HDFS-14821. Make HDFS-14617 (fsimage sub-sections) off by default. Contributed by Stephen O'Donnell.

Signed-off-by: Wei-Chiu Chuang <weichiu@apache.org>

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSConfigKeys.java

@@ -899,7 +899,7 @@ public class DFSConfigKeys extends CommonConfigurationKeys {
 
   public static final String DFS_IMAGE_PARALLEL_LOAD_KEY =
       "dfs.image.parallel.load";
-  public static final boolean DFS_IMAGE_PARALLEL_LOAD_DEFAULT = true;
+  public static final boolean DFS_IMAGE_PARALLEL_LOAD_DEFAULT = false;
 
   public static final String DFS_IMAGE_PARALLEL_TARGET_SECTIONS_KEY =
       "dfs.image.parallel.target.sections";

hadoop-hdfs-project/hadoop-hdfs/src/main/resources/hdfs-default.xml

@@ -1393,7 +1393,7 @@
 
 <property>
   <name>dfs.image.parallel.load</name>
-  <value>true</value>
+  <value>false</value>
   <description>
         If true, write sub-section entries to the fsimage index so it can
         be loaded in parallel. Also controls whether parallel loading
@@ -1403,6 +1403,11 @@
         Parallel loading is not compatible with image compression,
         so if dfs.image.compress is set to true this setting will be
         ignored and no parallel loading will occur.
+        Enabling this feature may impact rolling upgrades and downgrades if
+        the previous version does not support this feature. If the feature was
+        enabled and a downgrade is required, first set this parameter to
+        false and then save the namespace to create a fsimage with no
+        sub-sections and then perform the downgrade.
   </description>
 </property>