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Hi Nova,

I'm working on a blueprint to support Detach/Attach root volumes. The
blueprint has been proposed for quite a while since mitaka[1] in that
version of proposal, we only talked about instances in shelved_offloaded
status. And in Stein[2] the status of stopped was also added. But now we
realized that support detach/attach root volume on a stopped instance could
be problemastic since the underlying image could change which might
invalidate the current host.[3]

So Matt and Sean suggested maybe we could just do it for shelved_offloaded
instances, and I have updated the patch according to this comment. And I
will update the spec latter, so if anyone have thought on this, please let
me know.

Another thing I wanted to discuss is that in the proposal, we will reset
some fields in the root_bdm instead of delete the whole record, among those
fields, the tag field could be tricky. My idea was to reset it too. But
there also could be cases that the users might think that it would not
change[4].

Thoughts,

BR,

[1]
http://specs.openstack.org/openstack/nova-specs/specs/mitaka/approved/detach-boot-volume.html#proposed-change
[2]
http://specs.openstack.org/openstack/nova-specs/specs/stein/approved/detach-boot-volume.html#proposed-change
[3] https://review.openstack.org/#/c/614750/34/nova/compute/manager.py@5467
[4] https://review.openstack.org/#/c/614750/37/nova/objects/block_device.py
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