diff --git a/hadoop-mapreduce-project/CHANGES.txt b/hadoop-mapreduce-project/CHANGES.txt
index 7c1cea65716..3b8749d379c 100644
--- a/hadoop-mapreduce-project/CHANGES.txt
+++ b/hadoop-mapreduce-project/CHANGES.txt
@@ -273,6 +273,9 @@ Release 2.4.1 - UNRELEASED
     MAPREDUCE-5835. Killing Task might cause the job to go to ERROR state
     (Ming Ma via jlowe)
 
+    MAPREDUCE-5821. Avoid unintentional reallocation of byte arrays in segments
+    during merge. (Todd Lipcon via cdouglas)
+
 Release 2.4.0 - 2014-04-07 
 
   INCOMPATIBLE CHANGES
diff --git a/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapred/Merger.java b/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapred/Merger.java
index b4362ac5096..9493871138d 100644
--- a/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapred/Merger.java
+++ b/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapred/Merger.java
@@ -537,6 +537,8 @@ public class Merger {
         }
       }
       minSegment = top();
+      long startPos = minSegment.getPosition();
+      key = minSegment.getKey();
       if (!minSegment.inMemory()) {
         //When we load the value from an inmemory segment, we reset
         //the "value" DIB in this class to the inmem segment's byte[].
@@ -547,11 +549,11 @@ public class Merger {
         //segment, we reset the "value" DIB to the byte[] in that (so 
         //we reuse the disk segment DIB whenever we consider
         //a disk segment).
+        minSegment.getValue(diskIFileValue);
         value.reset(diskIFileValue.getData(), diskIFileValue.getLength());
+      } else {
+        minSegment.getValue(value);
       }
-      long startPos = minSegment.getPosition();
-      key = minSegment.getKey();
-      minSegment.getValue(value);
       long endPos = minSegment.getPosition();
       totalBytesProcessed += endPos - startPos;
       mergeProgress.set(totalBytesProcessed * progPerByte);