diff --git a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
index 5f3cc02e37..71ce48fc96 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
+++ b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
@@ -992,6 +992,9 @@ Release 2.7.0 - UNRELEASED
     HDFS-7808. Remove obsolete -ns options in in DFSHAAdmin.java.
     (Arshad Mohammad via wheat9)
 
+    HDFS-7788. Post-2.6 namenode may not start up with an image containing
+    inodes created with an old release. (Rushabh Shah via kihwal)
+
     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/namenode/INodeFile.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeFile.java
index 24e25ecf6b..3743bf0b45 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeFile.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeFile.java
@@ -107,6 +107,9 @@ static byte getStoragePolicyID(long header) {
     static long toLong(long preferredBlockSize, short replication,
         byte storagePolicyID) {
       long h = 0;
+      if (preferredBlockSize == 0) {
+        preferredBlockSize = PREFERRED_BLOCK_SIZE.BITS.getMin();
+      }
       h = PREFERRED_BLOCK_SIZE.BITS.combine(preferredBlockSize, h);
       h = REPLICATION.BITS.combine(replication, h);
       h = STORAGE_POLICY_ID.BITS.combine(storagePolicyID, h);
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/util/LongBitFormat.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/util/LongBitFormat.java
index 863d9f744f..9399d84f1e 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/util/LongBitFormat.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/util/LongBitFormat.java
@@ -64,4 +64,8 @@ public long combine(long value, long record) {
     }
     return (record & ~MASK) | (value << OFFSET);
   }
+  
+  public long getMin() {
+    return MIN;
+  }
 }
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestFSImage.java b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestFSImage.java
index c68ae046df..f7dad183c5 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestFSImage.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestFSImage.java
@@ -28,10 +28,13 @@
 
 import org.apache.hadoop.conf.Configuration;
 import org.apache.hadoop.fs.FSDataOutputStream;
+import org.apache.hadoop.fs.FileSystem;
+import org.apache.hadoop.fs.FileUtil;
 import org.apache.hadoop.fs.Path;
 import org.apache.hadoop.hdfs.DFSConfigKeys;
 import org.apache.hadoop.hdfs.DFSOutputStream;
 import org.apache.hadoop.hdfs.DistributedFileSystem;
+import org.apache.hadoop.hdfs.HdfsConfiguration;
 import org.apache.hadoop.hdfs.MiniDFSCluster;
 import org.apache.hadoop.hdfs.client.HdfsDataOutputStream.SyncFlag;
 import org.apache.hadoop.hdfs.protocol.HdfsConstants;
@@ -40,10 +43,14 @@
 import org.apache.hadoop.hdfs.server.common.HdfsServerConstants.BlockUCState;
 import org.apache.hadoop.hdfs.server.namenode.LeaseManager.Lease;
 import org.apache.hadoop.hdfs.util.MD5FileUtils;
+import org.apache.hadoop.test.GenericTestUtils;
+import org.apache.hadoop.test.PathUtils;
 import org.junit.Test;
 
 public class TestFSImage {
 
+  private static final String HADOOP_2_7_ZER0_BLOCK_SIZE_TGZ =
+      "image-with-zero-block-size.tar.gz";
   @Test
   public void testPersist() throws IOException {
     Configuration conf = new Configuration();
@@ -183,4 +190,45 @@ public void testLoadMtimeAtime() throws Exception {
       }
     }
   }
+  
+  /**
+   * In this test case, I have created an image with a file having
+   * preferredblockSize = 0. We are trying to read this image (since file with
+   * preferredblockSize = 0 was allowed pre 2.1.0-beta version. The namenode 
+   * after 2.6 version will not be able to read this particular file.
+   * See HDFS-7788 for more information.
+   * @throws Exception
+   */
+  @Test
+  public void testZeroBlockSize() throws Exception {
+    final Configuration conf = new HdfsConfiguration();
+    String tarFile = System.getProperty("test.cache.data", "build/test/cache")
+      + "/" + HADOOP_2_7_ZER0_BLOCK_SIZE_TGZ;
+    String testDir = PathUtils.getTestDirName(getClass());
+    File dfsDir = new File(testDir, "image-with-zero-block-size");
+    if (dfsDir.exists() && !FileUtil.fullyDelete(dfsDir)) {
+      throw new IOException("Could not delete dfs directory '" + dfsDir + "'");
+    }
+    FileUtil.unTar(new File(tarFile), new File(testDir));
+    File nameDir = new File(dfsDir, "name");
+    GenericTestUtils.assertExists(nameDir);
+    conf.set(DFSConfigKeys.DFS_NAMENODE_NAME_DIR_KEY, 
+        nameDir.getAbsolutePath());
+    MiniDFSCluster cluster = new MiniDFSCluster.Builder(conf).numDataNodes(1)
+        .format(false)
+        .manageDataDfsDirs(false)
+        .manageNameDfsDirs(false)
+        .waitSafeMode(false)
+        .build();
+    try {
+      FileSystem fs = cluster.getFileSystem();
+      Path testPath = new Path("/tmp/zeroBlockFile");
+      assertTrue("File /tmp/zeroBlockFile doesn't exist ", fs.exists(testPath));
+      assertTrue("Name node didn't come up", cluster.isNameNodeUp(0));
+    } finally {
+      cluster.shutdown();
+      //Clean up
+      FileUtil.fullyDelete(dfsDir);
+    }
+  }
 }
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/test/resources/image-with-zero-block-size.tar.gz b/hadoop-hdfs-project/hadoop-hdfs/src/test/resources/image-with-zero-block-size.tar.gz
new file mode 100644
index 0000000000..8bfc4547ff
Binary files /dev/null and b/hadoop-hdfs-project/hadoop-hdfs/src/test/resources/image-with-zero-block-size.tar.gz differ