We need to retrieve the total capacity of the backend devices per pool, like this:

• ceilometer_volume_total_capacity{pool="pool1"}
• ceilometer_volume_used_capacity{pool="pool1"}
• (ceilometer_volume_used_capacity / ceilometer_volume_total_capacity) * 100

ceilometer-central-agent could retrieve the total capacity of all pools by using the API endpoint: /v3/{project_id}/scheduler-stats/get_pools (https://docs.openstack.org/api-ref/block-storage/v3/#list-all-back-end-storage-pools). That lists the total capacity and the used capacity of all the pools managed by Cinder. The "project_id" parameter of the request does nothing, and it always return the full capacity of pools.

NOTE: this is currently being reported via notification, but it might be better to retrieve it via API