What is Split Brain in Oracle Clusterware and Real Application Cluster (Doc ID 1425586.1)

APPLIES TO:

Oracle Database - Enterprise Edition - Version 10.1.0.2 and later
Information in this document applies to any platform.

PURPOSE

This note is to explain what is split brain in an Oracle Real Application cluster and what errors/consequences are associated with it.

SCOPE

For DBA and Support engineer.

DETAILS

In generic term, split-brain indicates data inconsistencies originating from the maintenance of two separate data sets with overlap in scope, either because of servers
in a network design, or a failure condition based on servers not communicating and unifying their data to each other.
There are two components in Oracle Real Application Cluster implementation could experience split brain.

1. Clusterware layer

Cluster nodes maintain their heartbeat via private network and voting disk. When there is a private network disruption, cluster nodes can not communicate to each other via private network
for the time period of misscount setting, split brain will happen. In such case, voting disk will be used to determine which node(s) survive and which node(s) will be evicted. The common voting result will be:
a. The group with more cluster nodes survive

b. The group with lower node member in case of same number of node(s) available in each group

c. Some improvement has been made to ensure node(s) with lower load survive in case the eviction is caused by high system load.
Commonly, one will see messages similar to the followings in ocssd.log when split brain happens:

[ CSSD]2011-01-12 23:23:08.090 [1262557536] >TRACE: clssnmCheckDskInfo: Checking disk info...
[ CSSD]2011-01-12 23:23:08.090 [1262557536] >ERROR: clssnmCheckDskInfo: Aborting local node to avoid splitbrain.
[ CSSD]2011-01-12 23:23:08.090 [1262557536] >ERROR: : my node(2), Leader(2), Size(1) VS Node(1), Leader(1), Size(2)
[ CSSD]2011-01-12 23:23:08.090 [1262557536] >ERROR:
###################################
[ CSSD]2011-01-12 23:23:08.090 [1262557536] >ERROR: clssscExit: CSSD aborting
###################################

Above messages indicate the communication from node 2 to node 1 is not working, hence node 2 only sees 1 node, but node 1 is working fine and it can see two nodes in the cluster. To
avoid splitbrain, node 2 aborted itself.
Solution: Please engage network administrator to check private network layer to eliminate any network fault.

2. Real Application Cluster (database) layer

To ensure data consistency, each instance of a RAC database needs to keep heartbeat with the other instances. The heartbeat is maintained by background processes like LMON, LMD, LMS
and LCK. Any of these processes experience IPC Send time out will incur communication reconfiguration and instance eviction to avoid split brain. Controlfile is used similarly to voting disk in clusterware layer to determine which instance(s) survive and which
instance(s) evict. The voting result is similar to clusterware voting result. As the result, 1 or more instance(s) will be evicted.
Common messages in instance alert log are similar to:

alert log of instance 1:

---------

Mon Dec 07 19:43:05 2011

IPC Send timeout detected.Sender: ospid 26318

Receiver: inst 2 binc 554466600 ospid 29940

IPC Send timeout to 2.0 inc 8 for msg type 65521 from opid 20

Mon Dec 07 19:43:07 2011

Communications reconfiguration: instance_number 2

Mon Dec 07 19:43:07 2011

Trace dumping is performing id=[cdmp_20091207194307]

Waiting for clusterware split-brain resolution

Mon Dec 07 19:53:07 2011

Evicting instance 2 from cluster

Waiting for instances to leave: 

2 

...

alert log of instance 2:

---------

Mon Dec 07 19:42:18 2011

IPC Send timeout detected. Receiver ospid 29940

Mon Dec 07 19:42:18 2011

Errors in file 

/u01/app/oracle/diag/rdbms/bd/BD2/trace/BD2_lmd0_29940.trc:

Trace dumping is performing id=[cdmp_20091207194307]

Mon Dec 07 19:42:20 2011

Waiting for clusterware split-brain resolution

Mon Dec 07 19:44:45 2011

ERROR: LMS0 (ospid: 29942) detects an idle connection to instance 1

Mon Dec 07 19:44:51 2011

ERROR: LMD0 (ospid: 29940) detects an idle connection to instance 1

Mon Dec 07 19:45:38 2011

ERROR: LMS1 (ospid: 29954) detects an idle connection to instance 1

Mon Dec 07 19:52:27 2011

Errors in file 

/u01/app/oracle/diag/rdbms/bd/BD2/trace/PVBD2_lmon_29938.trc  

(incident=90153):

ORA-29740: evicted by member 0, group incarnation 10

Incident details in: 

/u01/app/oracle/diag/rdbms/bd/BD2/incident/incdir_90153/BD2_lmon_29938_i90153.trc
In above example, instance 2 LMD0 (pid 29940) is the receiver in IPC Send timeout. There could be variou
4000
s reasons causing IPC Send timeout. For example:
a. Network problem

b. Process hang

c. Bug etc
Please see Top 5 issues for Instance Eviction <Doc ID 1374110.1> for more information.
In case of instance eviction, alert log and all background traces need to be checked to determine the root cause.