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Vinay, good summary of the options and their pros-cons!

The simple solution does not waste IP addresses, same we have 2 servers and the first exhausts its address pool, then the second server is the only one with addresses left, we it will get used.  A negative is that if one of the servers were to go down, we have lost access to half the address space .. but if and when the server comes back, all shall be good. In the meantime, guess we would need to monitor the dhcp agents along with all other services/servers to see if they are alive.

- Split scope DHCP (two or more servers split the IP address and there is no overlap)
  pros: simple
  cons: wastes IP addresses,


From: Vinay Bannai [mailto:vbannai at gmail.com]
Sent: Friday, November 30, 2012 3:49 PM
To: OpenStack Development Mailing List
Subject: Re: [openstack-dev] [Quantum] continuing todays discussion about the l3 agents

Gary and Mark,

You brought up the issue of scaling horizontally and vertically in your earlier email. In the case of horizontal scaling, I would agree that it would have to be based on the "scheduler" approach proposed by Gong and Nachi.

On the issue of vertical scaling (I am using the DHCP redundancy as an example), I think it would be good to base our discussions on the various methods that have been discussed and do pro/con analysis in terms of scale, performance and other such metrics.

- Split scope DHCP (two or more servers split the IP address and there is no overlap)
  pros: simple
  cons: wastes IP addresses,

- Active/Standby model (might have run VRRP or hearbeats to dictate who is active)
  pros: load evenly shared
  cons: needs shared knowledge of address assignments,
            need hearbeats or VRRP to keep track of failovers

- LB method (use load balancer to fan out to multiple dhcp servers)
  pros: scales very well
  cons: the lb becomes the single point of failure,
           the lease assignments needs to be shared between the dhcp servers

I see that the DHCP agent and the quantum server communicate using RPC. Is the plan to leave it alone or migrate it towards something like AMQP based server in the future when the "scheduler" stuff is implemented.

Vinay


On Wed, Nov 28, 2012 at 8:03 AM, Mark McClain <mark.mcclain at dreamhost.com<mailto:mark.mcclain at dreamhost.com>> wrote:

On Nov 28, 2012, at 8:03 AM, gong yong sheng <gongysh at linux.vnet.ibm.com<mailto:gongysh at linux.vnet.ibm.com>> wrote:

> On 11/28/2012 08:11 AM, Mark McClain wrote:
>> On Nov 27, 2012, at 6:33 PM, gong yong sheng <gongysh at linux.vnet.ibm.com<mailto:gongysh at linux.vnet.ibm.com>> wrote:
>>
>> Just wanted to clarify two items:
>>
>>>> At the moment all of the dhcp agents receive all of the updates. I do not see why we need the quantum service to indicate which agent runs where. This will change the manner in which the dhcp agents work.
>>> No. currently, we can run only one dhcp agent  since we are using a topic queue for notification.
>> You are correct.  There is a bug in the underlying Oslo RPC implementation that sets the topic and queue names to be same value.  I didn't get a clear explanation of this problem until today and will have to figure out a fix to oslo.
>>
>>> And one problem with multiple agents serving the same ip is:
>>> we will have more than one agents want to update the ip's leasetime now and than.
>> This is not a problem.  The DHCP protocol was designed for multiple servers on a network.  When a client accepts a lease, the server that offered the accepted lease will be the only process attempting to update the lease for that port.  The other DHCP instances will not do anything, so there won't be any chance for a conflict.  Also, when a client renews it sends a unicast message to that previous DHCP server and so there will only be one writer in this scenario too.  Additionally, we don't have to worry about conflicting assignments because the dhcp agents use the same static allocations from the Quantum database.
> I mean dhcp agent is trying to update leasetime to quantum server. If we have more than one dhcp agents, this will cause confusion.
>    def update_lease(self, network_id, ip_address, time_remaining):
>        try:
>            self.plugin_rpc.update_lease_expiration(network_id, ip_address,
>                                                    time_remaining)
>        except:
>            self.needs_resync = True
>            LOG.exception(_('Unable to update lease'))
> I think it is our dhcp agent's defect. Why does our dhcp agent need the lease time? all the IPs are managed in our quantum server, there is not need for dynamic ip management in dhcp server managed by dhcp agent.
There cannot be confusion.  The dhcp client selects only one server to accept a lease, so only one agent will update this field at a time. (See RFC2131 section 4.3.2 for protocol specifics).  The dnsmasq allocation database is static in Quantum's setup, so the lease renewal needs to propagate to the Quantum Server.  The Quantum server then uses the lease time to avoid allocating IP addresses before the lease has expired.  In Quantum, we add an additional restriction that expired allocations are not reclaimed until the associated port has been deleted as well.

mark


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Vinay Bannai
Email: vbannai at gmail.com<mailto:vbannai at gmail.com>
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