Open Stack

I was talking about this problem with my management and had a good analogy that might make the flaw a little more visible.

Thought experiment: Imagine a VM as a physical box, imagine nova/quantum/cinder/glance as the IT personnel.

It seems like the IT personnel would go through the following likely overly simplistic steps when setting up a physical box:

1. Get physical box from supplier (equivalent to nova reserving a VM with libvirt) that matches desired specs (aka flavor).
   a. Place physical box in rack and ensure available power switches and so-on.
2. Connect hard-drives to physical box (equivalent to downloading the image from glance, or talking to cinder to do the same)
3. Connect network-cables to physical box (equivalent to talking to quantum, having quantum-agent plug things in…)
   a. Connect network-cables to router, configure router ports/flows and so-on.
4.  Turn on said box by pressing the power switch/button.
5. Report back to user that box is now ready for use.

>From the below it seems to be that the 3,4&5 can happen simultaneously (or out of order)? That seems pretty bad and is not something I would expect when setting up a physical box.

Jun would that be a correct 'simplification' of the problem?

From: Jun Cheol Park <jun.park.earth at gmail.com<mailto:jun.park.earth at gmail.com>>
Reply-To: OpenStack Development Mailing List <openstack-dev at lists.openstack.org<mailto:openstack-dev at lists.openstack.org>>
Date: Thursday, May 16, 2013 3:05 PM
To: OpenStack Development Mailing List <openstack-dev at lists.openstack.org<mailto:openstack-dev at lists.openstack.org>>
Subject: Re: [openstack-dev] [Nova][Quantum] Move quantum port creation to nova-api

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.

  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.

 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.

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<mailto: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>
> <mailto: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
>
>
>
> _______________________________________________
> OpenStack-dev mailing list
> OpenStack-dev at lists.openstack.org<mailto:OpenStack-dev at lists.openstack.org>
> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
>


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