Host aggregate - can it grow dynamically?
1) Can I Expose the host-aggregate as availability zone?
2) Is there anyway to make the Host Aggregate dynamically growing (with each VM creation- add the host to host aggregate if its not already there)?
I think that if you explain in more details your use case you may have more results. Alternatively post your request on the General mailing list.
The host aggregates are not meant to contain your VM's but the actual nodes that run your open stack cluster so that you can partition what portions clients can use and bill them separately.
So if you have one aggregate of SSD it's pricier than old HHD's both separated by aggregates/zones