shenxingfeng created HDFS-8221:
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Summary: HDFS have two Standby NNs because
ActiveStandbyElectorLock ephemeralOwner in ZK is different with the sessionId
stored in ZKFC
Key: HDFS-8221
URL: https://issues.apache.org/jira/browse/HDFS-8221
Project: Hadoop HDFS
Issue Type: Bug
Components: auto-failover
Affects Versions: 2.4.1
Reporter: shenxingfeng
Firstly, NN1 is active, NN2 is standby. When NN1 become standby due to some
reasons, NN2 then take over the active state imediately. But after NN2 becoming
active, It changed to standby again. And, HDFS got two standby NN forever.
After check the log, I found that NN2 become standby beacuse It have wrong
sessionID with ActiveStandbyElectorLock ephemeralOwner stored in Znode.
And the rootcause is when NN1 go to standby, NN2 create one session A with zk,
and become active. Ideally, NN2 should have the same sessionID with
ActiveStandbyElectorLock ephemeralOwner stored in Znode, but some network
reason can result in NN2'ZKFC sessionID changed.
So, I think when NN2 become standby due to different sessionid, NN2 should
unlock the state in Znode in order to failover again.
ActiveStandyElector.processResult
==================
Code code = Code.get(rc);
if (isSuccess(code)) {
// the following owner check completes verification in case the lock znode
// creation was retried
if (stat.getEphemeralOwner() == zkClient.getSessionId()) {
// we own the lock znode. so we are the leader
if (!becomeActive()) {
reJoinElectionAfterFailureToBecomeActive();
}
} else {
// we dont own the lock znode. so we are a standby.
becomeStandby();
}
// the watch set by us will notify about changes
return;
}
ActiveStandbyElectorLock content
==================
[zk: 160.149.0.114:24002(CONNECTED) 1] get
/hadoop-ha/hacluster/ActiveStandbyElectorLock
160-149-0-117 锟斤拷(锟斤拷
cZxid = 0x2000a38d9
ctime = Thu Apr 16 11:32:54 CST 2015
mZxid = 0x2000a38d9
mtime = Thu Apr 16 11:32:54 CST 2015
pZxid = 0x2000a38d9
cversion = 0
dataVersion = 0
aclVersion = 0
ephemeralOwner = 0x164cb2b3e4b36ae4
dataLength = 38
numChildren = 0
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