On 4/12/23 23:07, Han Zhou wrote:
> On Wed, Apr 12, 2023 at 8:01 AM <[email protected]> wrote:
>>
>> From: Lucas Alvares Gomes <[email protected]>
>>
>> In order for the CMS to know which Chassis a distributed gateway port
>> is bond to, this patch updates the ovn-northd daemon to populate the
>> Logical_Router_Port table with that information.
>>
>> To avoid changing the database schema, ovn-northd is setting a new key
>> called "hosting-chassis" in the options column from the LRP table. This
>> key value points to the name of the Chassis that is currently hosting
>> the distributed port.
>>
>> Reported-at: https://bugzilla.redhat.com/show_bug.cgi?id=2107515
>> Signed-off-by: Lucas Alvares Gomes <[email protected]>

Hi, Lucas, Han,

> 
> Thanks Lucas for the patch. However, in my opinion the chassis binding
> information belongs to SB and should stay there, otherwise we would make it
> consistent for LSPs and update the chassis information for them, too, which
> I think is not good in terms of clarity and extra control plane load. We'd
> better keep the separation between NB and SB clear and avoid propagating
> data between them back-and-forth.
> 

I partially agree with this but it also feels wrong that the CMS
accesses the SB directly.  In an ideal world (and I know that's not the
case today for neutron or ovn-k8s) the CMS should not care about what's
in the SB; that is internal OVN data.

I suggest a different approach if we want to go ahead and propagate such
information to the NB: can't we store the "active chassis" information
per Gateway_chassis/HA_Chassis_group instead?  That's
O(number-of-chassis) records that we need to update on chassis failover.
 We might even skip this for Gateway_chassis as I understand that this
is the "old" way of configuring things (*).

(*) Should we deprecate Gateway_chassis?

> For the problem mentioned in the bugzilla, it seems to me already a scale
> challenge that something other than ovn-controller is connecting to OVN SB
> from every node (if I understand correctly). Moving all these connections
> from SB to NB may just make it much worse, because NB DB is usually more
> heavily/frequently updated by the CMS. (For small scale, this may not
> matter, even if the agent connects to both NB and SB.)
> 

An alternative to address the scale issue without changing OVN could be
to use a dedicated SB relay to which all external (non-OVN) agents that
need access to SB information can connect.  Would that help?

Regards,
Dumitru

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