diff --git a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.HDFS-2802.txt b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.HDFS-2802.txt
index 2a7b10effba..96dceb70598 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.HDFS-2802.txt
+++ b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.HDFS-2802.txt
@@ -335,3 +335,6 @@ Branch-2802 Snapshot (Unreleased)
 
   HDFS-4791. Update and fix deletion of reference inode.  (Jing Zhao via
   szetszwo)
+
+  HDFS-4798. Update computeContentSummary() for the reference nodes in
+  snapshots.  (szetszwo)
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/Content.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/Content.java
index bb700051ccc..5dfa3e952f5 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/Content.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/Content.java
@@ -42,16 +42,20 @@ public enum Content {
 
   /** Content counts. */
   public static class Counts extends EnumCounters<Content> {
+    public static Counts newInstance() {
+      return new Counts();
+    }
+
     private Counts() {
       super(Content.values());
     }
   }
 
-  static final EnumCounters.Factory<Content, Counts> FACTORY
+  private static final EnumCounters.Factory<Content, Counts> FACTORY
       = new EnumCounters.Factory<Content, Counts>() {
     @Override
     public Counts newInstance() {
-      return new Counts();
+      return Counts.newInstance();
     }
   };
 
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INode.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INode.java
index 541f591113a..2cc2a9931e1 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INode.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INode.java
@@ -33,7 +33,6 @@ import org.apache.hadoop.fs.permission.PermissionStatus;
 import org.apache.hadoop.hdfs.DFSUtil;
 import org.apache.hadoop.hdfs.protocol.Block;
 import org.apache.hadoop.hdfs.protocol.QuotaExceededException;
-import org.apache.hadoop.hdfs.server.namenode.Content.CountsMap.Key;
 import org.apache.hadoop.hdfs.server.namenode.INodeReference.DstReference;
 import org.apache.hadoop.hdfs.server.namenode.INodeReference.WithName;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.FileWithSnapshot;
@@ -369,23 +368,14 @@ public abstract class INode implements Diff.Element<byte[]> {
 
   /** Compute {@link ContentSummary}. */
   public final ContentSummary computeContentSummary() {
-    final Content.Counts current = computeContentSummary(
-        new Content.CountsMap()).getCounts(Key.CURRENT);
-    return new ContentSummary(current.get(Content.LENGTH),
-        current.get(Content.FILE) + current.get(Content.SYMLINK),
-        current.get(Content.DIRECTORY), getNsQuota(),
-        current.get(Content.DISKSPACE), getDsQuota());
+    final Content.Counts counts = computeContentSummary(
+        Content.Counts.newInstance());
+    return new ContentSummary(counts.get(Content.LENGTH),
+        counts.get(Content.FILE) + counts.get(Content.SYMLINK),
+        counts.get(Content.DIRECTORY), getNsQuota(),
+        counts.get(Content.DISKSPACE), getDsQuota());
   }
 
-  /**
-   * Count subtree content summary with a {@link Content.CountsMap}.
-   *
-   * @param countsMap The subtree counts for returning.
-   * @return The same objects as the counts parameter.
-   */
-  public abstract Content.CountsMap computeContentSummary(
-      Content.CountsMap countsMap);
-
   /**
    * Count subtree content summary with a {@link Content.Counts}.
    *
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeDirectory.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeDirectory.java
index db7a27f4b6a..78aa46b30fd 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeDirectory.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeDirectory.java
@@ -29,7 +29,6 @@ import org.apache.hadoop.fs.UnresolvedLinkException;
 import org.apache.hadoop.fs.permission.PermissionStatus;
 import org.apache.hadoop.hdfs.DFSUtil;
 import org.apache.hadoop.hdfs.protocol.QuotaExceededException;
-import org.apache.hadoop.hdfs.server.namenode.Content.CountsMap.Key;
 import org.apache.hadoop.hdfs.server.namenode.INodeReference.WithCount;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeDirectorySnapshottable;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeDirectoryWithSnapshot;
@@ -473,16 +472,6 @@ public class INodeDirectory extends INodeWithAdditionalFields {
     return counts;
   }
 
