[nova] Per-instance serial number implementation question

Matt Riedemann mriedemos at gmail.com
Thu Jan 31 15:00:56 UTC 2019

I'm going to top post and try to summarize where we are on this thread 
since I have it on today's nova meeting agenda under the "stuck reviews" 

* The email started as a proposal to change the proposed image property 
and flavor extra spec from a confusing boolean to an enum.

* The question was raised why even have any other option than a unique 
serial number for all instances based on the instance UUID.

* Stephen asked Daniel Berrange (danpb) about the history of the 
[libvirt]/sysinfo_serial configuration option and it sounds like it was 
mostly added as a way to determine guests running on the same host, 
which can already be determined using the hostId parameter in the REST 
API (hostId is the hashed instance.host + instance.project_id so it's 
not exactly the same since it's unique per host and project, not just 
host). However, the hostId is not exposed to the guest in the metadata 
API / config drive - so that could be a regression for applications that 
used this somehow to calculate affinity within the guest based on the 
serial (note that mnaser has a patch to expose hostId in the metadata 
API / config drive [1]).

* danpb said the system.serial we set today should really be 
chassis.serial but that's only available in libvirt >= 4.1.0 and our 
current minimum required version of libvirt is 1.3.1 so setting 
chassis.serial would have to be conditional on the running version of 
libvirt (this is common in that driver).

* Applications that depend on the serial number within the guest were 
not guaranteed it would be unique or not change because migrating the 
guest to another host would change the serial number anyway (that's the 
point of the blueprint - to keep the serial unchanged for each guest), 
so if we just changed to always using unique serial numbers everywhere 
it should probably be OK (and tolerated/expected by guest applications).

* Clearly we would have a release note if we change this behavior but 
keep in mind that end users are not reading release notes, and none of 
this is documented today anyway outside of the [libvirt]/sysinfo_serial 
config option. So a release note would really only help an operator or 
support personal if they get a ticket due to the change in behavior 
(which we probably wouldn't hear about upstream for 2+ years given how 
slow openstack deployments upgrade).

So where are we? If we want the minimal amount of behavior change as 
possible then we just add the new image property / flavor extra spec / 
config option choice, but that arguably adds technical debt and 
virt-driver specific behavior to the API (again, that's not uncommon 

If we want to simplify, we don't add the image property / flavor extra 
spec. But what do we do about the existing config option?

Do we add the 'unique' choice, make it the default, and then deprecate 
the option to at least signal the change is coming in Train?

Or do we just deprecate the option in Stein and completely ignore it, 
always setting the unique serial number as the instance.uuid (and set 
the host serial in chassis.serial if libvirt>=4.1.0)?

In addition, do we expose hostId in the metadata API / config drive via 
[1] so there is a true alternative *from within the guest* to determine 
guest affinity on the same host? I'm personally OK with [1] if there is 
some user documentation around it (as noted in the review).

If we are not going to add the new image property / extra spec, my 
personal choice would be to:

- Add the 'unique' choice to the [libvirt]/sysinfo_serial config option 
and make it the default for new deployments.
- Deprecate the sysinfo_serial config option in Stein and remove it in 
Train. This would give at least some window of time for transition 
and/or raising a stink if someone thinks we should leave the old 
per-host behavior.
- Merge mnaser's patch to expose hostId in the metadata API and config 
drive so users still have a way within the guest to determine that 
affinity for servers in the same project on the same host.

What do others think?

[1] https://review.openstack.org/#/c/577933/

On 1/24/2019 9:09 AM, Matt Riedemann wrote:
> The proposal from the spec for this feature was to add an image property 
> (hw_unique_serial), flavor extra spec (hw:unique_serial) and new 
> "unique" choice to the [libvirt]/sysinfo_serial config option. The image 
> property and extra spec would be booleans but really only True values 
> make sense and False would be more or less ignored. There were no plans 
> to enforce strict checking of a boolean value, e.g. if the image 
> property was True but the flavor extra spec was False, we would not 
> raise an exception for incompatible values, we would just use OR logic 
> and take the image property True value.
> The boolean usage proposed is a bit confusing, as can be seen from 
> comments in the spec [1] and the proposed code change [2].
> After thinking about this a bit, I'm now thinking maybe we should just 
> use a single-value enum for the image property and flavor extra spec:
> image: hw_guest_serial=unique
> flavor: hw:guest_serial=unique
> If either are set, then we use a unique serial number for the guest. If 
> neither are set, then the serial number is based on the host 
> configuration as it is today.
> I think that's more clear usage, do others agree? Alex does. I can't 
> think of any cases where users would want hw_unique_serial=False, so 
> this removes that ability and confusion over whether or not to enforce 
> mismatching booleans.
> [1] 
> https://review.openstack.org/#/c/612531/2/specs/stein/approved/per-instance-libvirt-sysinfo-serial.rst@43 
> [2] 
> https://review.openstack.org/#/c/619953/7/nova/virt/libvirt/driver.py@4894




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