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Hi Florian,

For my own setup, I wanted to achieve highly-available network, and avoid
the loss of the gateway of every instances running if nova-network falls
down. I couldn't afford 2 dedicated nodes to put nova-network itself in an
highly available state. Now if I loose a nova-network on a compute node,
all my instances running on this compute node will loose their gateway but
this scenario is better than loosing all my VMs. The multi_host was the
best option and I think it's applicable to every setup. So precisely, every
compute node hosts those services:

   - nova-compute
   - nova-network - avoid networking SPOF
   - nova-api-metadata - you don't need the entire nova-api service. Each
   new instance only needs to reach the metadata. Running this from the
   compute node can also improve performance with the cloud-init service.

Of course this setup works with the multi_host parameter enable.

My 2 cts contribution ;)

On Tue, Jun 19, 2012 at 7:52 PM, Florian Haas <florian at hastexo.com> wrote:
> Hi everyone,
>
> perhaps someone can shed some light on a floating IP issue.
>
> I have 2 nova-compute nodes (call them alice and bob), one of them
> (alice) is also running nova-network. bob uses alice as its
> --metadata_host and --network_host.
>
> I assign a floating IP to a guest running on bob. Expectedly, that IP
> is bound to the NIC specified as the --public_interface on alice (my
> nova-network host).
>
> However, since alice has a route into the --fixed_range network over
> its local bridge, the incoming traffic for the floating IP is routed
> there, where there's no guest to answer it -- because the guest is,
> after all, running on bob.
>
> Now, this seems fairly logical to me in the combination of
>
> 1. a nova-network host also running nova-compute;
> 2. other nova-compute hosts being around;
> 3. those other nova-compute hosts _not_ also running nova-network (and
> hence there being no multi-host networking).
>
> If my reasoning is correct, is it safe to say that in order to be able
> to use floating IPs in an environment with multiple nova-compute
> hosts, you must
>
> 1. Either have a single nova-network host that is _not_ also running
> nova-compute (but has a network connection to the --fixed_range
> network, of course);
> 2. or run nova-network on all your nova-compute hosts, which however
> requires that you enable multi-host mode and also run nova-api there?
>
> Any help is much appreciated. Thanks!
>
> Cheers,
> Florian
>
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