-  @Override
-  public Content.CountsMap computeContentSummary(
-      final Content.CountsMap countsMap) {
-    for (INode child : getChildrenList(null)) {
-      child.computeContentSummary(countsMap);
-    }
-    countsMap.getCounts(Key.CURRENT).add(Content.DIRECTORY, 1);
-    return countsMap;
-  }
-
   /**
    * @param snapshot
    *          if it is not null, get the result from the given snapshot;
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeDirectoryWithQuota.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeDirectoryWithQuota.java
index 1148f320aef..b623fe5c06e 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeDirectoryWithQuota.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeDirectoryWithQuota.java
@@ -106,15 +106,6 @@ public class INodeDirectoryWithQuota extends INodeDirectory {
     return counts;
   }
 
-  @Override
-  public Content.CountsMap computeContentSummary(
-      final Content.CountsMap countsMap) {
-    final long original = countsMap.sum(Content.DISKSPACE);
-    super.computeContentSummary(countsMap);
-    checkDiskspace(countsMap.sum(Content.DISKSPACE) - original);
-    return countsMap;
-  }
-
   @Override
   public Content.Counts computeContentSummary(
       final Content.Counts counts) {
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 655438ee45c..ef1bcaeda4d 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
@@ -31,10 +31,9 @@ import org.apache.hadoop.hdfs.server.blockmanagement.BlockCollection;
 import org.apache.hadoop.hdfs.server.blockmanagement.BlockInfo;
 import org.apache.hadoop.hdfs.server.blockmanagement.BlockInfoUnderConstruction;
 import org.apache.hadoop.hdfs.server.blockmanagement.DatanodeDescriptor;
-import org.apache.hadoop.hdfs.server.namenode.Content.CountsMap.Key;
+import org.apache.hadoop.hdfs.server.namenode.snapshot.FileDiffList;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.FileWithSnapshot;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.FileWithSnapshot.FileDiff;
-import org.apache.hadoop.hdfs.server.namenode.snapshot.FileDiffList;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.FileWithSnapshot.Util;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeFileWithSnapshot;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.Snapshot;
@@ -359,15 +358,6 @@ public class INodeFile extends INodeWithAdditionalFields implements BlockCollect
     return counts;
   }
 
-
-  @Override
-  public final Content.CountsMap computeContentSummary(
-      final Content.CountsMap countsMap) {
-    computeContentSummary4Snapshot(countsMap.getCounts(Key.SNAPSHOT));
-    computeContentSummary4Current(countsMap.getCounts(Key.CURRENT));
-    return countsMap;
-  }
-
   @Override
   public final Content.Counts computeContentSummary(
       final Content.Counts counts) {
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeMap.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeMap.java
index e0e4cdf49ee..18c8adf3fa8 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeMap.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeMap.java
@@ -22,7 +22,6 @@ import java.util.List;
 import org.apache.hadoop.fs.permission.FsPermission;
 import org.apache.hadoop.fs.permission.PermissionStatus;
 import org.apache.hadoop.hdfs.protocol.QuotaExceededException;
-import org.apache.hadoop.hdfs.server.namenode.Content.CountsMap;
 import org.apache.hadoop.hdfs.server.namenode.Quota.Counts;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.Snapshot;
 import org.apache.hadoop.hdfs.util.GSet;
@@ -112,11 +111,6 @@ public class INodeMap {
         return null;
       }
       
-      @Override
-      public CountsMap computeContentSummary(CountsMap countsMap) {
-        return null;
-      }
-      
       @Override
       public Counts cleanSubtree(Snapshot snapshot, Snapshot prior,
           BlocksMapUpdateInfo collectedBlocks, List<INode> removedINodes)
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeReference.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeReference.java
index ef5366ef2b1..865d2098477 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeReference.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeReference.java
@@ -269,12 +269,7 @@ public abstract class INodeReference extends INode {
   }
 
