Thanks for the explanation. 

Openstack A is very old and at the end of life.. I am going to burn it to the ground. I just want to move some instances from Openstack A to Openstack B (This is all legacy applications and hard to rebuild).  Once I move the snapshot to Openstack B and spin up the vm after that I don't need that snapshot hanging around. I can just delete it and make the glance registry neat and clean. 

Do I need to flatten the image in Openstack A (old) ?  

On Wed, Jan 24, 2024 at 1:41 PM Eugen Block <eblock@nde.ag> wrote:
If I understand your workflow correctly you just need to import the 
snapshot to cluster B, on cluster B you then flatten the base (glance) 
image:

rbd flatten glance/<SOME_UUID>

This should remove all references from the parent image from cluster 
A. Glance doesn't really care about that (as far as I know). You can 
then create a new snapshot (and protect it) for that image so glance 
on cluster B can create children:

rbd snap create glance/<SOME_UUID>@snap
rbd snap protect glance/<SOME_UUID>@snap

That should basically be it, now you can launch instances on site B 
from that imported image. What I don't understand yet is this part: 
"and how does glance understand [...] and how to delete it from glance?"
I thought you wanted a new base image on site B, why deleting it from 
glance? Or are you referring to the snapshot on site A? If that's the 
case then you don't need to worry about that after the image has been 
flattened, it removes all references to site A, you should be able to 
delete the snapshot on site A.

Zitat von Satish Patel <satish.txt@gmail.com>:

> Hi Eugen,
>
> Could you give me example commands and processes to flatten? Do I need to
> flatten from ceph storage and how does glance understand that image is
> flatten and how to delete it from glance?
>
> On Wed, Jan 24, 2024 at 1:06 PM Eugen Block <eblock@nde.ag> wrote:
>
>> You can just flatten the image to make it independent from the parent
>> image. After flattening (rbd flatten) you could sparsify it to get
>> some storage capacity back.
>>
>> Zitat von Satish Patel <satish.txt@gmail.com>:
>>
>> > Folks,
>> >
>> > I have two openstack clouds and both have their own Ceph backend
>> storage.I
>> > am trying to migrate instances from openstack A to openstack B.
>> >
>> > 1. Take a snapshot from A
>> > 2.  Export snapshot and import to B
>> > 3. Create instance on B
>> > 4. Delete snapshot - (I am getting error because its in-use)
>> >
>> > Because it has a parent reference to that volume. How do I remove the
>> > reference so it will let me delete a snapshot. Reason I am asking is
>> > because I have so many VMs to migrate and don't want glance to have 100s
>> of
>> > entities in those snapshots.
>> >
>> > What is the alternative here? I can try qcow2 if that is the final
>> solution
>> > to make it clean.
>> >
>> > # rbd -p volumes info volume-f2b2aec2-cc57-49e5-aca1-54b5a7ee9f3a
>> > rbd image 'volume-f2b2aec2-cc57-49e5-aca1-54b5a7ee9f3a':
>> >       size 40 GiB in 5120 objects
>> >       order 23 (8 MiB objects)
>> >       snapshot_count: 0
>> >       id: 5473c827864fed
>> >       block_name_prefix: rbd_data.5473c827864fed
>> >       format: 2
>> >       features: layering, exclusive-lock, object-map, fast-diff,
>> deep-flatten
>> >       op_features:
>> >       flags:
>> >       create_timestamp: Fri Jan 19 19:03:25 2024
>> >       access_timestamp: Wed Jan 24 15:51:04 2024
>> >       modify_timestamp: Wed Jan 24 15:52:40 2024
>> >       parent: images/3708f961-fb74-49f1-ab9b-40cf7954abed@snap
>> >       overlap: 40 GiB
>>
>>
>>
>>