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From "Yongjun Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-10943) rollEditLog expects empty EditsDoubleBuffer.bufCurrent which is not guaranteed
Date Sat, 01 Oct 2016 04:13:20 GMT
Yongjun Zhang created HDFS-10943:

             Summary: rollEditLog expects empty EditsDoubleBuffer.bufCurrent which is not
                 Key: HDFS-10943
                 URL: https://issues.apache.org/jira/browse/HDFS-10943
             Project: Hadoop HDFS
          Issue Type: Bug
            Reporter: Yongjun Zhang

Per the following trace stack:
FATAL org.apache.hadoop.hdfs.server.namenode.FSEditLog: Error: finalize log segment 10562075963,
10562174157 failed for required journal (JournalAndStream(mgr=QJM to [,,,,], stream=QuorumOutputStream starting at txid 10562075963))
java.io.IOException: FSEditStream has 49708 bytes still to be flushed and cannot be closed.
        at org.apache.hadoop.hdfs.server.namenode.EditsDoubleBuffer.close(EditsDoubleBuffer.java:66)
        at org.apache.hadoop.hdfs.qjournal.client.QuorumOutputStream.close(QuorumOutputStream.java:65)
        at org.apache.hadoop.hdfs.server.namenode.JournalSet$JournalAndStream.closeStream(JournalSet.java:115)
        at org.apache.hadoop.hdfs.server.namenode.JournalSet$4.apply(JournalSet.java:235)
        at org.apache.hadoop.hdfs.server.namenode.JournalSet.mapJournalsAndReportErrors(JournalSet.java:393)
        at org.apache.hadoop.hdfs.server.namenode.JournalSet.finalizeLogSegment(JournalSet.java:231)
        at org.apache.hadoop.hdfs.server.namenode.FSEditLog.endCurrentLogSegment(FSEditLog.java:1243)
        at org.apache.hadoop.hdfs.server.namenode.FSEditLog.rollEditLog(FSEditLog.java:1172)
        at org.apache.hadoop.hdfs.server.namenode.FSImage.rollEditLog(FSImage.java:1243)
        at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.rollEditLog(FSNamesystem.java:6437)
        at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.rollEditLog(NameNodeRpcServer.java:1002)
        at org.apache.hadoop.hdfs.protocolPB.NamenodeProtocolServerSideTranslatorPB.rollEditLog(NamenodeProtocolServerSideTranslatorPB.java:142)
        at org.apache.hadoop.hdfs.protocol.proto.NamenodeProtocolProtos$NamenodeProtocolService$2.callBlockingMethod(NamenodeProtocolProtos.java:12025)
        at org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:617)
        at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:1060)
        at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2086)
        at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2082)
        at java.security.AccessController.doPrivileged(Native Method)
        at javax.security.auth.Subject.doAs(Subject.java:422)
        at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1671)
        at org.apache.hadoop.ipc.Server$Handler.run(Server.java:2080)
2016-09-23 21:40:59,618 WARN org.apache.hadoop.hdfs.qjournal.client.QuorumJournalManager:
Aborting QuorumOutputStream starting at txid 10562075963

The exception is from  EditsDoubleBuffer
 public void close() throws IOException {

    int bufSize = bufCurrent.size();
    if (bufSize != 0) {
      throw new IOException("FSEditStream has " + bufSize
          + " bytes still to be flushed and cannot be closed.");

    IOUtils.cleanup(null, bufCurrent, bufReady);
    bufCurrent = bufReady = null;

We can see that FSNamesystem.rollEditLog expects  EditsDoubleBuffer.bufCurrent to be empty.

Edits are recorded via FSEditLog$logSync, which does:
   * The data is double-buffered within each edit log implementation so that
   * in-memory writing can occur in parallel with the on-disk writing.
   * Each sync occurs in three steps:
   *   1. synchronized, it swaps the double buffer and sets the isSyncRunning
   *      flag.
   *   2. unsynchronized, it flushes the data to storage
   *   3. synchronized, it resets the flag and notifies anyone waiting on the
   *      sync.
   * The lack of synchronization on step 2 allows other threads to continue
   * to write into the memory buffer while the sync is in progress.
   * Because this step is unsynchronized, actions that need to avoid
   * concurrency with sync() should be synchronized and also call
   * waitForSyncToFinish() before assuming they are running alone.

We can see that step 2 is on-purposely not synchronized to let other threads to write into
the memory buffer, presumbaly EditsDoubleBuffer.bufCurrent. This means that the EditsDoubleBuffer.bufCurrent
 can be non-empty when logSync is done.

Now if rollEditLog happens, the above exception happens.

Another interesting observation is, the size of the EditsDoubleBuffer can be as large as "private
int outputBufferCapacity = 512 * 1024;", which means a lot of edits could get buffered before
they are flushed to JNs. 

How can rollEdit operation expect the EditsDoubleBuffer.bufCurrent to be empty? Or ask in
another way, ollEdit operation want to close the stream
where the close operation expects EditsDoubleBuffer.bufCurrent to be empty. It  seems rollEdit
should make sure the EditsDoubleBuffer is flushed in a synchronized way before it tries to
close the stream. Why it doesn't do that?

If my above theory is correct, wonder why this issue doesn't show up so often.

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