   @Override
-  public final Content.CountsMap computeContentSummary(Content.CountsMap countsMap) {
-    return referred.computeContentSummary(countsMap);
-  }
-
-  @Override
-  public final Content.Counts computeContentSummary(Content.Counts counts) {
+  public Content.Counts computeContentSummary(Content.Counts counts) {
     return referred.computeContentSummary(counts);
   }
 
@@ -462,6 +457,15 @@ public abstract class INodeReference extends INode {
       return lastSnapshotId;
     }
     
+    @Override
+    public final Content.Counts computeContentSummary(Content.Counts counts) {
+      //only count diskspace for WithName
+      final Quota.Counts q = Quota.Counts.newInstance();
+      computeQuotaUsage(q, false, lastSnapshotId);
+      counts.add(Content.DISKSPACE, q.get(Quota.DISKSPACE));
+      return counts;
+    }
+
     @Override
     public final Quota.Counts computeQuotaUsage(Quota.Counts counts,
         boolean useCache, int lastSnapshotId) {
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeSymlink.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeSymlink.java
index 96873fa5803..653ec8d3d97 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeSymlink.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeSymlink.java
@@ -24,7 +24,6 @@ import org.apache.hadoop.classification.InterfaceAudience;
 import org.apache.hadoop.fs.permission.PermissionStatus;
 import org.apache.hadoop.hdfs.DFSUtil;
 import org.apache.hadoop.hdfs.protocol.QuotaExceededException;
-import org.apache.hadoop.hdfs.server.namenode.Content.CountsMap.Key;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.Snapshot;
 
 /**
@@ -97,13 +96,6 @@ public class INodeSymlink extends INodeWithAdditionalFields {
     return counts;
   }
 
-  @Override
-  public Content.CountsMap computeContentSummary(
-      final Content.CountsMap countsMap) {
-    computeContentSummary(countsMap.getCounts(Key.CURRENT));
-    return countsMap;
-  }
-
   @Override
   public Content.Counts computeContentSummary(final Content.Counts counts) {
     counts.add(Content.SYMLINK, 1);
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/INodeDirectorySnapshottable.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/INodeDirectorySnapshottable.java
index 8db9a93c306..86bcf90047b 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/INodeDirectorySnapshottable.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/INodeDirectorySnapshottable.java
@@ -37,7 +37,6 @@ import org.apache.hadoop.hdfs.protocol.SnapshotDiffReport;
 import org.apache.hadoop.hdfs.protocol.SnapshotDiffReport.DiffReportEntry;
 import org.apache.hadoop.hdfs.protocol.SnapshotDiffReport.DiffType;
 import org.apache.hadoop.hdfs.server.namenode.Content;
-import org.apache.hadoop.hdfs.server.namenode.Content.CountsMap.Key;
 import org.apache.hadoop.hdfs.server.namenode.INode;
 import org.apache.hadoop.hdfs.server.namenode.INodeDirectory;
 import org.apache.hadoop.hdfs.server.namenode.INodeFile;
@@ -347,16 +346,6 @@ public class INodeDirectorySnapshottable extends INodeDirectoryWithSnapshot {
     return counts;
   }
 
-  @Override
-  public Content.CountsMap computeContentSummary(
-      final Content.CountsMap countsMap) {
-    super.computeContentSummary(countsMap);
-    countsMap.getCounts(Key.SNAPSHOT).add(Content.SNAPSHOT,
-        snapshotsByNames.size());
-    countsMap.getCounts(Key.CURRENT).add(Content.SNAPSHOTTABLE_DIRECTORY, 1);
-    return countsMap;
-  }
-
   /**
    * Compute the difference between two snapshots (or a snapshot and the current
    * directory) of the directory.
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/INodeDirectoryWithSnapshot.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/INodeDirectoryWithSnapshot.java
index 8310963c7e7..ca39e6f4ec4 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/INodeDirectoryWithSnapshot.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/INodeDirectoryWithSnapshot.java
@@ -30,7 +30,6 @@ import org.apache.hadoop.hdfs.protocol.QuotaExceededException;
 import org.apache.hadoop.hdfs.protocol.SnapshotDiffReport.DiffReportEntry;
 import org.apache.hadoop.hdfs.protocol.SnapshotDiffReport.DiffType;
 import org.apache.hadoop.hdfs.server.namenode.Content;
-import org.apache.hadoop.hdfs.server.namenode.Content.CountsMap.Key;
 import org.apache.hadoop.hdfs.server.namenode.FSImageSerialization;
 import org.apache.hadoop.hdfs.server.namenode.INode;
 import org.apache.hadoop.hdfs.server.namenode.INodeDirectory;
@@ -819,14 +818,6 @@ public class INodeDirectoryWithSnapshot extends INodeDirectoryWithQuota {
     return counts;
   }
 
