Apologies in advance if this seems trivial but I am looking for some direction on this and I may have found a bug while testing also.

Some background - I have 2 physical hosts which I am testing with (host1, host2) and I have 2 separate cinder backends (backend1, backend2). Backend1 can only be utilised by host1. Same for backend2 - it can only be utilised by host2. So they are paired together like: host1:backend1 host2:backend2

So I wanted to select a Cinder storage back-end based on nova availability zone and to do this when creating an instance through horizon (not creating a volume directly). Also I wanted to avoid the use of metadata input on each instance create or by using metadata from images (such as cinder_img_volume_type) [2] . Because I can foresee a necessity to then have a number of images which reference each AZ or backend individually. 

• Is it possible to select a backend based on nova AZ? If so, could anyone share any resources to me that could help me understand how to achieve it? 

Because I failed at achieving the above, I then decided to use one way which had worked for me in the past, which was to use the image metadata "cinder_img_volume_type". However I find that this is not working. The “default” volume type is selected (if cinder.conf has it) or if no default, then `__DEFAULT__` is being selected. The link at [2] states that first, a volume type is used based on the volume type selected and if not chosen/set then the 2nd method is "cinder_img_volume_type" from the image metadata and then the 3rd and final is the default from cinder.conf.

I have tested with fresh deployment using Kayobe as well as RDO’s packstack.

Openstack version is Train

Steps to reproduce:
1. Install packstack

2. Update cinder.conf with enabled_backends and the [backend]

3. Add the volume type to reference the backend (for reference, I call this volume type `number-1`)

4. Upload an image and add metadata `cinder_img_volume_type` and the name as mentioned in step 3: number-1

5. Try and create an instance using horizon. Source = image and create new volume

6. Result = volume type / backend as chosen in the image metadata is not used and instance goes into error status. 

After fresh-deploying the RDO Packstack, I enabled debug logs and tested again. In the cinder-api.log I see “"volume_type": null,” and then the next debug log immediately after logged as “Create volume request body:” has “volume_type': None”. 

I was searching for a list of the supported image metadata, in case it had changed but the pages seem empty one rocky/stein/train [3] or not yet updated. 

Selecting backend based on nova AZ:: 

I was searching how to achieve this and I came across this video on the subject of AZs [1]. Although it seems only in the context of creating volumes (not with creating instances with volume from an image, for example). 

I have tried creating a host aggregate in nova, with AZ name `host1az`. I've also created a backend in Cinder (cinder.conf) with `backend_availability_zone = host1az`. But this does not appear to achieve the desired result, either and the cinder api logs are showing “"availability_zone": null” during the volume create part of the launch instance from Horizon.

I also tried setting RESKEY [3] in the volume type, but again similar situation seen; although I dont think this option is the correct context for what I am attempting. 

Could anyone please nudge me in the right direction on this? Any pointers appreciated at this point. Thanks in advance.

References: 

[1] https://www.youtube.com/watch?v=a5332_Ew9JA

[2] https://docs.openstack.org/cinder/latest/cli/cli-manage-volumes.html 

[3] https://docs.openstack.org/cinder/train/contributor/api/cinder.api.schemas.volume_image_metadata.html 

[4] https://docs.openstack.org/cinder/rocky/admin/blockstorage-availability-zone-type.html 

Regards,

Tony Pearce