[openstack-dev] [nova] about rebuild instance booted from volume

Ben Nemec openstack at nemebean.com
Thu Mar 15 14:27:55 UTC 2018

On 03/14/2018 08:46 AM, Matt Riedemann wrote:
> On 3/14/2018 3:42 AM, 李杰 wrote:
>>              This is the spec about  rebuild a instance booted from 
>> volume.In the spec,there is a
>>        question about if we should delete the old root_volume.Anyone 
>> who is interested in
>>        booted from volume can help to review this. Any suggestion is 
>> welcome.Thank you!
>>        The link is here.
>>        Re:the rebuild spec:https://review.openstack.org/#/c/532407/
> Copying the operators list and giving some more context.
> This spec is proposing to add support for rebuild with a new image for 
> volume-backed servers, which today is just a 400 failure in the API 
> since the compute doesn't support that scenario.
> With the proposed solution, the backing root volume would be deleted and 
> a new volume would be created from the new image, similar to how boot 
> from volume works.
> The question raised in the spec is whether or not nova should delete the 
> root volume even if its delete_on_termination flag is set to False. The 
> semantics get a bit weird here since that flag was not meant for this 
> scenario, it's meant to be used when deleting the server to which the 
> volume is attached. Rebuilding a server is not deleting it, but we would 
> need to replace the root volume, so what do we do with the volume we're 
> replacing?
> Do we say that delete_on_termination only applies to deleting a server 
> and not rebuild and therefore nova can delete the root volume during a 
> rebuild?
> If we don't delete the volume during rebuild, we could end up leaving a 
> lot of volumes lying around that the user then has to clean up, 
> otherwise they'll eventually go over quota.
> We need user (and operator) feedback on this issue and what they would 
> expect to happen.

As a developer who would also be a user of this functionality, I don't 
want the volume left around after rebuild.  To me the data loss of the 
root disk is inherent in the rebuild operation.  I guess the one gotcha 
might be that I always create the root volume as part of the initial 
instance creation, if someone manually created a volume and then pointed 
Nova at it there's probably a better chance they don't want Nova to 
delete it on them.

Rather than overload delete_on_termination, could another flag like 
delete_on_rebuild be added?


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