HDFS-15610 Reduced datanode upgrade/hardlink thread from 12 to 6 (#2365)

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

@@ -1386,7 +1386,7 @@ public class DFSConfigKeys extends CommonConfigurationKeys {
       "dfs.datanode.parallel.volumes.load.threads.num";
   public static final String DFS_DATANODE_BLOCK_ID_LAYOUT_UPGRADE_THREADS_KEY =
       "dfs.datanode.block.id.layout.upgrade.threads";
-  public static final int DFS_DATANODE_BLOCK_ID_LAYOUT_UPGRADE_THREADS = 12;
+  public static final int DFS_DATANODE_BLOCK_ID_LAYOUT_UPGRADE_THREADS = 6;
 
   public static final String DFS_NAMENODE_INOTIFY_MAX_EVENTS_PER_RPC_KEY =
       "dfs.namenode.inotify.max.events.per.rpc";

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

@@ -3296,7 +3296,7 @@
 
 <property>
   <name>dfs.datanode.block.id.layout.upgrade.threads</name>
-  <value>12</value>
+  <value>6</value>
   <description>The number of threads to use when creating hard links from
     current to previous blocks during upgrade of a DataNode to block ID-based
     block layout (see HDFS-6482 for details on the layout).</description>