diff --git a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
index 6755f0041e..7fe585041b 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
+++ b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
@@ -1448,8 +1448,6 @@ Release 2.8.0 - UNRELEASED
     HDFS-9019. Adding informative message to sticky bit permission denied
     exception. (xyao)
 
-    HDFS-8860. Remove unused Replica copyOnWrite code (Lei (Eddy) Xu via Colin P. McCabe)
-
     HDFS-8716. Introduce a new config specifically for safe mode block count
     (Chang Li via kihwal)
 
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/FinalizedReplica.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/FinalizedReplica.java
index 8daeb51e0d..cc3287406a 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/FinalizedReplica.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/FinalizedReplica.java
@@ -27,6 +27,7 @@
  * This class describes a replica that has been finalized.
  */
 public class FinalizedReplica extends ReplicaInfo {
+  private boolean unlinked;      // copy-on-write done for block
 
   /**
    * Constructor
@@ -57,6 +58,7 @@ public FinalizedReplica(Block block, FsVolumeSpi vol, File dir) {
    */
   public FinalizedReplica(FinalizedReplica from) {
     super(from);
+    this.unlinked = from.isUnlinked();
   }
 
   @Override  // ReplicaInfo
@@ -64,6 +66,16 @@ public ReplicaState getState() {
     return ReplicaState.FINALIZED;
   }
   
+  @Override // ReplicaInfo
+  public boolean isUnlinked() {
+    return unlinked;
+  }
+
+  @Override  // ReplicaInfo
+  public void setUnlinked() {
+    unlinked = true;
+  }
+  
   @Override
   public long getVisibleLength() {
     return getNumBytes();       // all bytes are visible
@@ -86,6 +98,7 @@ public int hashCode() {
   
   @Override
   public String toString() {
-    return super.toString();
+    return super.toString()
+        + "\n  unlinked          =" + unlinked;
   }
 }
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/ReplicaInfo.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/ReplicaInfo.java
index d19e656c9d..e41cce0535 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/ReplicaInfo.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/ReplicaInfo.java
@@ -18,12 +18,18 @@
 package org.apache.hadoop.hdfs.server.datanode;
 
 import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
 import java.util.HashMap;
 import java.util.Map;
 
 import org.apache.hadoop.classification.InterfaceAudience;
+import org.apache.hadoop.fs.FileUtil;
+import org.apache.hadoop.fs.HardLink;
 import org.apache.hadoop.hdfs.protocol.Block;
 import org.apache.hadoop.hdfs.server.datanode.fsdataset.FsVolumeSpi;
+import org.apache.hadoop.io.IOUtils;
 import org.apache.hadoop.util.LightWeightResizableGSet;
 
 import com.google.common.annotations.VisibleForTesting;
@@ -193,6 +199,22 @@ public static ReplicaDirInfo parseBaseDir(File dir) {
     return new ReplicaDirInfo(currentDir.getAbsolutePath(), hasSubdirs);
   }
 
+  /**
+   * check if this replica has already been unlinked.
+   * @return true if the replica has already been unlinked 
+   *         or no need to be detached; false otherwise
+   */
+  public boolean isUnlinked() {
+    return true;                // no need to be unlinked
+  }
+
+  /**
+   * set that this replica is unlinked
+   */
+  public void setUnlinked() {
+    // no need to be unlinked
+  }
+
   /**
    * Number of bytes reserved for this replica on disk.
    */
@@ -210,6 +232,72 @@ public long getOriginalBytesReserved() {
     return 0;
   }
 
