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

Matthew Booth mbooth at redhat.com
Wed Mar 14 15:35:22 UTC 2018

On 14 March 2018 at 13:46, Matt Riedemann <mriedemos at gmail.com> 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.

My 2c was to overwrite, not delete the volume[1]. I believe this preserves
both sets of semantics: the server is rebuilt, and the volume is not
deleted. The user will still lose their data, of course, but that's implied
by the rebuild they explicitly requested. The volume id will remain the

[1] I suspect this would require new functionality in cinder to
re-initialize from image.

Matthew Booth
Red Hat OpenStack Engineer, Compute DFG

Phone: +442070094448 (UK)
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