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

[Simone Gotti <simone.gotti@gmail.com>]

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.

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


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