diff --git a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
index 346050c2aca..d10dd292a26 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
+++ b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
@@ -839,6 +839,9 @@ Release 2.7.0 - UNRELEASED
     HDFS-6833.  DirectoryScanner should not register a deleting block with
     memory of DataNode.  (Shinichi Yamashita via szetszwo)
 
+    HDFS-7926. NameNode implementation of ClientProtocol.truncate(..) is not 
+    idempotent (Tsz Wo Nicholas Sze via brandonli)
+
     BREAKDOWN OF HDFS-7584 SUBTASKS AND RELATED JIRAS
 
       HDFS-7720. Quota by Storage Type API, tools and ClientNameNode
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/BlockInfoContiguousUnderConstruction.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/BlockInfoContiguousUnderConstruction.java
index 91b76ccb0e2..ae809a5e919 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/BlockInfoContiguousUnderConstruction.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/BlockInfoContiguousUnderConstruction.java
@@ -383,6 +383,7 @@ public class BlockInfoContiguousUnderConstruction extends BlockInfoContiguous {
 
   private void appendUCParts(StringBuilder sb) {
     sb.append("{UCState=").append(blockUCState)
+      .append(", truncateBlock=" + truncateBlock)
       .append(", primaryNodeIndex=").append(primaryNodeIndex)
       .append(", replicas=[");
     if (replicas != null) {
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java
index 81d2b887df2..d49088597a4 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java
@@ -2021,6 +2021,21 @@ public class FSNamesystem implements Namesystem, FSNamesystemMBean,
       throw new UnsupportedOperationException(
           "Cannot truncate lazy persist file " + src);
     }
+
+    // Check if the file is already being truncated with the same length
+    final BlockInfoContiguous last = file.getLastBlock();
+    if (last != null && last.getBlockUCState() == BlockUCState.UNDER_RECOVERY) {
+      final Block truncateBlock
+          = ((BlockInfoContiguousUnderConstruction)last).getTruncateBlock();
+      if (truncateBlock != null) {
+        final long truncateLength = file.computeFileSize(false, false)
+            + truncateBlock.getNumBytes();
+        if (newLength == truncateLength) {
+          return false;
+        }
+      }
+    }
+
     // Opening an existing file for truncate. May need lease recovery.
     recoverLeaseInternal(RecoverLeaseOp.TRUNCATE_FILE,
         iip, src, clientName, clientMachine, false);
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestFileTruncate.java b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestFileTruncate.java
index b69d3456be0..260d8bbb496 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestFileTruncate.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestFileTruncate.java
@@ -166,6 +166,8 @@ public class TestFileTruncate {
       LOG.info("newLength=" + newLength + ", isReady=" + isReady);
       assertEquals("File must be closed for truncating at the block boundary",
           isReady, newLength % BLOCK_SIZE == 0);
+      assertEquals("Truncate is not idempotent",
+          isReady, fs.truncate(p, newLength));
       if (!isReady) {
         checkBlockRecovery(p);
       }