Yongjun Zhang created HDFS-10943:
------------------------------------
Summary: rollEditLog expects empty EditsDoubleBuffer.bufCurrent
which is not guaranteed
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:
{code}
FATAL org.apache.hadoop.hdfs.server.namenode.FSEditLog: Error: finalize log
segment 10562075963, 10562174157 failed for required journal
(JournalAndStream(mgr=QJM to [0.0.0.1:8485, 0.0.0.2:8485, 0.0.0.3:8485,
0.0.0.4:8485, 0.0.0.5:8485], 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
{code}
The exception is from EditsDoubleBuffer
{code}
public void close() throws IOException {
Preconditions.checkNotNull(bufCurrent);
Preconditions.checkNotNull(bufReady);
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;
}
{code}
We can see that FSNamesystem.rollEditLog expects EditsDoubleBuffer.bufCurrent
to be empty.
Edits are recorded via FSEditLog$logSync, which does:
{code}
* The data is double-buffered within each edit log implementation so that
* in-memory writing can occur in parallel with the on-disk writing.
*
* Each sync occurs in three steps:
* 1. synchronized, it swaps the double buffer and sets the isSyncRunning
* flag.
* 2. unsynchronized, it flushes the data to storage
* 3. synchronized, it resets the flag and notifies anyone waiting on the
* sync.
*
* The lack of synchronization on step 2 allows other threads to continue
* to write into the memory buffer while the sync is in progress.
* Because this step is unsynchronized, actions that need to avoid
* concurrency with sync() should be synchronized and also call
* waitForSyncToFinish() before assuming they are running alone.
*/
{code}
We can see that step 2 is on-purposely not synchronized to let other threads to
write into the memory buffer, presumbaly EditsDoubleBuffer.bufCurrent. This
means that the EditsDoubleBuffer.bufCurrent can be non-empty when logSync is
done.
Now if rollEditLog happens, the above exception happens.
Another interesting observation is, the size of the EditsDoubleBuffer can be as
large as "private int outputBufferCapacity = 512 * 1024;", which means a lot of
edits could get buffered before they are flushed to JNs.
How can rollEdit operation expect the EditsDoubleBuffer.bufCurrent to be empty?
Or ask in another way, ollEdit operation want to close the stream
{code}
org.apache.hadoop.hdfs.server.namenode.JournalSet$JournalAndStream.closeStream(JournalSet.java:115)
{code}
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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