Open Stack

> On 28 Apr 2015, at 11:15 pm, Bogdan Dobrelya <bdobrelia at mirantis.com> wrote:
> 
>> Hello,
> 
> Hello, Zhou
> 
>> 
>> I using Fuel 6.0.1 and find that RabbitMQ recover time is long after
>> power failure. I have a running HA environment, then I reset power of
>> all the machines at the same time. I observe that after reboot it
>> usually takes 10 minutes for RabittMQ cluster to appear running
>> master-slave mode in pacemaker. If I power off all the 3 controllers and
>> only start 2 of them, the downtime sometimes can be as long as 20 minutes.
> 
> Yes, this is a known issue [0]. Note, there were many bugfixes, like
> [1],[2],[3], merged for MQ OCF script, so you may want to try to
> backport them as well by the following guide [4]
> 
> [0] https://bugs.launchpad.net/fuel/+bug/1432603
> [1] https://review.openstack.org/#/c/175460/
> [2] https://review.openstack.org/#/c/175457/
> [3] https://review.openstack.org/#/c/175371/
> [4] https://review.openstack.org/#/c/170476/

Is there a reason you’re using a custom OCF script instead of the upstream[a] one?
Please have a chat with David (the maintainer, in CC) if there is something you believe is wrong with it.

[a] https://github.com/ClusterLabs/resource-agents/blob/master/heartbeat/rabbitmq-cluster

> 
>> 
>> I have a little investigation and find out there are some possible causes.
>> 
>> 1. MySQL Recovery Takes Too Long [1] and Blocking RabbitMQ Clustering in
>> Pacemaker
>> 
>> The pacemaker resource p_mysql start timeout is set to 475s. Sometimes
>> MySQL-wss fails to start after power failure, and pacemaker would wait
>> 475s before retry starting it. The problem is that pacemaker divides
>> resource state transitions into batches. Since RabbitMQ is master-slave
>> resource, I assume that starting all the slaves and promoting master are
>> put into two different batches. If unfortunately starting all RabbitMQ
>> slaves are put in the same batch as MySQL starting, even if RabbitMQ
>> slaves and all other resources are ready, pacemaker will not continue
>> but just wait for MySQL timeout.
> 
> Could you please elaborate the what is the same/different batches for MQ
> and DB? Note, there is a MQ clustering logic flow charts available here
> [5] and we're planning to release a dedicated technical bulletin for this.
> 
> [5] http://goo.gl/PPNrw7
> 
>> 
>> I can re-produce this by hard powering off all the controllers and start
>> them again. It's more likely to trigger MySQL failure in this way. Then
>> I observe that if there is one cloned mysql instance not starting, the
>> whole pacemaker cluster gets stuck and does not emit any log. On the
>> host of the failed instance, I can see a mysql resource agent process
>> calling the sleep command. If I kill that process, the pacemaker comes
>> back alive and RabbitMQ master gets promoted. In fact this long timeout
>> is blocking every resource from state transition in pacemaker.
>> 
>> This maybe a known problem of pacemaker and there are some discussions
>> in Linux-HA mailing list [2]. It might not be fixed in the near future.
>> It seems in generally it's bad to have long timeout in state transition
>> actions (start/stop/promote/demote). There maybe another way to
>> implement MySQL-wss resource agent to use a short start timeout and
>> monitor the wss cluster state using monitor action.
> 
> This is very interesting, thank you! I believe all commands for MySQL RA
> OCF script should be as well wrapped with timeout -SIGTERM or -SIGKILL
> as we did for MQ RA OCF. And there should no be any sleep calls. I
> created a bug for this [6].
> 
> [6] https://bugs.launchpad.net/fuel/+bug/1449542
> 
>> 
>> I also find a fix to improve MySQL start timeout [3]. It shortens the
>> timeout to 300s. At the time I sending this email, I can not find it in
>> stable/6.0 branch. Maybe the maintainer needs to cherry-pick it to
>> stable/6.0 ?
>> 
>> [1] https://bugs.launchpad.net/fuel/+bug/1441885
>> [2] http://lists.linux-ha.org/pipermail/linux-ha/2014-March/047989.html
>> [3] https://review.openstack.org/#/c/171333/
>> 
>> 
>> 2. RabbitMQ Resource Agent Breaks Existing Cluster
>> 
>> Read the code of the RabbitMQ resource agent, I find it does the
>> following to start RabbitMQ master-slave cluster.
