will proxy access to the underlying BlockDeviceMapping object allowing the virt layer to lookup the previously listed encrypted and encryption_* attributes.

While outside the scope of this spec the above highlights a huge amount of complexity and technical debt still residing in the codebase around how storage configurations are handled between the different layers. In the long term we should plan to remove block_device_info and replace it with direct access to BlockDeviceMapping based objects ensuring the entire configuration is always exposed to the virt layer.

Report that a disk is encrypted at rest through the metadata API

Extend the metadata API so that users can confirm that their ephemeral storage is encrypted at rest through the metadata API, accessible from within their instance.

    "devices": [
            "type": "nic",
            "bus": "pci",
            "address": "0000:00:02.0",
            "mac": "00:11:22:33:44:55",
            "tags": ["trusted"]
            "type": "disk",
            "bus": "virtio",
            "address": "0:0",
            "serial": "12352423",
            "path": "/dev/vda",
            "encrypted": "True"
            "type": "disk",
            "bus": "ide",
            "address": "0:0",
            "serial": "disk-vol-2352423",
            "path": "/dev/sda",
            "tags": ["baz"]

This should also be extended to cover disks provided by encrypted volumes but this is obviously out of scope for this implementation.

Block resize between flavors with different hw:ephemeral_encryption settings

Ephemeral data is expected to persist through a resize and as such any resize between flavors that differed in their configuration of ephemeral encryption (one enabled, another disabled or formats etc) would cause us to convert this data in place. This isn’t trivial and so for this initial implementation resizing between flavors that differ will be blocked.

Provide a migration path from the legacy implementation

New nova-manage and nova-status commands will be introduced to migrate any instances using the legacy libvirt virt driver implementation ahead of the removal of this in a future release.

The nova-manage command will ensure that any existing instances with ephemeral_key_uuid set will have their associated BlockDeviceMapping records updated to reference said secret key, the plain encryption format and configured options on the host before clearing ephemeral_key_uuid.

Additionally the libvirt virt driver will also attempt to migrate instances with ephemeral_key_uuid set during spawn. This should allow at least some of the instances to be moved during the W release ahead of X.

The nova-status command will simply report on the existence of any instances with ephemeral_key_uuid set that do not have the corresponding BlockDeviceMapping attributes enabled etc.

Deprecate the now legacy implementation

The legacy implementation within the libvirt virt driver will be deprecated for removal in a future release once the ability to migrate is in place.


Continue to use the transparent host configurables and expand support to other encryption formats such as LUKS.

Data model impact

See above for the various flavor extra spec, image property, BlockDeviceMapping and DriverBlockDevice object changes.

REST API impact

  • Flavor extra specs and image property validation will be introduced for the any ephemeral encryption provided options.

  • Attempts to resize between flavors that differ in their ephemeral encryption options will be rejected.

  • Attempts to rebuild between images that differ in their ephemeral encryption options will be allowed.

  • The metadata API will be changed to allow users to determine if their ephemeral storage is encrypted as discussed above.

Security impact

This should hopefully be positive given the unique secret per disk and user visible choice regarding how their ephemeral storage is encrypted at rest.

Additionally this should allow additional virt drivers to support ephemeral storage encryption while also allowing the libvirt virt driver to increase coverage of the feature across more imagebackends such as qcow2 and rbd.

Notifications impact


Other end user impact

Users will now need to opt-in to ephemeral storage encryption being used by their instances through their choice of image or flavors.

Performance Impact

The additional pre-filter will add a small amount of overhead when scheduling instances but this should fail fast if ephemeral encryption is not requested through the image or flavor.

The performance impact of increased use of ephemeral storage encryption by instances is left to be discussed in the virt driver specific specs as this will vary between hypervisors.

Other deployer impact


Developer impact

Virt driver developers will be able to indicate support for specific ephemeral storage encryption formats using the newly introduced compute compatibility traits.

Upgrade impact

The compute traits should ensure that requests to schedule instances using ephemeral storage encryption with mixed computes (N-1 and N) will work during a rolling upgrade.

As discussed earlier in the spec future upgrades will need to provide a path for existing ephemeral storage encryption users to migrate from the legacy implementation. This should be trivial but may require an additional grenade based job in CI during the W cycle to prove out the migration path.



Primary assignee:


Other contributors:

Feature Liaison

Feature liaison:


Work Items

  • Introduce hw_ephemeral_encryption* image properties and hw:ephemeral_encryption flavor extra specs.


  • Introduce a new encrypted. encryption_secret_uuid, encryption_format and encryption_options attributes to the BlockDeviceMapping Object.

  • Wire up the new BlockDeviceMapping object attributes through the Driver*BlockDevice layer and block_device_info dict.

  • Report ephemeral storage encryption through the metadata API.

  • Introduce new nova-manage and nova-status commands to allow existing users to migrate to this new implementation. This should however be blocked outside of testing until a virt driver implementation is landed.

  • Validate all of the above in functional tests ahead of any virt driver implementation landing.




At present without a virt driver implementation this will be tested entirely within our unit and functional test suites.

Once a virt driver implementation is available additional integration tests in Tempest and whitebox tests can be written.

Testing of the migration path from the legacy implementation will require an additional grenade job but this will require the libvirt virt driver implementation to be completed first.

Documentation Impact

  • The new host configurables, flavor extra specs and image properties should be documented.

  • New user documentation should be written covering the overall use of the feature from a Nova point of view.

  • Reference documentation around BlockDeviceMapping objects etc should be updated to make note of the new encryption attributes.



Optional section intended to be used each time the spec is updated to describe new design, API or any database schema updated. Useful to let reader understand what’s happened along the time.


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