Ming Ma created HDFS-6306:
-----------------------------

             Summary: Standby NN can hold FSDirectory's writeLock for a long 
time under heavy load
                 Key: HDFS-6306
                 URL: https://issues.apache.org/jira/browse/HDFS-6306
             Project: Hadoop HDFS
          Issue Type: Bug
            Reporter: Ming Ma


Standby NN uses FSEditLogLoader to update its namespace.  It can hold 
FSDirectory's writeLock for a long time when active NN generates lots of edits.

{noformat}

loadEditRecords

    fsNamesys.writeLock();
    fsDir.writeLock();
    ...
    try {
      while (true) {
        try {
          FSEditLogOp op;
          try {
            op = in.readOp();
        ...
          }
       }
    } finally {
      ...
      fsDir.writeUnlock();
      fsNamesys.writeUnlock();
    }
{noformat}

With the fix in https://issues.apache.org/jira/browse/HDFS-5693, JMX response 
time is good for active NN as it no longer requires FSnamesystem's lock, even 
though it still need to acquire FSDirectory's readlock during FSDirectory's 
totalInodes. That isn't an issue for active NN as each client RPC request might 
only acquire FSDirectory lock for short period of time. But Standby NN could 
hold the lock for a longer period of time.

There are two ways to fix these:

1. Fix standby NN to acquire FSDirectory's writeLock for each edit record.
2. Fix FSDirectory's totalInodes to not take readLock so JMX can still go 
through.



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