diff --git a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
index 9dfa3d7536d..2f178e67dd3 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
+++ b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
@@ -149,6 +149,9 @@ Release 2.7.1 - UNRELEASED
     HDFS-8626. Reserved RBW space is not released if creation of RBW File
     fails. (kanaka kumar avvaru via Arpit Agarwal)
 
+    HDFS-8681. BlockScanner is incorrectly disabled by default.
+    (Arpit Agarwal)
+
 Release 2.7.0 - 2015-04-20
 
   INCOMPATIBLE CHANGES
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSConfigKeys.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSConfigKeys.java
index b8d203e3cd5..a8d7159b991 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSConfigKeys.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSConfigKeys.java
@@ -453,7 +453,7 @@ public class DFSConfigKeys extends CommonConfigurationKeys {
   public static final String  DFS_DATANODE_MAX_RECEIVER_THREADS_KEY = "dfs.datanode.max.transfer.threads";
   public static final int     DFS_DATANODE_MAX_RECEIVER_THREADS_DEFAULT = 4096;
   public static final String  DFS_DATANODE_SCAN_PERIOD_HOURS_KEY = "dfs.datanode.scan.period.hours";
-  public static final int     DFS_DATANODE_SCAN_PERIOD_HOURS_DEFAULT = 0;
+  public static final int     DFS_DATANODE_SCAN_PERIOD_HOURS_DEFAULT = 21 * 24;  // 3 weeks.
   public static final String  DFS_BLOCK_SCANNER_VOLUME_BYTES_PER_SECOND = "dfs.block.scanner.volume.bytes.per.second";
   public static final long    DFS_BLOCK_SCANNER_VOLUME_BYTES_PER_SECOND_DEFAULT = 1048576L;
   public static final String  DFS_DATANODE_TRANSFERTO_ALLOWED_KEY = "dfs.datanode.transferTo.allowed";
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/BlockScanner.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/BlockScanner.java
index b0248c50d9c..9c4dd10f005 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/BlockScanner.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/BlockScanner.java
@@ -115,6 +115,34 @@ public class BlockScanner {
       }
     }
 
+    /**
+     * Determine the configured block scanner interval.
+     *
+     * For compatibility with prior releases of HDFS, if the
+     * configured value is zero then the scan period is
+     * set to 3 weeks.
+     *
+     * If the configured value is less than zero then the scanner
+     * is disabled.
+     *
+     * @param conf Configuration object.
+     * @return block scan period in milliseconds.
+     */
+    private static long getConfiguredScanPeriodMs(Configuration conf) {
+      long tempScanPeriodMs = getUnitTestLong(
+          conf, INTERNAL_DFS_DATANODE_SCAN_PERIOD_MS,
+              TimeUnit.MILLISECONDS.convert(conf.getLong(
+                  DFS_DATANODE_SCAN_PERIOD_HOURS_KEY,
+                  DFS_DATANODE_SCAN_PERIOD_HOURS_DEFAULT), TimeUnit.HOURS));
+
+      if (tempScanPeriodMs == 0) {
+        tempScanPeriodMs = TimeUnit.MILLISECONDS.convert(
+            DFS_DATANODE_SCAN_PERIOD_HOURS_DEFAULT, TimeUnit.HOURS);
+      }
+
+      return tempScanPeriodMs;
+    }
+
     @SuppressWarnings("unchecked")
     Conf(Configuration conf) {
       this.targetBytesPerSec = Math.max(0L, conf.getLong(
@@ -123,11 +151,7 @@ public class BlockScanner {
       this.maxStalenessMs = Math.max(0L, getUnitTestLong(conf,
           INTERNAL_DFS_BLOCK_SCANNER_MAX_STALENESS_MS,
           INTERNAL_DFS_BLOCK_SCANNER_MAX_STALENESS_MS_DEFAULT));
-      this.scanPeriodMs = Math.max(0L,
-          getUnitTestLong(conf, INTERNAL_DFS_DATANODE_SCAN_PERIOD_MS,
-              TimeUnit.MILLISECONDS.convert(conf.getLong(
-                  DFS_DATANODE_SCAN_PERIOD_HOURS_KEY,
-                  DFS_DATANODE_SCAN_PERIOD_HOURS_DEFAULT), TimeUnit.HOURS)));
+      this.scanPeriodMs = getConfiguredScanPeriodMs(conf);
       this.cursorSaveMs = Math.max(0L, getUnitTestLong(conf,
           INTERNAL_DFS_BLOCK_SCANNER_CURSOR_SAVE_INTERVAL_MS,
           INTERNAL_DFS_BLOCK_SCANNER_CURSOR_SAVE_INTERVAL_MS_DEFAULT));
@@ -159,7 +183,7 @@ public class BlockScanner {
    * no threads will start.
    */
   public boolean isEnabled() {
-    return (conf.scanPeriodMs) > 0 && (conf.targetBytesPerSec > 0);
+    return (conf.scanPeriodMs > 0) && (conf.targetBytesPerSec > 0);
   }
 
  /**
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/resources/hdfs-default.xml b/hadoop-hdfs-project/hadoop-hdfs/src/main/resources/hdfs-default.xml
index f50bdc19efb..41997981c3e 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/resources/hdfs-default.xml
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/resources/hdfs-default.xml
@@ -1024,11 +1024,14 @@
 
 <property>
   <name>dfs.datanode.scan.period.hours</name>
-  <value>0</value>
+  <value>504</value>
   <description>
-        If this is 0 or negative, the DataNode's block scanner will be
-        disabled.  If this is positive, the DataNode will not scan any
+        If this is positive, the DataNode will not scan any
         individual block more than once in the specified scan period.
+        If this is negative, the block scanner is disabled.
+        If this is set to zero, then the default value of 504 hours
+        or 3 weeks is used. Prior versions of HDFS incorrectly documented
+        that setting this key to zero will disable the block scanner.
   </description>
 </property>
 
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 03e5dc5505d..5946080a4fb 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
@@ -264,6 +264,7 @@ public class TestFsDatasetImpl {
   public void testChangeVolumeWithRunningCheckDirs() throws IOException {
     RoundRobinVolumeChoosingPolicy<FsVolumeImpl> blockChooser =
         new RoundRobinVolumeChoosingPolicy<>();
+    conf.setLong(DFSConfigKeys.DFS_DATANODE_SCAN_PERIOD_HOURS_KEY, -1);
     final BlockScanner blockScanner = new BlockScanner(datanode, conf);
     final FsVolumeList volumeList = new FsVolumeList(
         Collections.<VolumeFailureInfo>emptyList(), blockScanner, blockChooser);