Hi Ingo,
If you're not already using it, you will want to take a look at the Database Schema page which is within the Documentation interface on the DWH web portal. I usually have at least one eye on this when working in MySQL.
If you're in dwh_capacity, you can probably use something like this to combine storage with volume:
select * from storage_dimension
join volume_dimension on volume_dimension.storageIdentifier=storage_dimension.identifier and storage_dimension.latest=1;
If your annotation is published, it will be a field in the volume table.
If working in dwh_inventory, the join is simpler:
select * from dwh_inventory.storage
join dwh_inventory.volume on volume.storageId=storage.id;
But you'll have to join in the annotation_value and object_to_annotaion tables as in the earlier example with tiers.