+   /**
+   * Copy specified file into a temporary file. Then rename the
+   * temporary file to the original name. This will cause any
+   * hardlinks to the original file to be removed. The temporary
+   * files are created in the same directory. The temporary files will
+   * be recovered (especially on Windows) on datanode restart.
+   */
+  private void unlinkFile(File file, Block b) throws IOException {
+    File tmpFile = DatanodeUtil.createTmpFile(b, DatanodeUtil.getUnlinkTmpFile(file));
+    try {
+      FileInputStream in = new FileInputStream(file);
+      try {
+        FileOutputStream out = new FileOutputStream(tmpFile);
+        try {
+          IOUtils.copyBytes(in, out, 16*1024);
+        } finally {
+          out.close();
+        }
+      } finally {
+        in.close();
+      }
+      if (file.length() != tmpFile.length()) {
+        throw new IOException("Copy of file " + file + " size " + file.length()+
+                              " into file " + tmpFile +
+                              " resulted in a size of " + tmpFile.length());
+      }
+      FileUtil.replaceFile(tmpFile, file);
+    } catch (IOException e) {
+      boolean done = tmpFile.delete();
+      if (!done) {
+        DataNode.LOG.info("detachFile failed to delete temporary file " +
+                          tmpFile);
+      }
+      throw e;
+    }
+  }
+
+  /**
+   * Remove a hard link by copying the block to a temporary place and 
+   * then moving it back
+   * @param numLinks number of hard links
+   * @return true if copy is successful; 
+   *         false if it is already detached or no need to be detached
+   * @throws IOException if there is any copy error
+   */
+  public boolean unlinkBlock(int numLinks) throws IOException {
+    if (isUnlinked()) {
+      return false;
+    }
+    File file = getBlockFile();
+    if (file == null || getVolume() == null) {
+      throw new IOException("detachBlock:Block not found. " + this);
+    }
+    File meta = getMetaFile();
+
+    if (HardLink.getLinkCount(file) > numLinks) {
+      DataNode.LOG.info("CopyOnWrite for block " + this);
+      unlinkFile(file, this);
+    }
+    if (HardLink.getLinkCount(meta) > numLinks) {
+      unlinkFile(meta, this);
+    }
+    setUnlinked();
+    return true;
+  }
+
   @Override  //Object
   public String toString() {
     return getClass().getSimpleName()
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/ReplicaUnderRecovery.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/ReplicaUnderRecovery.java
index 558ee21753..2cd8a01218 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/ReplicaUnderRecovery.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/ReplicaUnderRecovery.java
@@ -85,6 +85,16 @@ public void setRecoveryID(long recoveryId) {
   public ReplicaInfo getOriginalReplica() {
     return original;
   }
+
+  @Override //ReplicaInfo
+  public boolean isUnlinked() {
+    return original.isUnlinked();
+  }
+
+  @Override //ReplicaInfo
+  public void setUnlinked() {
+    original.setUnlinked();
+  }
   
   @Override //ReplicaInfo
   public ReplicaState getState() {
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/ReplicaWaitingToBeRecovered.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/ReplicaWaitingToBeRecovered.java
index 220649d1eb..26ab3dbe24 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/ReplicaWaitingToBeRecovered.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/ReplicaWaitingToBeRecovered.java
@@ -33,6 +33,7 @@
  * lease recovery.
  */
 public class ReplicaWaitingToBeRecovered extends ReplicaInfo {
+  private boolean unlinked;      // copy-on-write done for block
 
   /**
    * Constructor
@@ -63,6 +64,7 @@ public ReplicaWaitingToBeRecovered(Block block, FsVolumeSpi vol, File dir) {
    */
   public ReplicaWaitingToBeRecovered(ReplicaWaitingToBeRecovered from) {
     super(from);
+    this.unlinked = from.isUnlinked();
   }
 
   @Override //ReplicaInfo
@@ -70,6 +72,16 @@ public ReplicaState getState() {
     return ReplicaState.RWR;
   }
   
+  @Override //ReplicaInfo
+  public boolean isUnlinked() {
+    return unlinked;
+  }
+
+  @Override //ReplicaInfo
+  public void setUnlinked() {
+    unlinked = true;
+  }
+  
   @Override //ReplicaInfo
   public long getVisibleLength() {
     return -1;  //no bytes are visible
@@ -92,6 +104,7 @@ public int hashCode() {
 
   @Override
   public String toString() {
-    return super.toString();
+    return super.toString()
+        + "\n  unlinked=" + unlinked;
   }
 }
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetImpl.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetImpl.java
index 466c7e9325..1d8c705f7b 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetImpl.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetImpl.java
@@ -1109,6 +1109,8 @@ private synchronized ReplicaBeingWritten append(String bpid,
       throws IOException {
     // If the block is cached, start uncaching it.
     cacheManager.uncacheBlock(bpid, replicaInfo.getBlockId());
+    // unlink the finalized replica
+    replicaInfo.unlinkBlock(1);
     
     // construct a RBW replica with the new GS
     File blkfile = replicaInfo.getBlockFile();
@@ -2478,6 +2480,7 @@ private FinalizedReplica updateReplicaUnderRecovery(
           + ", rur=" + rur);
     }
     if (rur.getNumBytes() > newlength) {
+      rur.unlinkBlock(1);
       truncateBlock(blockFile, metaFile, rur.getNumBytes(), newlength);
       if(!copyOnTruncate) {
         // update RUR with the new length
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestFileAppend.java b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestFileAppend.java
index 7b7f41573b..85d92c91b3 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestFileAppend.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestFileAppend.java
@@ -109,6 +109,78 @@ private void checkFile(DistributedFileSystem fileSys, Path name, int repl)
         expected, "Read 1", false);
   }
 
