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From Chris Nauroth <cnaur...@hortonworks.com>
Subject Re: hedged read bug
Date Fri, 06 Jun 2014 19:44:20 GMT
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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