[Pacemaker] [Cluster-devel] [PATCH] Fix pacemaker's wrong quorum view in a CMAN+pacemaker cluster

[Simone Gotti]

On 03/14/2011 08:36 AM, Andrew Beekhof wrote:
> On Sun, Mar 13, 2011 at 2:17 PM, Simone Gotti <simone.gotti at gmail.com> wrote:
>> Hi all,
>>
>> Testing a cman+pacemaker cluster on rhel6 I noticed a very nasty
>> behavior when some nodes were leaving and rejoining the cluster. When a
>> nodes starts leaving and rejoining the cluster the quorum view of
>> pacemaker starts becoming sometimes different from the quorum view of
>> cman. The one not telling the truth was pacemaker.
> Do they ever start agreeing again?  In other words, is the situation
> transient or is Pacemaker always 1 (or more) behind after that?

Looks like it will be always behind and even more at every new cluster
change. I don't see any other part in the code where CMAN's events are
dequeued other than this one.

The event just says if we have or not quorum. So can happen that an old
dequeued message says the same as the current real state but it's just a
coincidence.



>> I reproduced the problem with a simple test case made of 2 nodes using
>> cman (no two_nodes flag) and pacemaker (started only on the first node:
>> pcmk01).
>>
>> For the tests I was using the latest version of pacemaker (1.1.5) while
>> keeping the original versions of corosync and cluster (cman) packages
>> provided by the rhel6 (corosync-1.2.3-21.el6.x86_64,
>> cman-3.0.12-23.el6.4.x86_64)
>>
>> The problem is that when a node joins a cluster (starting cman) the cman
>> on the other nodes emits not one but 2 events (I didn't investigated if
>> this is normal or present only in some versions of cman) but when crmd
>> calls cman_dispatch it's using the flag CMAN_DISPATCH_ONE so only one of
>> the two events is dequeued. In the subsequent cluster event the old one
>> is dequeued.
>>
>> The fix I tried used CMAN_DISPATCH_ALL instead of CMAN_DISPATCH_ONE and
>> looks like its working.
>>
>> I'm CCing the cluster-devel list as they can be interested in the double
>> event emitted by cman.
>>
>>
>> Thanks.
>>
>> Bye!
>>
>>
>> == Test case ==
>>
>> === Without the patch ===
>>
>> Start with both nodes with cman started (so the cluster is quorate).
>>
>>
>> Now stop cman on pcmk02. Output on pcmk01:
>>
>> pcmk01 corosync[16793]:   [CMAN  ] quorum lost, blocking activity
>> pcmk01 corosync[16793]:   [QUORUM] This node is within the non-primary
>> component and will NOT provide any services.
>> pcmk01 corosync[16793]:   [QUORUM] Members[1]: 1
>> pcmk01 corosync[16793]:   [TOTEM ] A processor joined or left the
>> membership and a new membership was formed.
>> pcmk01 corosync[16793]:   [CPG   ] downlist received left_list: 1
>> pcmk01 corosync[16793]:   [CPG   ] chosen downlist from node r(0)
>> ip(192.168.200.71)
>> pcmk01 corosync[16793]:   [MAIN  ] Completed service synchronization,
>> ready to provide service.
>> pcmk01 crmd: [16993]: notice: cman_event_callback: Membership 668:
>> quorum lost
>>
>> Only one event is enqueued.
>>
>> Now start again cman on pcmk02. Output on pcmk01:
>>
>> pcmk01 corosync[16793]:   [TOTEM ] A processor joined or left the
>> membership and a new membership was formed.
>> pcmk01 corosync[16793]:   [CMAN  ] quorum regained, resuming activity
>> pcmk01 corosync[16793]:   [QUORUM] This node is within the primary
>> component and will provide service.
>> pcmk01 corosync[16793]:   [QUORUM] Members[2]: 1 2
>> pcmk01 corosync[16793]:   [QUORUM] Members[2]: 1 2
>> pcmk01 crmd: [16993]: notice: cman_event_callback: Membership 672:
>> quorum acquired
>> pcmk01 corosync[16793]:   [CPG   ] downlist received left_list: 0
>> pcmk01 corosync[16793]:   [CPG   ] downlist received left_list: 0
>> pcmk01 corosync[16793]:   [CPG   ] chosen downlist from node r(0)
>> ip(192.168.200.71)
>> pcmk01 corosync[16793]:   [MAIN  ] Completed service synchronization,
>> ready to provide service.
>>
>> As you can see two events are enqueued and only one si dequeued (due to
>> the CMAN_DISPATCH_ONE flag passed to cman_dispatch).
