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From Chris Nauroth <cnaur...@hortonworks.com>
Subject Re: hedged read bug
Date Mon, 09 Jun 2014 19:06:31 GMT
Hi Lei,

I just reviewed this code path on trunk again, and I couldn't find a
problem.  It appears to me that if one future fails, then the exception
handling logic will allow the other future to proceed without canceling.
 Also, I haven't been able to reproduce the infinite loop that you reported
with the test case that you gave.

However, if you're still seeing a bug on your side, then I recommend filing
a new jira issue with a full description.  We can continue troubleshooting
there.

Chris Nauroth
Hortonworks
http://hortonworks.com/



On Sun, Jun 8, 2014 at 8:16 PM, lei liu <liulei412@gmail.com> wrote:

> Hi Chris,
>
> I review the patch, I think there is problem in the patch.
>
> Example  there are two futures, if the first return futrue is failure  and
> then the the second future will be cancled.
>
>
> 2014-06-07 3:44 GMT+08:00 Chris Nauroth <cnauroth@hortonworks.com>:
>
> > Hello Lei,
> >
> > There is a known bug in 2.4.0 that can cause hedged reads to hang.  I
> fixed
> > it in HDFS-6231:
> >
> > https://issues.apache.org/jira/browse/HDFS-6231
> >
> > This patch will be included in the forthcoming 2.4.1 release.  I'm
> curious
> > to see if applying this patch fixes the problem for you.  Can you try it
> > and let us know?  Thank you!
> >
> > Chris Nauroth
> > Hortonworks
> > http://hortonworks.com/
> >
> >
> >
> > On Thu, Jun 5, 2014 at 8:34 PM, lei liu <liulei412@gmail.com> wrote:
> >
> > > I use hadoop2.4.
> > >
> > > When I use "hedged read", If there is only one live datanode, the
> reading
> > > from  the datanode throw TimeoutException and ChecksumException., the
> > > Client will infinite wait.
> > >
> > > Example below test case:
> > >   @Test
> > >   public void testException() throws IOException, InterruptedException,
> > > ExecutionException {
> > >     Configuration conf = new Configuration();
> > >     int numHedgedReadPoolThreads = 5;
> > >     final int hedgedReadTimeoutMillis = 50;
> > >
> conf.setInt(DFSConfigKeys.DFS_DFSCLIENT_HEDGED_READ_THREADPOOL_SIZE,
> > > numHedgedReadPoolThreads);
> > >
> > conf.setLong(DFSConfigKeys.DFS_DFSCLIENT_HEDGED_READ_THRESHOLD_MILLIS,
> > >       hedgedReadTimeoutMillis);
> > >     // Set up the InjectionHandler
> > >     DFSClientFaultInjector.instance =
> > > Mockito.mock(DFSClientFaultInjector.class);
> > >     DFSClientFaultInjector injector = DFSClientFaultInjector.instance;
> > >     // make preads ChecksumException
> > >     Mockito.doAnswer(new Answer<Void>() {
> > >       @Override
> > >       public Void answer(InvocationOnMock invocation) throws Throwable
> {
> > >         if(true) {
> > >           Thread.sleep(hedgedReadTimeoutMillis + 10);
> > >           throw new ChecksumException("test", 100);
> > >         }
> > >         return null;
> > >       }
> > >     }*).when(injector).fetchFromDatanodeException();*
> > >
> > >     MiniDFSCluster cluster = new
> > > MiniDFSCluster.Builder(conf).numDataNodes(3).format(true).build();
> > >     DistributedFileSystem fileSys = cluster.getFileSystem();
> > >     DFSClient dfsClient = fileSys.getClient();
> > >     DFSHedgedReadMetrics metrics = dfsClient.getHedgedReadMetrics();
> > >
> > >     try {
> > >       Path file = new Path("/hedgedReadException.dat");
> > >       FSDataOutputStream  output = fileSys.create(file,(short)1);
> > >       byte[] data = new byte[64 * 1024];
> > >       output.write(data);
> > >       output.flush();
> > >       output.write(data);
> > >       output.flush();
> > >       output.write(data);
> > >       output.flush();
> > >       output.close();
> > >       byte[] buffer = new byte[64 * 1024];
> > >       FSDataInputStream  input = fileSys.open(file);
> > >       input.read(0, buffer, 0, 1024);
> > >       input.close();
> > >       assertTrue(metrics.getHedgedReadOps() == 1);
> > >       assertTrue(metrics.getHedgedReadWins() == 1);
> > >     } finally {
> > >       fileSys.close();
> > >       cluster.shutdown();
> > >       Mockito.reset(injector);
> > >     }
> > >   }
> > >
> > >
> > > *The code of actualGetFromOneDataNode() method call
> > > **fetchFromDatanodeException()
> > > method as below:*
> > >       try {
> > >         *DFSClientFaultInjector.get().fetchFromDatanodeException();*
> > >         Token<BlockTokenIdentifier> blockToken = block.getBlockToken();
> > >         int len = (int) (end - start + 1);
> > >         reader = new BlockReaderFactory(dfsClient.getConf()).
> > >             setInetSocketAddress(targetAddr).
> > >             setRemotePeerFactory(dfsClient).
> > >             setDatanodeInfo(chosenNode).
> > >             setFileName(src).
> > >             setBlock(block.getBlock()).
> > >             setBlockToken(blockToken).
> > >             setStartOffset(start).
> > >             setVerifyChecksum(verifyChecksum).
> > >             setClientName(dfsClient.clientName).
> > >             setLength(len).
> > >             setCachingStrategy(curCachingStrategy).
> > >
> setAllowShortCircuitLocalReads(allowShortCircuitLocalReads).
> > >             setClientCacheContext(dfsClient.getClientContext()).
> > >             setUserGroupInformation(dfsClient.ugi).
> > >             setConfiguration(dfsClient.getConfiguration()).
> > >             build();
> > >         int nread = reader.readAll(buf, offset, len);
> > >         if (nread != len) {
> > >           throw new IOException("truncated return from reader.read():
> " +
> > >                                 "excpected " + len + ", got " + nread);
> > >         }
> > >         return;
> > >       } catch (ChecksumException e) {
> > >         String msg = "fetchBlockByteRange(). Got a checksum exception
> > for "
> > >             + src + " at " + block.getBlock() + ":" + e.getPos() + "
> > from "
> > >             + chosenNode;
> > >         DFSClient.LOG.warn(msg);
> > >         // we want to remember what we have tried
> > >         addIntoCorruptedBlockMap(block.getBlock(), chosenNode,
> > > corruptedBlockMap);
> > >         addToDeadNodes(chosenNode);
> > >         throw new IOException(msg);
> > >       }
> > >
> >
> > --
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-- 
CONFIDENTIALITY NOTICE
NOTICE: This message is intended for the use of the individual or entity to 
which it is addressed and may contain information that is confidential, 
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of this message is not the intended recipient, you are hereby notified that 
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received this communication in error, please contact the sender immediately 
and delete it from your system. Thank You.

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