diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/blockmanagement/TestDatanodeManager.java b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/blockmanagement/TestDatanodeManager.java
new file mode 100644
index 00000000000..75034932acd
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+++ b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/blockmanagement/TestDatanodeManager.java
@@ -0,0 +1,148 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hadoop.hdfs.server.blockmanagement;
+
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Random;
+
+import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.hdfs.server.namenode.FSNamesystem;
+import org.apache.hadoop.hdfs.server.protocol.DatanodeRegistration;
+import org.junit.Test;
+import org.mockito.Mockito;
+import org.mortbay.log.Log;
+
+import static org.junit.Assert.*;
+
+public class TestDatanodeManager {
+
+  //The number of times the registration / removal of nodes should happen
+  final int NUM_ITERATIONS = 500;
+
+  /**
+   * This test sends a random sequence of node registrations and node removals
+   * to the DatanodeManager (of nodes with different IDs and versions), and
+   * checks that the DatanodeManager keeps a correct count of different software
+   * versions at all times.
+   */
+  @Test
+  public void testNumVersionsReportedCorrect() throws IOException {
+    //Create the DatanodeManager which will be tested
+    FSNamesystem fsn = Mockito.mock(FSNamesystem.class);
+    Mockito.when(fsn.hasWriteLock()).thenReturn(true);
+    DatanodeManager dm = new DatanodeManager(Mockito.mock(BlockManager.class),
+      fsn, new Configuration());
+
+    //Seed the RNG with a known value so test failures are easier to reproduce
+    Random rng = new Random();
+    int seed = rng.nextInt();
+    rng = new Random(seed);
+    Log.info("Using seed " + seed + " for testing");
+
+    //A map of the Storage IDs to the DN registration it was registered with
+    HashMap <String, DatanodeRegistration> sIdToDnReg =
+      new HashMap<String, DatanodeRegistration>();
+
+    for(int i=0; i<NUM_ITERATIONS; ++i) {
+
+      //If true, remove a node for every 3rd time (if there's one)
+      if(rng.nextBoolean() && i%3 == 0 && sIdToDnReg.size()!=0) {
+        //Pick a random node.
+        int randomIndex = rng.nextInt() % sIdToDnReg.size();
+        //Iterate to that random position 
+        Iterator<Map.Entry<String, DatanodeRegistration>> it =
+          sIdToDnReg.entrySet().iterator();
+        for(int j=0; j<randomIndex-1; ++j) {
+          it.next();
+        }
+        DatanodeRegistration toRemove = it.next().getValue();
+        Log.info("Removing node " + toRemove.getStorageID() + " ip " +
+        toRemove.getXferAddr() + " version : " + toRemove.getSoftwareVersion());
+
+        //Remove that random node
+        dm.removeDatanode(toRemove);
+        it.remove();
+      }
+
+      // Otherwise register a node. This node may be a new / an old one
+      else {
+        //Pick a random storageID to register.
+        String storageID = "someStorageID" + rng.nextInt(5000);
+
+        DatanodeRegistration dr = Mockito.mock(DatanodeRegistration.class);
+        Mockito.when(dr.getStorageID()).thenReturn(storageID);
+
+        //If this storageID had already been registered before
+        if(sIdToDnReg.containsKey(storageID)) {
+          dr = sIdToDnReg.get(storageID);
+          //Half of the times, change the IP address
+          if(rng.nextBoolean()) {
+            dr.setIpAddr(dr.getIpAddr() + "newIP");
+          }
+        } else { //This storageID has never been registered
+          //Ensure IP address is unique to storageID
+          String ip = "someIP" + storageID;
+          Mockito.when(dr.getIpAddr()).thenReturn(ip);
+          Mockito.when(dr.getXferAddr()).thenReturn(ip + ":9000");
+          Mockito.when(dr.getXferPort()).thenReturn(9000);
+        }
+        //Pick a random version to register with
+        Mockito.when(dr.getSoftwareVersion()).thenReturn(
+          "version" + rng.nextInt(5));
+
+        Log.info("Registering node storageID: " + dr.getStorageID() +
+          ", version: " + dr.getSoftwareVersion() + ", IP address: "
+          + dr.getXferAddr());
+
+        //Register this random node
+        dm.registerDatanode(dr);
+        sIdToDnReg.put(storageID, dr);
+      }
+
+      //Verify DatanodeManager still has the right count
+      Map<String, Integer> mapToCheck = dm.getDatanodesSoftwareVersions();
+
+      //Remove counts from versions and make sure that after removing all nodes
+      //mapToCheck is empty
+      for(Entry<String, DatanodeRegistration> it: sIdToDnReg.entrySet()) {
+        String ver = it.getValue().getSoftwareVersion();
+        if(!mapToCheck.containsKey(ver)) {
+          throw new AssertionError("The correct number of datanodes of a "
+            + "version was not found on iteration " + i);
+        }
+        mapToCheck.put(ver, mapToCheck.get(ver) - 1);
+        if(mapToCheck.get(ver) == 0) {
+          mapToCheck.remove(ver);
+        }
+      }
+      for(Entry <String, Integer> entry: mapToCheck.entrySet()) {
+        Log.info("Still in map: " + entry.getKey() + " has "
+          + entry.getValue());
+      }
+      assertEquals("The map of version counts returned by DatanodeManager was"
+        + " not what it was expected to be on iteration " + i, 0,
+        mapToCheck.size());
+    }
+  }
+
+}