HDFS-11738. Hedged pread takes more time when block moved from initial locations. Contributed by Vinayakumar B.

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/DFSClientFaultInjector.java

@@ -61,4 +61,6 @@ public class DFSClientFaultInjector {
   public boolean skipRollingRestartWait() {
     return false;
   }
+
+  public void sleepBeforeHedgedGet() {}
 }

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/DFSInputStream.java

@@ -830,11 +830,36 @@ public class DFSInputStream extends FSInputStream
 
   private DNAddrPair chooseDataNode(LocatedBlock block,
       Collection<DatanodeInfo> ignoredNodes) throws IOException {
+    return chooseDataNode(block, ignoredNodes, true);
+  }
+
+  /**
+   * Choose datanode to read from.
+   *
+   * @param block             Block to choose datanode addr from
+   * @param ignoredNodes      Ignored nodes inside.
+   * @param refetchIfRequired Whether to refetch if no nodes to chose
+   *                          from.
+   * @return Returns chosen DNAddrPair; Can be null if refetchIfRequired is
+   * false.
+   */
+  private DNAddrPair chooseDataNode(LocatedBlock block,
+      Collection<DatanodeInfo> ignoredNodes, boolean refetchIfRequired)
+      throws IOException {
     while (true) {
       DNAddrPair result = getBestNodeDNAddrPair(block, ignoredNodes);
       if (result != null) {
         return result;
+      } else if (refetchIfRequired) {
+        block = refetchLocations(block, ignoredNodes);
       } else {
+        return null;
+      }
+    }
+  }
+
+  private LocatedBlock refetchLocations(LocatedBlock block,
+      Collection<DatanodeInfo> ignoredNodes) throws IOException {
     String errMsg = getBestNodeDNAddrPairErrorString(block.getLocations(),
         deadNodes, ignoredNodes);
     String blockInfo = block.getBlock() + " file=" + src;
@@ -864,7 +889,8 @@ public class DFSInputStream extends FSInputStream
       // will wait 6000ms grace period before retry and the waiting window is
       // expanded to 9000ms.
       final int timeWindow = dfsClient.getConf().getTimeWindow();
-          double waitTime = timeWindow * failures +       // grace period for the last round of attempt
+      // grace period for the last round of attempt
+      double waitTime = timeWindow * failures +
           // expanding time window for each failure
           timeWindow * (failures + 1) *
           ThreadLocalRandom.current().nextDouble();
@@ -880,8 +906,7 @@ public class DFSInputStream extends FSInputStream
     openInfo(true);
     block = refreshLocatedBlock(block);
     failures++;
-      }
-    }
+    return block;
   }
 
   /**
@@ -985,6 +1010,7 @@ public class DFSInputStream extends FSInputStream
     return new Callable<ByteBuffer>() {
       @Override
       public ByteBuffer call() throws Exception {
+        DFSClientFaultInjector.get().sleepBeforeHedgedGet();
         try (TraceScope ignored = dfsClient.getTracer().
             newScope("hedgedRead" + hedgedReadId, parentSpanId)) {
           actualGetFromOneDataNode(datanode, start, end, bb, corruptedBlocks);
@@ -1159,20 +1185,22 @@ public class DFSInputStream extends FSInputStream
         // We are starting up a 'hedged' read. We have a read already
         // ongoing. Call getBestNodeDNAddrPair instead of chooseDataNode.
         // If no nodes to do hedged reads against, pass.
+        boolean refetch = false;
         try {
-          chosenNode = getBestNodeDNAddrPair(block, ignored);
-          if (chosenNode == null) {
-            chosenNode = chooseDataNode(block, ignored);
-          }
+          chosenNode = chooseDataNode(block, ignored, false);
+          if (chosenNode != null) {
             // Latest block, if refreshed internally
             block = chosenNode.block;
             bb = ByteBuffer.allocate(len);
-          Callable<ByteBuffer> getFromDataNodeCallable = getFromOneDataNode(
-              chosenNode, block, start, end, bb,
+            Callable<ByteBuffer> getFromDataNodeCallable =
+                getFromOneDataNode(chosenNode, block, start, end, bb,
                     corruptedBlocks, hedgedReadId++);
-          Future<ByteBuffer> oneMoreRequest = hedgedService
-              .submit(getFromDataNodeCallable);
+            Future<ByteBuffer> oneMoreRequest =
+                hedgedService.submit(getFromDataNodeCallable);
             futures.add(oneMoreRequest);
+          } else {
+            refetch = true;
+          }
         } catch (IOException ioe) {
           DFSClient.LOG.debug("Failed getting node for hedged read: {}",
               ioe.getMessage());
@@ -1190,6 +1218,9 @@ public class DFSInputStream extends FSInputStream
         } catch (InterruptedException ie) {
           // Ignore and retry
         }
+        if (refetch) {
+          refetchLocations(block, ignored);
+        }
         // We got here if exception. Ignore this node on next go around IFF
         // we found a chosenNode to hedge read against.
         if (chosenNode != null && chosenNode.info != null) {

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestPread.java

@@ -626,7 +626,7 @@ public class TestPread {
    */
   @Test
   public void testPreadFailureWithChangedBlockLocations() throws Exception {
-    doPreadTestWithChangedLocations();
+    doPreadTestWithChangedLocations(1);
   }
 
   /**
@@ -639,21 +639,36 @@ public class TestPread {
    * 7. Consider next calls to getBlockLocations() always returns DN3 as last
    * location.<br>
    */
-  @Test
+  @Test(timeout = 60000)
   public void testPreadHedgedFailureWithChangedBlockLocations()
       throws Exception {
     isHedgedRead = true;
-    doPreadTestWithChangedLocations();
+    DFSClientFaultInjector old = DFSClientFaultInjector.get();
+    try {
+      DFSClientFaultInjector.set(new DFSClientFaultInjector() {
+        public void sleepBeforeHedgedGet() {
+          try {
+            Thread.sleep(500);
+          } catch (InterruptedException e) {
+          }
+        }
+      });
+      doPreadTestWithChangedLocations(2);
+    } finally {
+      DFSClientFaultInjector.set(old);
+    }
   }
 
-  private void doPreadTestWithChangedLocations()
+  private void doPreadTestWithChangedLocations(int maxFailures)
       throws IOException, TimeoutException, InterruptedException {
     GenericTestUtils.setLogLevel(DFSClient.LOG, Level.DEBUG);
     Configuration conf = new HdfsConfiguration();
     conf.setInt(DFSConfigKeys.DFS_REPLICATION_KEY, 2);
     conf.setInt(DFSConfigKeys.DFS_HEARTBEAT_INTERVAL_KEY, 1);
     if (isHedgedRead) {
+      conf.setInt(HdfsClientConfigKeys.HedgedRead.THRESHOLD_MILLIS_KEY, 100);
       conf.setInt(HdfsClientConfigKeys.HedgedRead.THREADPOOL_SIZE_KEY, 2);
+      conf.setInt(HdfsClientConfigKeys.Retry.WINDOW_BASE_KEY, 1000);
     }
     try (MiniDFSCluster cluster =
         new MiniDFSCluster.Builder(conf).numDataNodes(3).build()) {
@@ -747,6 +762,9 @@ public class TestPread {
       int n = din.read(0, buf, 0, data.length());
       assertEquals(data.length(), n);
       assertEquals("Data should be read", data, new String(buf, 0, n));
+      assertTrue("Read should complete with maximum " + maxFailures
+              + " failures, but completed with " + din.failures,
+          din.failures <= maxFailures);
       DFSClient.LOG.info("Read completed");
     }
   }