[openstack-dev] [nova] Interesting bug when unshelving an instance in an AZ and the AZ is gone

Matt Riedemann mriedemos at gmail.com
Mon Oct 16 16:32:19 UTC 2017

On 10/16/2017 11:00 AM, Dean Troyer wrote:
> [not having a dog in this hunt, this is what I would expect as a cloud consumer]

Thanks for the user perspective, that's what I'm looking for here, and 
operator perspective of course.

> On Mon, Oct 16, 2017 at 10:22 AM, Matt Riedemann <mriedemos at gmail.com> wrote:
>> - The user creates an instance in a non-default AZ.
>> - They shelve offload the instance.
>> - The admin deletes the AZ that the instance was using, for whatever reason.
>> - The user unshelves the instance which goes back through scheduling and
>> fails with NoValidHost because the AZ on the original request spec no longer
>> exists.
>> 1. How reasonable is it for a user to expect in a stable production
>> environment that AZs are going to be deleted from under them? We actually
>> have a spec related to this but with AZ renames:
> Change happens...
>> 2. Should we null out the instance.availability_zone when it's shelved
>> offloaded like we do for the instance.host and instance.node attributes?
>> Similarly, we would not take into account the RequestSpec.availability_zone
>> when scheduling during unshelve. I tend to prefer this option because once
>> you unshelve offload an instance, it's no longer associated with a host and
>> therefore no longer associated with an AZ. However, is it reasonable to
>> assume that the user doesn't care that the instance, once unshelved, is no
>> longer in the originally requested AZ? Probably not a safe assumption.
> Agreed, unless we keep track that the user specified a default or no
> AZ at create.

We do keep track of what the user originally requested, that is this 
RequestSpec object thing I keep referring to.

> I think nulling the AZ when the original doesn't exist would be
> reasonable from a user standpoint, but I'd feel handcuffed if that
> happens and I can not select a new AZ. Or throwing a specific error
> and letting the user handle it in #3 below:

At the point of failure, the API has done an RPC cast and returned a 202 
to the user, so the only way to provide a message like this to the user 
would be to check if the original AZ still exists in the API. We could 
do that, it would just be something to be aware of.

>> 3. When a user unshelves, they can't propose a new AZ (and I don't think we
>> want to add that capability to the unshelve API). So if the original AZ is
> Here is my question... if I can specify an AZ on create, why not on
> unshelve?  Is it the image location movement under the hood?

I just don't think it's ever come up. The reason I hesitate to add the 
ability to the unshelve API is more or less rooted in my bias toward not 
liking shelve/unshelve in general because of how complicated and 
half-baked it is (we've had a lot of bugs from these APIs, some of which 
are still unresolved). That's not the user's fault though, so one could 
argue that if we're not going to deprecate these APIs, we need to make 
them more robust. We, as developers, also don't have any idea how many 
users are actually using the shelve API, so it's hard to know if we 
should spend any time on improving it.

>> gone, should we automatically remove the RequestSpec.availability_zone when
>> scheduling? I tend to not like this as it's very implicit and the user could
>> see the AZ on their instance change before and after unshelve and be
>> confused.
> Agreed that explicit is better than implicit.
>> 4. We could simply do nothing about this specific bug and assert the
>> behavior is correct. The user requested an instance in a specific AZ,
>> shelved that instance and when they wanted to unshelve it, it's no longer
>> available so it fails. The user would have to delete the instance and create
>> a new instance from the shelve snapshot image in a new AZ. If we implemented
> I do not have the list of things in my head that are preserved in
> shelve/unshelve that would be lost in a recreate, but that's where my
> worry would come.  Presumably that is why I shelved in the first place
> rather than snapshotting the server and removing it.  Depends on the
> cost models too, if I lose my grandfathered-in pricing by being forced
> to recreate I amy be unhappy.

The volumes and ports remain attached to the shelved instance, only the 
guest on the hypervisor is destroyed. It doesn't change anything about 
quota - you retain quota usage for a shelved instance so you have room 
in your quota to unshelve it later.

 From what I can tell, the os-simple-tenant-usage API will still count 
the instance and it's consumed disk/ram/cpu against you even though the 
guest is deleted from the hypervisor while the instance is shelved 
offloaded. So the operator is happy about shelved offloaded instances 
because that means they have more free capacity for new instances and 
moving things, but the user is still getting charged the same, if your 
billing model is based on os-simple-tenant-usage (which Telemetry uses I 

>> Sylvain's spec in #1 above, maybe we don't have this problem going forward
>> since you couldn't remove/delete an AZ when there are even shelved offloaded
>> instances still tied to it.
> As a user I probably do not mind this, as an operator I'd likely be unhappy.
> dt




More information about the OpenStack-dev mailing list