关于机房交换机故障导致HDFS NameNode挂掉的问题

HDFS HA中，一个active NN，一个standby NN，三个JNs，共涉及三台机器146.66/67/68。其中66上有一个JN，67上一个JN和一个active NN，68上一个JN和一个standby NN。67和68在一个机房，66在不同的机房。


发生故障的机房交换机，导致67和68都无法与66通信。所以，67上的active NN只能写入67和68上的JNs，68上的standby NN只能读取67和68上的JNs。
根据Hadoop官方文档（http://hadoop.apache.org/docs/stable/hadoop-project-dist/hadoop-hdfs/HDFSHighAvailabilityWithQJM.html），这种情况NNs和JN quorum应该正常工作：There must be at least 3 JournalNode daemons, since edit log modifications must be written to a majority of JNs. This will allow the system to tolerate the failure of a single machine. You may also run more than 3 JournalNodes, but in order to actually increase the number of failures the system can tolerate, you should run an odd number of JNs, (i.e. 3, 5, 7, etc.). Note that when running with N JournalNodes, the system can tolerate at most (N - 1) / 2 failures and continue to function normally。


但现实是，NN挂掉了。相关日志节选：
2015-11-05 03:01:37,135 FATAL namenode.FSEditLog (JournalSet.java:mapJournalsAndReportErrors(364)) - Error: flush failed for required journal (JournalAndStream(mgr=QJM to [192.168.146.66:8485, 192.168.146.67:8485, 192.168.146.68:8485], stream=QuorumOutputStream starting at txid 4354654650))
java.io.IOException: Timed out waiting 20000ms for a quorum of nodes to respond.
at org.apache.hadoop.hdfs.qjournal.client.AsyncLoggerSet.waitForWriteQuorum(AsyncLoggerSet.java:137)
at org.apache.hadoop.hdfs.qjournal.client.QuorumOutputStream.flushAndSync(QuorumOutputStream.java:107)
at org.apache.hadoop.hdfs.server.namenode.EditLogOutputStream.flush(EditLogOutputStream.java:113)
at org.apache.hadoop.hdfs.server.namenode.EditLogOutputStream.flush(EditLogOutputStream.java:107)
at org.apache.hadoop.hdfs.server.namenode.JournalSet$JournalSetOutputStream$8.apply(JournalSet.java:499)
at org.apache.hadoop.hdfs.server.namenode.JournalSet.mapJournalsAndReportErrors(JournalSet.java:359)
at org.apache.hadoop.hdfs.server.namenode.JournalSet.access$100(JournalSet.java:57)
at org.apache.hadoop.hdfs.server.namenode.JournalSet$JournalSetOutputStream.flush(JournalSet.java:495)
at org.apache.hadoop.hdfs.server.namenode.FSEditLog.logSync(FSEditLog.java:623)
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.getAdditionalBlock(FSNamesystem.java:2748)
at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.addBlock(NameNodeRpcServer.java:590)
at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolServerSideTranslatorPB.addBlock(ClientNamenodeProtocolServerSideTranslatorPB.java:440)
at org.apache.hadoop.hdfs.protocol.proto.ClientNamenodeProtocolProtos$ClientNamenodeProtocol$2.callBlockingMethod(ClientNamenodeProtocolProtos.java)
at org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:585)
at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:928)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2013)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2009)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:415)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1594)
at org.apache.hadoop.ipc.Server$Handler.run(Server.java:2007)
2015-11-05 03:01:37,135 WARN  client.QuorumJournalManager (QuorumOutputStream.java:abort(72)) - Aborting QuorumOutputStream starting at txid 4354654650
2015-11-05 03:01:37,139 INFO  util.ExitUtil (ExitUtil.java:terminate(124)) - Exiting with status 1
2015-11-05 03:01:37,145 INFO  namenode.NameNode (StringUtils.java:run(640)) - SHUTDOWN_MSG:
/************************************************************
SHUTDOWN_MSG: Shutting down NameNode at storm14667/192.168.146.67


查看HDFS的源码，与上面的行为一致。那么问题来了，是官方文档描述有问题（“at most”！？），还是我们的理解有误，进而我们的配置有问题？


从调用栈看，NN为处理client的请求，正在分配新的block，JournalSet.java：
  /**
   * The client would like to obtain an additional block for the indicated
   * filename (which is being written-to).  Return an array that consists
   * of the block, plus a set of machines.  The