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From "Dmytro Molkov (JIRA)" <j...@apache.org>
Subject [jira] Created: (HDFS-1194) Secondary namenode fails to fetch the image from the primary
Date Tue, 08 Jun 2010 23:21:12 GMT
Secondary namenode fails to fetch the image from the primary
------------------------------------------------------------

                 Key: HDFS-1194
                 URL: https://issues.apache.org/jira/browse/HDFS-1194
             Project: Hadoop HDFS
          Issue Type: Bug
    Affects Versions: 0.20.2, 0.20.1, 0.21.0, 0.22.0, 0.20-append
         Environment: Java(TM) SE Runtime Environment (build 1.6.0_14-b08)
Java HotSpot(TM) 64-Bit Server VM (build 14.0-b16, mixed mode)

CentOS 5
            Reporter: Dmytro Molkov
            Assignee: Dmytro Molkov


We just hit the problem described in HDFS-1024 again.
After more investigation of the underlying problems with CancelledKeyException there are some
findings:

One of the symptoms: the transfer becomes really slow (it does 700 kb/s) when I am doing the
fetch using wget. At the same time disk and network are OK since I can copy at 50 mb/s using
scp.
I was taking jstacks of the namenode while the transfer is in process and we found that every
stack trace has one thread of jetty sitting in this place:

{code}
   java.lang.Thread.State: TIMED_WAITING (sleeping)
	at java.lang.Thread.sleep(Native Method)
	at org.mortbay.io.nio.SelectorManager$SelectSet.doSelect(SelectorManager.java:452)
	at org.mortbay.io.nio.SelectorManager.doSelect(SelectorManager.java:185)
	at org.mortbay.jetty.nio.SelectChannelConnector.accept(SelectChannelConnector.java:124)
	at org.mortbay.jetty.AbstractConnector$Acceptor.run(AbstractConnector.java:707)
	at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:522)
{code}

Here is a jetty code that corresponds to this:

{code}
// Look for JVM bug 
                    if (selected==0 && wait>0 && (now-before)<wait/2
&& _selector.selectedKeys().size()==0)
                    {
                        if (_jvmBug++>5)  // TODO tune or configure this
                        {
                            // Probably JVM BUG!
                            
                            Iterator iter = _selector.keys().iterator();
                            while(iter.hasNext())
                            {
                                key = (SelectionKey) iter.next();
                                if (key.isValid()&&key.interestOps()==0)
                                {
                                    key.cancel();
                                }
                            }
                            try
                            {
                                Thread.sleep(20);  // tune or configure this
                            }
                            catch (InterruptedException e)
                            {
                                Log.ignore(e);
                            }
                        } 
                    }
{code}

Based on this it is obvious we are hitting a jetty workaround for a JVM bug that doesn't handle
select() properly.

There is a jetty JIRA for this http://jira.codehaus.org/browse/JETTY-937 (it actually introduces
the workaround for the JVM bug that we are hitting)

They say that the problem was fixed in 6.1.22, there is a person on that JIRA also saying
that switching to using SocketConnector instead of SelectChannelConnector helped in their
case.
Since we are hitting the same bug in our world we should either adopt the newer Jetty version
where there is a better workaround, but it might not help if we are still hitting that bug
constantly, the workaround might be better though.
Another approach is to switch to using SocketConnector which will eliminate the problem completely,
although I am not sure what problems that will bring.

The java version we are running is in Environment

Any thoughts

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