>> On all the controllers:
>> (1) Start Erlang beam process
>> (2) Start RabbitMQ App (If failed, reset mnesia DB and cluster state)
>> (3) Stop RabbitMQ App but do not stop the beam process
>> 
>> Then in pacemaker, all the RabbitMQ instances are in slave state. After
>> pacemaker determines the master, it does the following.
>> On the to-be-master host:
>> (4) Start RabbitMQ App (If failed, reset mnesia DB and cluster state)
>> On the slaves hosts:
>> (5) Start RabbitMQ App (If failed, reset mnesia DB and cluster state)
>> (6) Join RabbitMQ cluster of the master host
>> 
> 
> Yes, something like that. As I mentioned, there were several bug fixes
> in the 6.1 dev, and you can also check the MQ clustering flow charts.
> 
>> As far as I can understand, this process is to make sure the master
>> determined by pacemaker is the same as the master determined in RabbitMQ
>> cluster. If there is no existing cluster, it's fine. If it is run
> after
> 
> Not exactly. There is no master in mirrored MQ cluster. We define the
> rabbit_hosts configuration option from Oslo.messaging. What ensures all
> queue masters will be spread around all of MQ nodes in a long run. And
> we use a master abstraction only for the Pacemaker RA clustering layer.
> Here, a "master" is the MQ node what joins the rest of the MQ nodes.
> 
>> power failure and recovery, it introduces the a new problem.
> 
> We do erase the node master attribute in CIB for such cases. This should
> not bring problems into the master election logic.
> 
>> 
>> After power recovery, if some of the RabbitMQ instances reach step (2)
>> roughly at the same time (within 30s which is hard coded in RabbitMQ) as
>> the original RabbitMQ master instance, they form the original cluster
>> again and then shutdown. The other instances would have to wait for 30s
>> before it reports failure waiting for tables, and be  reset to a
>> standalone cluster.
>> 
>> In RabbitMQ documentation [4], it is also mentioned that if we shutdown
>> RabbitMQ master, a new master is elected from the rest of slaves. If we
> 
> (Note, the RabbitMQ documentation mentions *queue* masters and slaves,
> which are not the case for the Pacemaker RA clustering abstraction layer.)
> 
>> continue to shutdown nodes in step (3), we reach a point that the last
>> node is the RabbitMQ master, and pacemaker is not aware of it. I can see
>> there is code to bookkeeping a "rabbit-start-time" attribute in
>> pacemaker to record the most long lived instance to help pacemaker
>> determine the master, but it does not cover the case mentioned above.
> 
> We made an assumption what the node with the highest MQ uptime should
> know the most about recent cluster state, so other nodes must join it.
> RA OCF does not work with queue masters directly.
> 
>> A
>> recent patch [5] checks existing "rabbit-master" attribute but it
>> neither cover the above case.
>> 
>> So in step (4), pacemaker determines a different master which was a
>> RabbitMQ slave last time. It would wait for its original RabbitMQ master
>> for 30s and fail, then it gets reset to a standalone cluster. Here we
>> get some different clusters, so in step (5) and (6), it is likely to
>> report error in log saying timeout waiting for tables or fail to merge
>> mnesia database schema, then the those instances get reset. You can
>> easily re-produce the case by hard resetting power of all the controllers.
>> 
>> As you can see, if you are unlucky, there would be several "30s timeout
>> and reset" before you finally get a healthy RabbitMQ cluster.
> 
> The full MQ cluster reassemble logic is far from the perfect state,
> indeed. This might erase all mnesia files, hence any custom entities,
> like users or vhosts, would be removed as well. Note, we do not
> configure durable queues for Openstack so there is nothing to care about
> here - the full cluster downtime assumes there will be no AMQP messages
> stored at all.
> 
>> 
>> I find three possible solutions.
>> A. Using rabbitmqctl force_boot option [6]
>> It will skips waiting for 30s and resetting cluster, but just assume the
>> current node is the master and continue to operate. This is feasible
>> because the original RabbitMQ master would discards the local state and
>> sync with the new master after it joins a new cluster [7]. So we can be
>> sure that after step (4) and (6), the pacemaker determined master
>> instance is started unconditionally, and it will be the same as RabbitMQ
>> master, and all operations run without 30s timeout. I find this option
>> is only available in newer RabbitMQ release, and updating RabbitMQ might
>> introduce other compatibility problems.
