Open Stack

Yes we will have to monitor the dhcp agents.

To scale "vertically" (as Mark put it), we need to adopt some of these
redundancy and failover techniques. We may end up having to provide all the
 options to the users and let them pick at the time of creation of their
networks.

Vinay

On Nov 30, 2012, at 4:54 PM, "Bhandaru, Malini K" <
malini.k.bhandaru at intel.com> wrote:

  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 <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>
wrote:


On Nov 28, 2012, at 8:03 AM, gong yong sheng <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>
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
Google Voice: 415 938 7576

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