HDFS-15253 Default checkpoint transfer speed, 50mb per second (#2366)

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

@@ -983,7 +983,7 @@ public class DFSConfigKeys extends CommonConfigurationKeys {
 
   public static final String DFS_IMAGE_TRANSFER_RATE_KEY =
                                            "dfs.image.transfer.bandwidthPerSec";
-  public static final long DFS_IMAGE_TRANSFER_RATE_DEFAULT = 0;  //no throttling
+  public static final long DFS_IMAGE_TRANSFER_RATE_DEFAULT = 52428800;
 
   public static final String DFS_IMAGE_TRANSFER_BOOTSTRAP_STANDBY_RATE_KEY =
       "dfs.image.transfer-bootstrap-standby.bandwidthPerSec";

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

@@ -1436,13 +1436,13 @@
 
 <property>
   <name>dfs.image.transfer.bandwidthPerSec</name>
-  <value>0</value>
+  <value>52428800</value>
   <description>
         Maximum bandwidth used for regular image transfers (instead of
         bootstrapping the standby namenode), in bytes per second.
         This can help keep normal namenode operations responsive during
         checkpointing.
-        A default value of 0 indicates that throttling is disabled.
+        A default value is 50mb per second.
         The maximum bandwidth used for bootstrapping standby namenode is
         configured with dfs.image.transfer-bootstrap-standby.bandwidthPerSec.
         Support multiple size unit suffix(case insensitive), as described