diff --git a/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/TimeRangeTracker.java b/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/TimeRangeTracker.java
index 5c0eee5c335..d995fc6f404 100644
--- a/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/TimeRangeTracker.java
+++ b/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/TimeRangeTracker.java
@@ -19,7 +19,9 @@
 package org.apache.hadoop.hbase.regionserver;
 
 import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
 import java.io.DataInputStream;
+import java.io.DataOutputStream;
 import java.io.IOException;
 import java.util.concurrent.atomic.AtomicLong;
 
@@ -206,13 +208,24 @@ public abstract class TimeRangeTracker {
     }
   }
 
-  public static byte[] toByteArray(TimeRangeTracker tracker) {
-    return ProtobufUtil.prependPBMagic(
-        HBaseProtos.TimeRangeTracker.newBuilder()
-          .setFrom(tracker.getMin())
-          .setTo(tracker.getMax())
-          .build()
-          .toByteArray());
+  /**
+   * This method used to serialize TimeRangeTracker (TRT) by protobuf while this breaks the
+   * forward compatibility on HFile.(See HBASE-21008) In previous hbase version ( < 2.0.0 ) we use
+   * DataOutput to serialize TRT, these old versions don't have capability to deserialize TRT
+   * which is serialized by protobuf. So we need to revert the change of serializing
+   * TimeRangeTracker back to DataOutput. For more information, please check HBASE-21012.
+   * @param tracker TimeRangeTracker needed to be serialized.
+   * @return byte array filled with serialized TimeRangeTracker.
+   * @throws IOException if something goes wrong in writeLong.
+   */
+  public static byte[] toByteArray(TimeRangeTracker tracker) throws IOException {
+    try (ByteArrayOutputStream bos = new ByteArrayOutputStream()) {
+      try (DataOutputStream dos = new DataOutputStream(bos)) {
+        dos.writeLong(tracker.getMin());
+        dos.writeLong(tracker.getMax());
+        return bos.toByteArray();
+      }
+    }
   }
 
   /**
diff --git a/src/main/asciidoc/_chapters/upgrading.adoc b/src/main/asciidoc/_chapters/upgrading.adoc
index bc2ec1c103b..6dc788a50fa 100644
--- a/src/main/asciidoc/_chapters/upgrading.adoc
+++ b/src/main/asciidoc/_chapters/upgrading.adoc
@@ -588,6 +588,18 @@ The internal changes to HBase during this upgrade were sufficient for compilatio
 
 If you previously relied on client side tracing integrated with HBase operations, it is recommended that you upgrade your usage to HTrace 4 as well.
 
+[[upgrade2.0.hfile.compatability]]
+.HFile lose forward compatability
+
+HFiles generated by 2.0.0, 2.0.1, 2.1.0 are not forward compatible to 1.4.6-, 1.3.2.1-, 1.2.6.1-,
+and other inactive releases. Why HFile lose compatability is hbase in new versions
+(2.0.0, 2.0.1, 2.1.0) use protobuf to serialize/deserialize TimeRangeTracker (TRT) while old
+versions use DataInput/DataOutput. To solve this, We have to put
+link:https://jira.apache.org/jira/browse/HBASE-21012[HBASE-21012]
+to 2.x and put link:https://jira.apache.org/jira/browse/HBASE-21013[HBASE-21013] in 1.x.
+For more information, please check
+link:https://jira.apache.org/jira/browse/HBASE-21008[HBASE-21008].
+
 [[upgrade2.0.perf]]
 .Performance