Expose store specific information


Problem description

When we upload a volume from cinder’s RBD backend as an image to glance’s RBD backend, the generic code path to copy data chunk by chunk is executed, which makes the operation very slow. This can be optimized by using COW cloning for rbd backends and for this Cinder requires store type and rbd specific store info from glance side.

Proposed change

At the moment the Discovery (GET /v2/info/stores) API provides the list of multiple-backend present and the default store but not the type of the stores.

We will be extending the functionality of Discovery (GET /v2/info/stores) API by adding a new API (GET /v2/info/stores/detail) which will expose the store specific details about the store like store type and other specific store properties.

Since the store specific information is mostly intended for other services (like cinder) or operators to consume and not for end users, this operation will be admin only and we will introduce a new policy stores_info_detail, that will default to admin only rule, to restrict this for non-admin users. In future, when the service role will be in place in keystone to facilitate service to service interaction, this policy rule will be adjusted accordingly.

We will use the existing method get_store_from_store_identifier which returns the store class instance and will utilize it to fetch the store specific information to return it via API in a defined format. For more details see REST API impact section.



Data model impact


REST API impact

We are going to add a new API GET /v2/info/stores/detail which will return the store details like store type and store specific properties. It will be validated by the new policy rule stores_info_detail which defaults to admin only and then the detailed info related to the stores will be returned.

GET /v2/info/stores/detail

The output will be as follows:

    "stores": [
            "type": "rbd",
            "description": "More expensive store with data redundancy",
            "default": true,
            "properties": {
                "pool": "pool1"
            "type": "file",
            "description": "Less expensive store for seldom-used images",
            "properties": {}

We are going to add a field type to specify the type of store. Also we will add a field properties which will be a JSON object type and contain the store specific properties. For the current usecase we are only going to add RBD store info and leave the properties for other stores as empty JSON objects {}.

Security impact

Since this optimization skips the writing of image that happens on the glance side, it will also skip the checksum and hash value calculated in that scenario. Due to the above case, we will add a new config option on the cinder side to enable/disable this optimization. By default, it will be disabled.

Another case is we require direct-url and image locations in the image detail response to take benefit of any optimization in nova-glance or cinder-glance interactions which is a security concern described in OSSN-0065.

Notifications impact


Other end user impact

A new optional parameter --detail will be added to the stores info command on glanceclient side.

The parameter will accept boolean values. If True, it will expose store specific information else will show the non-detailed information about stores as it works currently.

  • Stores Detail: glance stores-info --detail


    With sufficient permissions, display additional information about the stores.

Performance Impact

Uploading volume to image incase of cinder RBD to glance RBD will be significantly improved.

Image size




Time without COW clone




Time with COW clone







Other deployer impact


Developer impact


Developer impact




Primary assignee:

mrjoshi, whoami-rajat

Other contributors:


Work Items

  • Add an optional parameter --detail in the stores-info command on the glanceclient side.

  • Add a new API to glance GET /v2/info/stores/detail.

  • Create a new policy stores_info_detail that will default to admin only rule and enforce it if detail flag is passed.




  • Unit Tests

  • Functional Tests

  • Tempest Tests

Documentation Impact

A new section for the new API GET /v2/info/stores/detail needs to be added in the api-ref.