Support clone_image from Glance cinder backend

When Glance cinder store is enabled, the glance image can store locations of cinder volume. When a raw image is cinder volume backed, we can efficiently create a new volume from the image by cloning the backing volume, instead of downloading the image from Glance. We can also make upload-to-image from a volume efficient by creating a cloned volume and add its location to an image.

Note that downloading the cinder-backed image from Glance is not currently supported, but this feature is proposed. [1]

Problem description

  • Creating a new volume from an image, and uploading a volume to an image take long time due to image transfer.

  • Sharing volume data among tenant is not currently supported in Cinder. To share the volume data, we need to upload it to Glance once.

Use Cases

  • Share volume templates (e.g. operating system volumes) among tenants via cinder volume backed images.

  • Each projects can create new instances via the image rapidly. If storage array supports thin-provisioning, the disk space could also be efficient.

Proposed change

When the allowed_direct_url_schemes in cinder.conf contain ‘cinder’, on creating a volume from an image:

  • Check if the image metadata satisfy the following conditions:
    • The image format is raw.

    • The image has locations in the format of ‘cinder://<volume-id>’

    • Any of the specified volume is on the same host as a new volume.

    • The image owner and the volume owner is the same (to avoid accessing volumes not shared by the image owner).

  • If all the conditions are met, clone the specified volume. When the requested size is larger than the image, extend the new volume.

  • Otherwise, download the image to a new volume.

On uploading a volume to an image:

  • If image format is raw and image_upload_use_cinder_backend is enabled in cinder.conf, clone the volume and register its location to the image [2].

    • If image_upload_use_internal_tenant is set to True, the cloned volume is placed in the internal tenant.

  • Otherwise, upload the volume data to Glance.


Generic image cache has the same effect in some situations. However, the proposed feature and generic image cache can co-exist to cover the different situations.

  • Image cache does not speed up uploading, but this feature make it efficient.

  • image cache can handle non-raw format images (it stores the converted image in image volumes), but this proposed feature doesn’t handle it so far.

Data model impact


REST API impact


Security impact

When the cloned volume is stored in the internal tenant, it could potentially be a risk if someone was able to get a hold of its credentials or access the image-volumes. Care will have to be taken to ensure it isn’t accessible by normal users.

Notifications impact


Other end user impact


Performance Impact

This improves the performance of uploading and download of the image, especially storage array can provide efficient volume cloning.

Other deployer impact

Some configuration options need to be set to enable this feature.

  • glance_api_version = 2

  • allowed_direct_url_schemes must contain ‘cinder’

  • image_upload_use_cinder_backend = True (new)

  • image_upload_use_internal_tenant = True / False (new)

In Glance, cinder backend must be enabled and show_multiple_locations must be set to True.

Developer impact




Primary assignee:


Other contributors:


Work Items

  • Efficient download from cinder volume backed image

  • Efficient upload to image



To enabling the other components to download the cinder volume backed images, Glance should have the changes [1].


  • Unit tests for download from cinder backed image

  • Unit tests for upload to image

Documentation Impact

Documentation about how to enable this feature should be added.