> 
> Yes, this option is only supported for newest RabbitMQ versions. But we
> definitely should look how this could help.
> 
>> 
>> B. Turn RabbitMQ into cloned instance and use pause_minority instead of
>> autoheal [8]
> 
> Indeed, there are cases when MQ's autoheal can do nothing with existing
> partitions and remains partitioned for ever, for example:
> 
> Masters: [ node-1 ]
> Slaves: [ node-2 node-3 ]
> root at node-1:~# rabbitmqctl cluster_status
> Cluster status of node 'rabbit at node-1' ...
> [{nodes,[{disc,['rabbit at node-1','rabbit at node-2']}]},
> {running_nodes,['rabbit at node-1']},
> {cluster_name,<<"rabbit at node-2">>},
> {partitions,[]}]
> ...done.
> root at node-2:~# rabbitmqctl cluster_status
> Cluster status of node 'rabbit at node-2' ...
> [{nodes,[{disc,['rabbit at node-2']}]}]
> ...done.
> root at node-3:~# rabbitmqctl cluster_status
> Cluster status of node 'rabbit at node-3' ...
> [{nodes,[{disc,['rabbit at node-1','rabbit at node-2','rabbit at node-3']}]},
> {running_nodes,['rabbit at node-3']},
> {cluster_name,<<"rabbit at node-2">>},
> {partitions,[]}]
> 
> So we should test the pause-minority value as well.
> But I strongly believe we should make MQ multi-state clone to support
> many masters, related bp [7]
> 
> [7]
> https://blueprints.launchpad.net/fuel/+spec/rabbitmq-pacemaker-multimaster-clone
> 
>> This works like MySQL-wss. It let RabbitMQ cluster itself deal with
>> partition in a manner similar to pacemaker quorum mechanism. When there
>> is network partition, instances in the minority partition pauses
>> themselves automatically. Pacemaker does not have to track who is the
>> RabbitMQ master, who lives longest, who to promote... It just starts all
>> the clones, done. This leads to huge change in RabbitMQ resource agent,
>> and the stability and other impact is to be tested.
> 
> Well, we should not mess the queue masters and multi-clone master for MQ
> resource in the pacemaker.
> As I said, pacemaker RA has nothing to do with queue masters. And we
> introduced this "master" mostly in order to support the full cluster
> reassemble case - there must be a node promoted and other nodes should join.
> 
>> 
>> C. Creating a "force_load" file
>> After reading RabbitMQ source code, I find that the actual thing it does
>> in solution A is just creating an empty file named "force_load" in
>> mnesia database dir, then mnesia thinks it is the last node shut down in
>> the last time and boot itself as the master. This implementation keeps
>> the same from v3.1.4 to the latest RabbitMQ master branch. I think we
>> can make use of this little trick. The change is adding just one line in
>> "try_to_start_rmq_app()" function.
>> 
>> touch "${MNESIA_FILES}/force_load" && \
>>  chown rabbitmq:rabbitmq "${MNESIA_FILES}/force_load"
> 
> This is a very good point, thank you.
> 
>> 
>> [4] http://www.rabbitmq.com/ha.html
>> [5] https://review.openstack.org/#/c/169291/
>> [6] https://www.rabbitmq.com/clustering.html
>> [7] http://www.rabbitmq.com/partitions.html#recovering
>> [8] http://www.rabbitmq.com/partitions.html#automatic-handling
>> 
>> Maybe you have better ideas on this. Please share your thoughts.
> 
> Thank you for a thorough feedback! This was a really great job.
> 
>> 
>> ----
>> Best wishes!
>> Zhou Zheng Sheng / ???  Software Engineer
>> Beijing AWcloud Software Co., Ltd.
>> 
> 
> 
> -- 
> Best regards,
> Bogdan Dobrelya,
> Skype #bogdando_at_yahoo.com
> Irc #bogdando
> 
> __________________________________________________________________________
> OpenStack Development Mailing List (not for usage questions)
> Unsubscribe: OpenStack-dev-request at lists.openstack.org?subject:unsubscribe
> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev