diff --git a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
index 6dbd6665d7c..b2766bd3b0c 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
+++ b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
@@ -368,6 +368,8 @@ Release 2.0.3-alpha - Unreleased
 
     HDFS-4059. Add number of stale DataNodes to metrics. (Jing Zhao via suresh)
 
+    HDFS-4155. libhdfs implementation of hsync API (Liang Xie via todd)
+
   IMPROVEMENTS
   
     HDFS-3925. Prettify PipelineAck#toString() for printing to a log
@@ -556,6 +558,9 @@ Release 2.0.3-alpha - Unreleased
 
     HDFS-1331. dfs -test should work like /bin/test (Andy Isaacson via daryn)
 
+    HDFS-3979. For hsync, datanode should wait for the local sync to complete
+    before sending ack. (Lars Hofhansl via szetszwo)
+
 Release 2.0.2-alpha - 2012-09-07 
 
   INCOMPATIBLE CHANGES
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/BlockReceiver.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/BlockReceiver.java
index 0f1ccb94351..6995fd2d4ed 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/BlockReceiver.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/BlockReceiver.java
@@ -319,9 +319,6 @@ class BlockReceiver implements Closeable {
    * @throws IOException
    */
   void flushOrSync(boolean isSync) throws IOException {
-    if (isSync && (out != null || checksumOut != null)) {
-      datanode.metrics.incrFsyncCount();      
-    }
     long flushTotalNanos = 0;
     if (checksumOut != null) {
       long flushStartNanos = System.nanoTime();
@@ -347,6 +344,9 @@ class BlockReceiver implements Closeable {
     }
     if (checksumOut != null || out != null) {
       datanode.metrics.addFlushNanos(flushTotalNanos);
+      if (isSync) {
+    	  datanode.metrics.incrFsyncCount();      
+      }
     }
   }
 
@@ -438,8 +438,10 @@ class BlockReceiver implements Closeable {
     int len = header.getDataLen();
     boolean syncBlock = header.getSyncBlock();
 
-    // make sure the block gets sync'ed upon close
-    this.syncOnClose |= syncBlock && lastPacketInBlock;
+    // avoid double sync'ing on close
+    if (syncBlock && lastPacketInBlock) {
+      this.syncOnClose = false;
+    }
 
     // update received bytes
     long firstByteInBlock = offsetInBlock;
@@ -448,11 +450,11 @@ class BlockReceiver implements Closeable {
       replicaInfo.setNumBytes(offsetInBlock);
     }
     
-    // put in queue for pending acks
-    if (responder != null) {
-      ((PacketResponder)responder.getRunnable()).enqueue(seqno,
-                                      lastPacketInBlock, offsetInBlock); 
-    }  
+    // put in queue for pending acks, unless sync was requested
+    if (responder != null && !syncBlock) {
+      ((PacketResponder) responder.getRunnable()).enqueue(seqno,
+          lastPacketInBlock, offsetInBlock);
+    }
 
     //First write the packet to the mirror:
     if (mirrorOut != null && !mirrorError) {
@@ -471,8 +473,8 @@ class BlockReceiver implements Closeable {
       if(LOG.isDebugEnabled()) {
         LOG.debug("Receiving an empty packet or the end of the block " + block);
       }
-      // flush unless close() would flush anyway
-      if (syncBlock && !lastPacketInBlock) {
+      // sync block if requested
+      if (syncBlock) {
         flushOrSync(true);
       }
     } else {
@@ -563,8 +565,8 @@ class BlockReceiver implements Closeable {
                 checksumBuf.arrayOffset() + checksumBuf.position(),
                 checksumLen);
           }
-          /// flush entire packet, sync unless close() will sync
-          flushOrSync(syncBlock && !lastPacketInBlock);
+          /// flush entire packet, sync if requested
+          flushOrSync(syncBlock);
           
           replicaInfo.setLastChecksumAndDataLen(
             offsetInBlock, lastChunkChecksum
@@ -580,6 +582,13 @@ class BlockReceiver implements Closeable {
       }
     }
 
+    // if sync was requested, put in queue for pending acks here
+    // (after the fsync finished)
+    if (responder != null && syncBlock) {
+      ((PacketResponder) responder.getRunnable()).enqueue(seqno,
+          lastPacketInBlock, offsetInBlock);
+    }
+
     if (throttler != null) { // throttle I/O
       throttler.throttle(len);
     }
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/native/libhdfs/hdfs.c b/hadoop-hdfs-project/hadoop-hdfs/src/main/native/libhdfs/hdfs.c
index a180dd24c73..ba980a7a539 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/native/libhdfs/hdfs.c
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/native/libhdfs/hdfs.c
@@ -1388,6 +1388,32 @@ int hdfsHFlush(hdfsFS fs, hdfsFile f)
     return 0;
 }
 
+int hdfsHSync(hdfsFS fs, hdfsFile f)
+{
+    //Get the JNIEnv* corresponding to current thread
+    JNIEnv* env = getJNIEnv();
+    if (env == NULL) {
+      errno = EINTERNAL;
+      return -1;
+    }
+
+    //Sanity check
+    if (!f || f->type != OUTPUT) {
+        errno = EBADF;
+        return -1;
+    }
+
+    jobject jOutputStream = f->file;
+    jthrowable jthr = invokeMethod(env, NULL, INSTANCE, jOutputStream,
+                     HADOOP_OSTRM, "hsync", "()V");
+    if (jthr) {
+        errno = printExceptionAndFree(env, jthr, PRINT_EXC_ALL,
+            "hdfsHSync: FSDataOutputStream#hsync");
+        return -1;
+    }
+    return 0;
+}
+
 int hdfsAvailable(hdfsFS fs, hdfsFile f)
 {
     // JAVA EQUIVALENT
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/native/libhdfs/hdfs.h b/hadoop-hdfs-project/hadoop-hdfs/src/main/native/libhdfs/hdfs.h
index fa71c8384c8..7973e0a5e3f 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/native/libhdfs/hdfs.h
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/native/libhdfs/hdfs.h
@@ -393,6 +393,17 @@ extern  "C" {
     int hdfsHFlush(hdfsFS fs, hdfsFile file);
 
 
+    /**
+     * hdfsHSync - Similar to posix fsync, Flush out the data in client's 
+     * user buffer. all the way to the disk device (but the disk may have 
+     * it in its cache).
+     * @param fs configured filesystem handle
+     * @param file file handle
+     * @return 0 on success, -1 on error and sets errno
+     */
+    int hdfsHSync(hdfsFS fs, hdfsFile file);
+
+
     /**
      * hdfsAvailable - Number of bytes that can be read from this
      * input stream without blocking.
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/native/libhdfs/test_libhdfs_threaded.c b/hadoop-hdfs-project/hadoop-hdfs/src/main/native/libhdfs/test_libhdfs_threaded.c
index d9cb0d9648d..c56c89300f7 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/native/libhdfs/test_libhdfs_threaded.c
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/native/libhdfs/test_libhdfs_threaded.c
@@ -150,6 +150,7 @@ static int doTestHdfsOperations(struct tlhThreadInfo *ti, hdfsFS fs)
         return EIO;
     }
     EXPECT_ZERO(hdfsFlush(fs, file));
+    EXPECT_ZERO(hdfsHSync(fs, file));
     EXPECT_ZERO(hdfsCloseFile(fs, file));
 
     /* Let's re-open the file for reading */
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/TestStartSecureDataNode.java b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/TestStartSecureDataNode.java
index d1b2d668ded..ba5587276c4 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/TestStartSecureDataNode.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/TestStartSecureDataNode.java
@@ -15,7 +15,7 @@
  * the License.
  */
 
-package org.apache.hadoop.hdfs.server.namenode;
+package org.apache.hadoop.hdfs.server.datanode;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;