[openstack-dev] [nova] Boot from volume 's instance of rebuild operation

Jaze Lee jazeltq at gmail.com
Thu Dec 7 06:14:24 UTC 2017

2017-12-07 10:27 GMT+08:00 Mathieu Gagné <mgagne at calavera.ca>:
> On Wed, Dec 6, 2017 at 9:13 PM, Jaze Lee <jazeltq at gmail.com> wrote:
>> 2017-12-07 7:12 GMT+08:00 Matt Riedemann <mriedemos at gmail.com>:
>>> On 12/6/2017 2:11 AM, 李杰 wrote:
>>>> Hi,all
>>>>         Now the boot from volume 's instance of rebuild operation has a
>>>> problem.For example,after the rebuild operation,the instance 's root disk is
>>>> not replace.
>>>> I found the reason is that when we use the "_build_resources" function to
>>>> prepare source,it obtains the block devices according to the previous
>>>> instance 's uuid and attaches them to instance.So boot from volume 's
>>>> instance of rebuild operation doesn't update data.
>>>>         To solve it,I plan to use CLI 's "metadata" option,to increase a
>>>> key name "source_type".The "source_type" includes "snapshot" and "image".We
>>>> can judge from "source_type".If the "source_type" is "snapshot",we can
>>>> transform the given snapshot to a volume and attach this volume to
>>>> instance.If the "source_type" is "image",we don't handle it.
>>>>         Can you give me some advice?Help in troubleshooting this issue
>>>> will be appreciated.
>>> See: https://review.openstack.org/#/c/520660/
>>> We just started disallowing rebuilding a volume-backed instance where the
>>> image changes during the rebuild. We don't support it in the compute
>>> service, as you've found out, so we're going to make it a fast failure in
>>> the API.
>> Well, are there some reasons for you want to disallowing rebuilding a
>> volume-backend instance.
>> Just give the the review is not a convincing answer.
> Rebuilding a volume-backed instance never worked. While the API
> currently accepts your request, nothing will happen. The instance
> won't be rebuilt as expected.
> There had been a couple of proposals/changes to add support:
> * https://review.openstack.org/#/c/442295/
> * https://review.openstack.org/#/c/305079/
> But technical challenges have been uncovered, making it harder to
> implement a proper solution.
> The change [1] linked by Matt Riedemann proposes failing rapidly so
> the user isn't under the false impression that a rebuild is being
> performed when in fact, nothing will happen.
> I do agree that being able to rebuild a volume-backed instance would
> be a great addition. We have been offering volume-backed instance for
> more than 4 years and our users love it.
> But for now, rebuild just doesn't work at all. It's better to send the
> right message through the API by failing fast.
> [1] https://review.openstack.org/#/c/520660/

Oh, I think it is not a proper solution. Adding a fast failure is much easier
than to support rebuilding of volume-backend instance.
I found the last reason to abandon the review at
is because the design is not very clear and lack of bp.
So everyone who wants to add this feature to nova is welcome. Please go ahead.

> --
> Mathieu
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