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I'm not sure it is good in this thread to continue to talk about the design
flaw, alleged by a few people including Mike and me and recognized by a few
other people including Joshua and more. However, since I got a response
from Aaron, I would like to follow up.

On Mon, May 20, 2013 at 10:39 AM, Aaron Rosen <arosen at nicira.com> wrote:

>
>
>
> 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.
>

Aaron, I'm really trying to understand what you described here, but
unfortunately failing. So please help us. You already mentioned several
times in this thread that you think that the task of creating OVS taps
needs to be done by nova-compute. But, I don't think you explained why or
what benefits in doing so. More importantly, I think we should know what is
your suggestion then. How can we resolve such an inconsistency problem that
I described while keeping (as you think it's a right thing to do) the
functionality of creating OVS taps belong to nova-compute?


>>   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.
>

As I explained, I don't think this problem is simply a bug. I would say
more of a design issue because this happens due to a circular dependency
between libvirtd and nova-compute regarding the automated restart of VMs.


>
>>  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.
>

As I questioned above, again why? Is there any other reason than my
suggestion appeared to be complex?

Regards,

-Jun
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