Snapshot support!/story/2008033

Problem description

Snapshot is not a new thing, it was available for virtual machines for a long time. Snapshot is useful for instance backup, image reusing, etc, but there is no such support for bare metals as it’s more complex than passing the request to libvirt.

Bare metal snapshot may not match with virtual machines in speed and efficiency, but it could address following requirements:

As an operator, I want to be able to back up a bare metal instance periodically and when there is hardware failure, the same image can be applied to another machine.

As an operator, I want to be able to build a master image from a post customized instance, capture the system into an image and apply to other similar machines.

Proposed change

The proposal is to implement a similar process like deployment, when a node is requested to do a snapshot, ironic prepares boot configuration then triggers a netboot to the node, IPA lookups and ironic instructs IPA to streaming the data of root disk to a remote storage.

The remote storage could be the host where the conductor is running if proper service like NFS, HTTP, etc is configured. Or another host as long as both conductor and IPA are able to access. The remote storage will act as an intermediate storage, when the streaming is done, conductor will upload the image to the image service and remove the image in the intermediate storage.

The initial proposal is to utlilize HTTP WebDAV as remote storage, NFS and other storage can be extended when this feature is implemented. When the direct deploy interface is configured to download images from HTTP service, it can be updated with WebDAV support.



Data model impact

A new hardware interface snapshot_interface will be added to support different snapshot implementations.

State Machine Impact

Add following states to the state machine:

  • snapshot wait

  • snapshotting

  • snapshot failed

With following transitions:

  • active -> snapshotting (on snapshot) Prepare boot and network

  • snapshotting -> snapshot failed (on fail) On error

  • snapshotting -> snapshot wait (on wait) Wait for ramdisk alive

  • snapshot wait -> snapshotting (on resume) Image streaming and uploading

  • snapshot wait -> snapshot failed (on fail) On timeout

  • snapshot wait -> snapshot failed (on abort) On abort request

  • snapshotting -> active (on done) Snapshot complete

  • snapshot failed -> active (on abort) Abort a failed snapshot

  • snapshot failed -> snapshotting (on snapshot) Retrigger a failed snapshot

  • snapshot wait -> deleting (on deleted) Abort snapshot and undeploy

  • snapshot failed -> deleting (on deleted) Abort and undeploy

REST API impact

The existing provision endpoint is used, a new verb snapshot will be guarded by microversions.

  • PUT /v1/nodes/{node_ident}/states/provision

    When the requested version is permitted, and node is in the active state, start to do a snapshot. An image_ref argument is required, which refers to a location where the snapshot image should be stored.

    If the image_ref is an UUID, it refers to the UUID of an image in the Image serivce, the image should be precreated before snapshot request and acts as a holder for receiving image data. When integrated with Compute service, the image_id is an argument passed to the driver interface snapshot().

    Example request:

       "target": "snapshot",
       "image_ref": "66498c26-a9b5-496c-97a8-5bc08f256155"
    The ``image_ref`` could be an URL in other forms for standalone usage,
    but not covered in this spec.

Other provision state transitions will use existing verbs.

Client (CLI) impact

“openstack baremetal” CLI

The impact to CLI should be trivial, will have a new command:

  • openstack baremetal node snapshot <node> ...


openstacksdk will be enhanced to know the snapshot API version.

RPC API impact

Will introduce a new rpc interface do_node_snapshot.

Driver API impact

Adds an optional interface snapshot_interface, initially implements fake, no-snapshot, and agent.

For the agent implementation, conductor host needs a WebDAV service to receive image data from the IPA. When the first heartbeat is received, ironic conductor sends command snapshot.stream_image with an URL to the IPA to start the snapshotting. IPA find the root device and use proper methods to dump disk data to the remote storage. The criterial for choosing the root device is same with deployment, on the time of this writing, the root device is the first available block device not less than 4GiB, or the first matched device if the root device hint is specified.

There are two methods to utilize the WebDAV service:

  • The WebDAV directory is mounted to the ramdisk, IPA uses qemu-img for snapshot, empty blocks will be bypassed so it provides better performance.

  • IPA uses dd to dump the disk data to the WebDAV URL, when finished, ironic conductor needs to convert the raw image into proper format.

As the root device could be quite large and even larger than the available space of conductor host, dd is not considered practical in the spec.

After the disk data is successfully retrieved, the conductor is responsible to upload the image to the Image Service using the specified image_id and remove the intermediate image.

Nova driver impact

Nova driver will need to implement the driver interface snapshot to integrate with ironic. But before the integration the feature can be consumed by ironic as long as the Image Service is available.

Ramdisk impact

For the agent implementation, a snapshot extension will be added to the ironic-python-agent, when the WebDAV directory could be mounted, qemu-img is used for streaming disk data to the URL.

The snapshot extension will be defined as:

Example arguments for an HTTP WebDAV connection:

IPA will generate an URL to mount:

mount -t davfs /tmp/tmp5fjchif0


NFS connection is not covered but can be implemented after this feature is implemented. It will look like:

mount -t nfs /tmp/tmp5fjchif0

Security impact

The transfer of instance image from the target bare metal to image service could have security risk since the data could be tampered or retrieved during this process.

Other end user impact


Scalability impact


Performance Impact


Other deployer impact


Developer impact




Primary assignee:


Other contributors:


Work Items

  • Equiping IPA with the ability of streaming disk data

  • Spawning states and transitions to Ironic

  • DB, RPC and API change

  • Make Client/SDK aware of the feature

  • Plug in with nova




Will be covered by unit tests and tempest.

Upgrades and Backwards Compatibility

Should be backwards compatible.

Documentation Impact

Documentation will be updated on this feature.