>>
>> The quorum is ragained both on cman and crmd. But there's another event
>> saying that the quorum is regained in the queue.
>>
>>
>> Now stop again cman on pcmk02. Output on pcmk01:
>>
>> pcmk01 corosync[16793]:   [CMAN  ] quorum lost, blocking activity
>> pcmk01 corosync[16793]:   [QUORUM] This node is within the non-primary
>> component and will NOT provide any services.
>> pcmk01 corosync[16793]:   [QUORUM] Members[1]: 1
>> pcmk01 corosync[16793]:   [TOTEM ] A processor joined or left the
>> membership and a new membership was formed.
>> pcmk01 corosync[16793]:   [CPG   ] downlist received left_list: 1
>> pcmk01 corosync[16793]:   [CPG   ] chosen downlist from node r(0)
>> ip(192.168.200.71)
>> pcmk01 corosync[16793]:   [MAIN  ] Completed service synchronization,
>> ready to provide service.
>> pcmk01 crmd: [16993]: info: cman_event_callback: Membership 676: quorum
>> retained
>>
>> CMAN says that the quorum is lost and only one event is dispatched. But
>> crmd dequeued the previous event and thinks that we have the quorum.
>>
>>
>> Now start again cman on pcmk02. Output on pcmk01:
>>
>> pcmk01 corosync[16793]:   [TOTEM ] A processor joined or left the
>> membership and a new membership was formed.
>> pcmk01 corosync[16793]:   [CMAN  ] quorum regained, resuming activity
>> pcmk01 corosync[16793]:   [QUORUM] This node is within the primary
>> component and will provide service.
>> pcmk01 corosync[16793]:   [QUORUM] Members[2]: 1 2
>> pcmk01 corosync[16793]:   [QUORUM] Members[2]: 1 2
>> pcmk01 crmd: [16993]: notice: cman_event_callback: Membership 680:
>> quorum lost
>> pcmk01 corosync[16793]:   [CPG   ] downlist received left_list: 0
>> pcmk01 corosync[16793]:   [CPG   ] downlist received left_list: 0
>> pcmk01 corosync[16793]:   [CPG   ] chosen downlist from node r(0)
>> ip(192.168.200.71)
>> pcmk01 corosync[16793]:   [MAIN  ] Completed service synchronization,
>> ready to provide service.
>>
>> CMAN says that the quorum is regained but crmd dequeued again the old
>> event and now it says that the quorum is lost. And so on...
>>
>>
>>
>> === With the patch ===
>>
>> stop cman on pcmk02. Output on pcmk01:
>>
>> pcmk01 corosync[13149]:   [CMAN  ] quorum lost, blocking activity
>> pcmk01 corosync[13149]:   [QUORUM] This node is within the non-primary
>> component and will NOT provide any services.
>> pcmk01 corosync[13149]:   [QUORUM] Members[1]: 1
>> pcmk01 corosync[13149]:   [TOTEM ] A processor joined or left the
>> membership and a new membership was formed.
>> pcmk01 corosync[13149]:   [CPG   ] downlist received left_list: 1
>> pcmk01 corosync[13149]:   [CPG   ] chosen downlist from node r(0)
>> ip(192.168.200.71)
>> pcmk01 corosync[13149]:   [MAIN  ] Completed service synchronization,
>> ready to provide service.
>>
>>  pcmk01 crmd: [13351]: notice: cman_event_callback: Membership 648:
>> quorum lost
>>
>> Only one event is enqued.
>>
>>
>> Now start again cman on pcmk02. Output on pcmk01:
>>
>> pcmk01 corosync[13149]:   [TOTEM ] A processor joined or left the
>> membership and a new membership was formed.
>> pcmk01 corosync[13149]:   [CMAN  ] quorum regained, resuming activity
>> pcmk01 corosync[13149]:   [QUORUM] This node is within the primary
>> component and will provide service.
>> pcmk01 corosync[13149]:   [QUORUM] Members[2]: 1 2
>> pcmk01 corosync[13149]:   [QUORUM] Members[2]: 1 2
>> pcmk01 crmd: [13351]: notice: cman_event_callback: Membership 652:
>> quorum acquired
>> pcmk01 corosync[13149]:   [CPG   ] downlist received left_list: 0
>> pcmk01 corosync[13149]:   [CPG   ] downlist received left_list: 0
>> pcmk01 corosync[13149]:   [CPG   ] chosen downlist from node r(0)
>> ip(192.168.200.71)
>> pcmk01 corosync[13149]:   [MAIN  ] Completed service synchronization,
>> ready to provide service.
>> pcmk01 crmd: [13351]: info: cman_event_callback: Membership 652: quorum
>> retained
>>
>> As you can see two events are enqued and both are dequeued.
>>
>>
>>
>> --
>> Simone Gotti
>>
>>
>>


-- 
Simone Gotti