+  /**
+   * Test that copy on write for blocks works correctly
+   * @throws IOException an exception might be thrown
+   */
+  @Test
+  public void testCopyOnWrite() throws IOException {
+    Configuration conf = new HdfsConfiguration();
+    if (simulatedStorage) {
+      SimulatedFSDataset.setFactory(conf);
+    }
+    MiniDFSCluster cluster = new MiniDFSCluster.Builder(conf).build();
+    FileSystem fs = cluster.getFileSystem();
+    InetSocketAddress addr = new InetSocketAddress("localhost",
+                                                   cluster.getNameNodePort());
+    DFSClient client = new DFSClient(addr, conf);
+    try {
+
+      // create a new file, write to it and close it.
+      //
+      Path file1 = new Path("/filestatus.dat");
+      FSDataOutputStream stm = AppendTestUtil.createFile(fs, file1, 1);
+      writeFile(stm);
+      stm.close();
+
+      // Get a handle to the datanode
+      DataNode[] dn = cluster.listDataNodes();
+      assertTrue("There should be only one datanode but found " + dn.length,
+                  dn.length == 1);
+
+      LocatedBlocks locations = client.getNamenode().getBlockLocations(
+                                  file1.toString(), 0, Long.MAX_VALUE);
+      List<LocatedBlock> blocks = locations.getLocatedBlocks();
+
+      //
+      // Create hard links for a few of the blocks
+      //
+      for (int i = 0; i < blocks.size(); i = i + 2) {
+        ExtendedBlock b = blocks.get(i).getBlock();
+        final File f = DataNodeTestUtils.getFile(dn[0],
+            b.getBlockPoolId(), b.getLocalBlock().getBlockId());
+        File link = new File(f.toString() + ".link");
+        System.out.println("Creating hardlink for File " + f + " to " + link);
+        HardLink.createHardLink(f, link);
+      }
+
+      //
+      // Detach all blocks. This should remove hardlinks (if any)
+      //
+      for (int i = 0; i < blocks.size(); i++) {
+        ExtendedBlock b = blocks.get(i).getBlock();
+        System.out.println("testCopyOnWrite detaching block " + b);
+        assertTrue("Detaching block " + b + " should have returned true",
+            DataNodeTestUtils.unlinkBlock(dn[0], b, 1));
+      }
+
+      // Since the blocks were already detached earlier, these calls should
+      // return false
+      //
+      for (int i = 0; i < blocks.size(); i++) {
+        ExtendedBlock b = blocks.get(i).getBlock();
+        System.out.println("testCopyOnWrite detaching block " + b);
+        assertTrue("Detaching block " + b + " should have returned false",
+            !DataNodeTestUtils.unlinkBlock(dn[0], b, 1));
+      }
+
+    } finally {
+      client.close();
+      fs.close();
+      cluster.shutdown();
+    }
+  }
+
   /**
    * Test a simple flush on a simple HDFS file.
    * @throws IOException an exception might be thrown
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/DataNodeTestUtils.java b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/DataNodeTestUtils.java
index 1d4719279f..6bcbb1d3be 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/DataNodeTestUtils.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/DataNodeTestUtils.java
@@ -35,6 +35,7 @@
 import org.apache.hadoop.hdfs.MiniDFSCluster;
 import org.apache.hadoop.hdfs.protocol.Block;
 import org.apache.hadoop.hdfs.protocol.DatanodeID;
+import org.apache.hadoop.hdfs.protocol.ExtendedBlock;
 import org.apache.hadoop.hdfs.protocolPB.DatanodeProtocolClientSideTranslatorPB;
 import org.apache.hadoop.hdfs.server.datanode.fsdataset.FsDatasetSpi;
 import org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetTestUtil;
@@ -158,6 +159,20 @@ public static FsDatasetSpi<?> getFSDataset(DataNode dn) {
     return dn.getFSDataset();
   }
 
