diff --git a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
index 04c0f4c1967..47ec910e92b 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
+++ b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
@@ -33,6 +33,8 @@ Release 2.6.1 - UNRELEASED
     HDFS-7733. NFS: readdir/readdirplus return null directory
     attribute on failure. (Arpit Agarwal)
 
+    HDFS-8486. DN startup may cause severe data loss. (daryn via cmccabe)
+
 Release 2.6.0 - 2014-11-18
 
   INCOMPATIBLE CHANGES
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DataNode.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DataNode.java
index a7dd9223584..fe57bc35975 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DataNode.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DataNode.java
@@ -1279,9 +1279,8 @@ public class DataNode extends ReconfigurableBase
     // failures.
     checkDiskError();
 
-    initPeriodicScanners(conf);
-    
     data.addBlockPool(nsInfo.getBlockPoolID(), conf);
+    initDirectoryScanner(conf);
   }
 
   BPOfferService[] getAllBpOs() {
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/BlockPoolSlice.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/BlockPoolSlice.java
index e3d1607a92a..58e57f9221b 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/BlockPoolSlice.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/BlockPoolSlice.java
@@ -531,10 +531,28 @@ class BlockPoolSlice {
       // Leave both block replicas in place.
       return replica1;
     }
+    final ReplicaInfo replicaToDelete =
+        selectReplicaToDelete(replica1, replica2);
+    final ReplicaInfo replicaToKeep =
+        (replicaToDelete != replica1) ? replica1 : replica2;
+    // Update volumeMap and delete the replica
+    volumeMap.add(bpid, replicaToKeep);
+    if (replicaToDelete != null) {
+      deleteReplica(replicaToDelete);
+    }
+    return replicaToKeep;
+  }
 
+  static ReplicaInfo selectReplicaToDelete(final ReplicaInfo replica1,
+      final ReplicaInfo replica2) {
     ReplicaInfo replicaToKeep;
     ReplicaInfo replicaToDelete;
 
+    // it's the same block so don't ever delete it, even if GS or size
+    // differs.  caller should keep the one it just discovered on disk
+    if (replica1.getBlockFile().equals(replica2.getBlockFile())) {
+      return null;
+    }
     if (replica1.getGenerationStamp() != replica2.getGenerationStamp()) {
       replicaToKeep = replica1.getGenerationStamp() > replica2.getGenerationStamp()
           ? replica1 : replica2;
@@ -554,10 +572,10 @@ class BlockPoolSlice {
       LOG.debug("resolveDuplicateReplicas decide to keep " + replicaToKeep
           + ".  Will try to delete " + replicaToDelete);
     }
+    return replicaToDelete;
+  }
 
-    // Update volumeMap.
-    volumeMap.add(bpid, replicaToKeep);
-
+  private void deleteReplica(final ReplicaInfo replicaToDelete) {
     // Delete the files on disk. Failure here is okay.
     final File blockFile = replicaToDelete.getBlockFile();
     if (!blockFile.delete()) {
@@ -567,10 +585,8 @@ class BlockPoolSlice {
     if (!metaFile.delete()) {
       LOG.warn("Failed to delete meta file " + metaFile);
     }
-
-    return replicaToKeep;
   }
-  
+
   /**
    * Find out the number of bytes in the block that match its crc.
    * 
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/TestFsDatasetImpl.java b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/TestFsDatasetImpl.java
index 7c39ca55680..9a8ce9c42fb 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/TestFsDatasetImpl.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/TestFsDatasetImpl.java
@@ -30,6 +30,8 @@ import org.apache.hadoop.hdfs.server.datanode.DNConf;
 import org.apache.hadoop.hdfs.server.datanode.DataBlockScanner;
 import org.apache.hadoop.hdfs.server.datanode.DataNode;
 import org.apache.hadoop.hdfs.server.datanode.DataStorage;
+import org.apache.hadoop.hdfs.server.datanode.FinalizedReplica;
+import org.apache.hadoop.hdfs.server.datanode.ReplicaInfo;
 import org.apache.hadoop.hdfs.server.datanode.StorageLocation;
 import org.apache.hadoop.test.GenericTestUtils;
 import org.apache.hadoop.util.StringUtils;
@@ -46,6 +48,8 @@ import java.util.List;
 import java.util.Set;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
 import static org.junit.Assert.fail;
 import static org.mockito.Matchers.any;
 import static org.mockito.Matchers.anyString;
@@ -186,4 +190,35 @@ public class TestFsDatasetImpl {
     verify(scanner, times(BLOCK_POOL_IDS.length))
         .deleteBlocks(anyString(), any(Block[].class));
   }
+
+  @Test
+  public void testDuplicateReplicaResolution() throws IOException {
+    FsVolumeImpl fsv1 = Mockito.mock(FsVolumeImpl.class);
+    FsVolumeImpl fsv2 = Mockito.mock(FsVolumeImpl.class);
+
+    File f1 = new File("d1/block");
+    File f2 = new File("d2/block");
+
+    ReplicaInfo replicaOlder = new FinalizedReplica(1,1,1,fsv1,f1);
+    ReplicaInfo replica = new FinalizedReplica(1,2,2,fsv1,f1);
+    ReplicaInfo replicaSame = new FinalizedReplica(1,2,2,fsv1,f1);
+    ReplicaInfo replicaNewer = new FinalizedReplica(1,3,3,fsv1,f1);
+
+    ReplicaInfo replicaOtherOlder = new FinalizedReplica(1,1,1,fsv2,f2);
+    ReplicaInfo replicaOtherSame = new FinalizedReplica(1,2,2,fsv2,f2);
+    ReplicaInfo replicaOtherNewer = new FinalizedReplica(1,3,3,fsv2,f2);
+
+    // equivalent path so don't remove either
+    assertNull(BlockPoolSlice.selectReplicaToDelete(replicaSame, replica));
+    assertNull(BlockPoolSlice.selectReplicaToDelete(replicaOlder, replica));
+    assertNull(BlockPoolSlice.selectReplicaToDelete(replicaNewer, replica));
+
+    // keep latest found replica
+    assertSame(replica,
+        BlockPoolSlice.selectReplicaToDelete(replicaOtherSame, replica));
+    assertSame(replicaOtherOlder,
+        BlockPoolSlice.selectReplicaToDelete(replicaOtherOlder, replica));
+    assertSame(replica,
+        BlockPoolSlice.selectReplicaToDelete(replicaOtherNewer, replica));
+  }
 }