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On 15:51 Fri 06 Feb     , Nilesh P Bhosale wrote:
<snip>
> I understand this is as per design, but curious to understand logic behind 
> this.
<snip>
> Why not allow deletion of volumes form the CG? at least when there are no 
> dependent snapshots.

>From the review [1], this is because allowing a volume that's part of
a consistency group to be deleted is error prone for both the user and the
storage backend. It assumes the storage backend will register the volume not
being part of the consistency group. It also assumes the user is keeping
tracking of what's part of a consistency group.

> With the current implementation, only way to delete the volume is to 
> delete the complete CG, deleting all the volumes in that, which I feel is 
> not right.

The plan in Kilo is to allow adding/removing volumes from a consistency group
[2][3]. The user now has to explicitly remove the volume from a consistency
group, which in my opinion is better than implicit with delete.

I'm open to rediscussing this issue with vendors and seeing about making sure
things in the backend to be cleaned up properly, but I think this solution
helps prevent the issue for both users and backends.

[1] - https://review.openstack.org/#/c/149095/
[2] - https://blueprints.launchpad.net/cinder/+spec/consistency-groups-kilo-update
[3] - https://review.openstack.org/#/c/144561/

-- 
Mike Perez