+  public static File getFile(DataNode dn, String bpid, long bid) {
+    return FsDatasetTestUtil.getFile(dn.getFSDataset(), bpid, bid);
+  }
+
+  public static File getBlockFile(DataNode dn, String bpid, Block b
+      ) throws IOException {
+    return FsDatasetTestUtil.getBlockFile(dn.getFSDataset(), bpid, b);
+  }
+  
+  public static boolean unlinkBlock(DataNode dn, ExtendedBlock bk, int numLinks
+      ) throws IOException {
+    return FsDatasetTestUtil.unlinkBlock(dn.getFSDataset(), bk, numLinks);
+  }
+
   /**
    * Fetch a copy of ReplicaInfo from a datanode by block id
    * @param dn datanode to retrieve a replicainfo object from
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetTestUtil.java b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetTestUtil.java
index f4480a16b9..9c297e881c 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetTestUtil.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetTestUtil.java
@@ -54,6 +54,12 @@ public static File getMetaFile(FsDatasetSpi<?> fsd, String bpid, Block b)
     return FsDatasetUtil.getMetaFile(getBlockFile(fsd, bpid, b), b
         .getGenerationStamp());
   }
+  
+  public static boolean unlinkBlock(FsDatasetSpi<?> fsd,
+      ExtendedBlock block, int numLinks) throws IOException {
+    final ReplicaInfo info = ((FsDatasetImpl)fsd).getReplicaInfo(block);
+    return info.unlinkBlock(numLinks);
+  }
 
   public static ReplicaInfo fetchReplicaInfo (final FsDatasetSpi<?> fsd,
       final String bpid, final long blockId) {
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/TestDatanodeRestart.java b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/TestDatanodeRestart.java
index 8bbac9f2f0..4516696f1f 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/TestDatanodeRestart.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/TestDatanodeRestart.java
@@ -143,7 +143,79 @@ private void testRbwReplicas(MiniDFSCluster cluster, boolean isCorrupt)
     }      
   }
 
+  // test recovering unlinked tmp replicas
+  @Test public void testRecoverReplicas() throws Exception {
+    Configuration conf = new HdfsConfiguration();
+    conf.setLong(DFSConfigKeys.DFS_BLOCK_SIZE_KEY, 1024L);
+    conf.setInt(HdfsClientConfigKeys.DFS_CLIENT_WRITE_PACKET_SIZE_KEY, 512);
+    MiniDFSCluster cluster = new MiniDFSCluster.Builder(conf).build();
+    cluster.waitActive();
+    try {
+      FileSystem fs = cluster.getFileSystem();
+      for (int i=0; i<4; i++) {
+        Path fileName = new Path("/test"+i);
+        DFSTestUtil.createFile(fs, fileName, 1, (short)1, 0L);
+        DFSTestUtil.waitReplication(fs, fileName, (short)1);
+      }
+      String bpid = cluster.getNamesystem().getBlockPoolId();
+      DataNode dn = cluster.getDataNodes().get(0);
+      Iterator<ReplicaInfo> replicasItor = 
+          dataset(dn).volumeMap.replicas(bpid).iterator();
+      ReplicaInfo replica = replicasItor.next();
+      createUnlinkTmpFile(replica, true, true); // rename block file
+      createUnlinkTmpFile(replica, false, true); // rename meta file
+      replica = replicasItor.next();
+      createUnlinkTmpFile(replica, true, false); // copy block file
+      createUnlinkTmpFile(replica, false, false); // copy meta file
+      replica = replicasItor.next();
+      createUnlinkTmpFile(replica, true, true); // rename block file
+      createUnlinkTmpFile(replica, false, false); // copy meta file
+
+      cluster.restartDataNodes();
+      cluster.waitActive();
+      dn = cluster.getDataNodes().get(0);
+
+      // check volumeMap: 4 finalized replica
+      Collection<ReplicaInfo> replicas = dataset(dn).volumeMap.replicas(bpid);
+      Assert.assertEquals(4, replicas.size());
+      replicasItor = replicas.iterator();
+      while (replicasItor.hasNext()) {
+        Assert.assertEquals(ReplicaState.FINALIZED, 
+            replicasItor.next().getState());
+      }
+    } finally {
+      cluster.shutdown();
+    }
+  }
+
   private static FsDatasetImpl dataset(DataNode dn) {
     return (FsDatasetImpl)DataNodeTestUtils.getFSDataset(dn);
   }
+
+  private static void createUnlinkTmpFile(ReplicaInfo replicaInfo, 
+      boolean changeBlockFile, 
+      boolean isRename) throws IOException {
+    File src;
+    if (changeBlockFile) {
+      src = replicaInfo.getBlockFile();
+    } else {
+      src = replicaInfo.getMetaFile();
+    }
+    File dst = DatanodeUtil.getUnlinkTmpFile(src);
+    if (isRename) {
+      src.renameTo(dst);
+    } else {
+      FileInputStream in = new FileInputStream(src);
+      try {
+        FileOutputStream out = new FileOutputStream(dst);
+        try {
+          IOUtils.copyBytes(in, out, 1);
+        } finally {
+          out.close();
+        }
+      } finally {
+        in.close();
+      }
+    }
+  }
 }