Open Stack

On Thu, May 16, 2013 at 3:05 PM, Jun Cheol Park <jun.park.earth at gmail.com>wrote:

> Aaron,
>
> >@Mike - I think we'd still want to leave nova-compute to create the tap
> interfaces and sticking  external-ids on them though.
>
> Sorry, I don't get this. Why do we need to leave nova-compute to create
> tap interfaces? That behavior has been a serious problem and a design flaw
> in dealing with ports, as Mike and I presented in Portland (Title: Using
> OpenStack In A Traditional Hosting Environment).
>
> All,
>
> Please, let me share the problems that we ran into due to such a design
> flaw between nova-compute and quantum-agent.
>
> 1. Using external-ids as an implicit triggering mechanism for deploying
> OVS flows (we used OVS plugin on hosts) causes inconsistencies between
> quantum DB and actual OVS tap interfaces on hosts. For example, even when
> necessary OVS flows have not been set up or failed for whatever reason
> (messaging system unstable, or quantum-server down, etc), nova-compute
> unknowingly declares that a VM is in the "active" state as long as
> nova-compute successfully creates OVS taps and sets up external-ids. But,
> the VM does not have actual network connectivity until somebody (here
> quantum-agent) deploys desired OVS flows. At this point, it is very hard to
> track down what goes wrong because nova list shows the VM is "active." This
> kind of inconsistency happens a lot because a quantum API (which
> quantum-server provided, here e.g., create_port()) only manages its quantum
> DB, but does not deal with actual network objects (e.g., OVS taps on
> hosts). In this design, there is no way to verify the actual state of
> targeting network objects.
>

Sure. The same thing happens though if you boot a machine and plug it into
a switch where the physical switch isn't configured. In my opinion
quantum's job is to handle the programming of the network from the tap
interface downwards (not actually creating the interfaces as those are part
of the server). The tap interfaces are currently created when the instance
is started via (kvm,etc). Changing this so that quantum would be creating
the tap interfaces in my opinion seems like it will make things more
complicated as we'll then add another ordered component.

>
>   Q. What if a quantum API really deals with network objects (e.g., OVS
> taps), not only updating quantum DB?
>   A. Nova-compute now can call a truly abstracted quantum API for creating
> a real port (or an OVS tap interface) on a targeting host, and then wait
> for a response from the call to see if an OVS tap interface is really
> created on the host. This way, nova-compute is able to make sure what is
> going on before proceeding the rest of tasks for creating a new VM. When
> there are some tasks that need to be taken care of regarding ports such as
> QoS (as Henry mentioned), quota (as this thread was invoked from), etc,
> nova-compute then decides what would be a next step (at least it would not
> blindly say that the VM is active).
>
> 2. Another example as the side effect of tap being created by
> nova-compute. When a host is rebooted, we expect all the VMs are
> automatically restarted. However, it's not possible. Here is why. When
> nova-compute restarts, it expects to see libvirtd running. Otherwise,
> nova-compute immediately stops. So we have to first start libvirtd before
> nova-compute. Now when libvirtd starts, it expects that all the OVS taps
> exist so that it can successfully start all the VMs that are supposed to
> use OVS taps. However, at this point since we have not started nova-compute
> that would create OVS taps, restarting libvirtd fails to restart VMs due to
> no taps found. So I ended up adding "restart libvirtd" in rc.local so that
> we can make libvirtd retry to restart VMs after nova-compute creates OVS
> taps.
>

This sounds like a bug to me. I'll play around with trying to reproduce
this later.  Feel free to create a launchpad bug.

>
>  Q. Again, what if quantum-agent itself is able to deal with actual ports
> without relying on nova-compute at all?
>  A. We can start quantum-agent which would create all the necessary OVS
> taps in its own way. Then, restart libvirtd which then would start all the
> VMs with the created OVS taps. This is a good example how to make quantum
> truly independent of nova-compute without using any dependency on
> external-ids.
>

I don't think this is the right approach. Answered above.

>
>

> 3. Not only all those problems above, it is not desired that nova-compute
> should have all the code of dealing with OVS specifics (e.g,, all the
> wrapping functions of ovs-related commands such as ovs-vsctl) although
> quantum-agent already has all the same code of OVS specifics to deal with
> OVS taps.
>
> In summary, all these problems above occur due to the fact that quantum
> API only manages quantum DB, leaving all the functionality in dealing with
> actual network objects dispersed across nova-compute (e.g., OVS tap
> creation) and quantum-agent (e.g., OVS flows deployment).
>
> > nova-compute should call port-update to set binding:host_id
>
> This could be also a very good use case. If a quantum API really creates
> an actual port on a host as I have been suggesting here, nova-compute
> simply gets the return values for the newly created port from that API
> call. The return values would include all the detailed information
> including host_id, vif_type, etc. And nova-compute can use them to update
> ports, or maybe create_port() API itself already updates necessary info and
> simply return the current info such as mapping of binding:host_id.
>
> I'm not sure how effectively I have been explaining what I meant to say
> regarding a desirable design between nova-compute and quantum (both
> quantum-server and quantum-agent). Based on comments I would get from this
> thread, I may start to write a blueprint proposal.
>
> Please, let me know anything that I missed or misunderstood.
>
> Thanks,
>
> -Jun
>
> On Thu, May 16, 2013 at 1:47 PM, Robert Kukura <rkukura at redhat.com> wrote:
>
>> On 05/16/2013 02:40 PM, Mike Wilson wrote:
>> >
>> >
>> >
>> > On Thu, May 16, 2013 at 12:28 PM, Robert Kukura <rkukura at redhat.com
>> > <mailto:rkukura at redhat.com>> wrote:
>> >
>> >     >
>> >     > @Mike - I think we'd still want to leave nova-compute to create
>> >     the tap
>> >     > interfaces and sticking  external-ids on them though.
>> >
>> >     It also seems nova-compute should call port-update to set
>> >     binding:host_id and then use the returned binding:vif_type, since
>> the
>> >     vif_type might vary depending on the host, at least with ml2. The
>> Arista
>> >     top-of-rack switch hardware driver functionality also depends on the
>> >     binding:host_id being set.
>> >
>> >     -Bob
>> >
>> >
>> > Hmmm, is that really nova-compute's job? Again, that seems to be the
>> > networking abstraction's job to me. We have all these quantum agents,
>> > they have the device_id (instance_uuid). Why not have a quantum
>> > component (agent maybe?) query nova for the host_id and then it calls
>> > port-update?
>>
>> I believe this is the final step of an attempt to cleanup the
>> abstraction between nova and quantum. The idea is to have quantum decide
>> on the VIF driver, rather than having this knowledge built into the nova
>> configuration.
>>
>> In some cases, quantum will need to know what host the port is being
>> bound on so it can determine which VIF driver to use (possibly based on
>> what agent is running on that host). Also, a quantum L2 agent (if there
>> is one) cannot notice the that the port is binding bound until after the
>> VIF driver has been selected and done its thing.
>>
>> The nova code for this has been in review for a while, but may have
>> expired. Gerrit is offline at the moment, so I can't search for it.
>>
>> -Bob
>>
>> >
>> > -Mike
>> >
>> >
>> >
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>> >
>>
>>
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