-  @Override
-  public Content.CountsMap computeContentSummary(
-      final Content.CountsMap countsMap) {
-    super.computeContentSummary(countsMap);
-    computeContentSummary4Snapshot(countsMap.getCounts(Key.SNAPSHOT));
-    return countsMap;
-  }
-
   @Override
   public Content.Counts computeContentSummary(final Content.Counts counts) {
     super.computeContentSummary(counts);
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/util/Diff.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/util/Diff.java
index fff53fb5e05..667221d8574 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/util/Diff.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/util/Diff.java
@@ -353,13 +353,50 @@ public class Diff<K, E extends Diff.Element<K>> {
 
   private static <K, E extends Diff.Element<K>> List<E> apply2Previous(
       final List<E> previous, final List<E> clist, final List<E> dlist) {
-    final List<E> current = new ArrayList<E>(previous);
-    for(E d : dlist) {
-      current.remove(d);
+    // Assumptions:
+    // (A1) All lists are sorted.
+    // (A2) All elements in dlist must be in previous.
+    // (A3) All elements in clist must be not in tmp = previous - dlist.
+    final List<E> tmp = new ArrayList<E>();
+    {
+      // tmp = previous - dlist
+      final Iterator<E> i = previous.iterator();
+      for(E deleted : dlist) {
+        E e = i.next(); //since dlist is non-empty, e must exist by (A2).
+        int cmp = 0;
+        for(; (cmp = e.compareTo(deleted.getKey())) < 0; e = i.next()) {
+          tmp.add(e);
+        }
+        Preconditions.checkState(cmp == 0); // check (A2)
+      }
+      for(; i.hasNext(); ) {
+        tmp.add(i.next());
+      }
     }
-    for(E c : clist) {
-      final int i = search(current, c.getKey());
-      current.add(-i - 1, c);
+
+    final List<E> current = new ArrayList<E>();
+    {
+      // current = tmp + clist
+      final Iterator<E> tmpIterator = tmp.iterator();
+      final Iterator<E> cIterator = clist.iterator();
+
+      E t = tmpIterator.hasNext()? tmpIterator.next(): null;
+      E c = cIterator.hasNext()? cIterator.next(): null;
+      for(; t != null || c != null; ) {
+        final int cmp = c == null? 1
+            : t == null? -1
+            : c.compareTo(t.getKey());
+
+        if (cmp < 0) {
+          current.add(c);
+          c = cIterator.hasNext()? cIterator.next(): null;
+        } else if (cmp > 0) {
+          current.add(t);
+          t = tmpIterator.hasNext()? tmpIterator.next(): null;
+        } else {
+          throw new AssertionError("Violated assumption (A3).");
+        }
+      }
     }
     return current;
   }
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/util/TestDiff.java b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/util/TestDiff.java
index d44ea43f654..22fc8998106 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/util/TestDiff.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/util/TestDiff.java
@@ -48,7 +48,7 @@ public class TestDiff {
   /** Test directory diff. */
   @Test(timeout=60000)
   public void testDiff() throws Exception {
-    for(int startSize = 0; startSize <= 1000; startSize = nextStep(startSize)) {
+    for(int startSize = 0; startSize <= 10000; startSize = nextStep(startSize)) {
       for(int m = 0; m <= 10000; m = nextStep(m)) {
         runDiffTest(startSize, m);
       }