HDFS-11494. Log message when DN is not selected for block replication. Contributed by Yiqun Lin.

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeDescriptor.java

@@ -637,6 +637,10 @@ public class DatanodeDescriptor extends DatanodeInfo {
       }
     }
     if (requiredSize > remaining - scheduledSize) {
+      LOG.debug(
+          "The node {} does not have enough {} space (required={},"
+          + " scheduled={}, remaining={}).",
+          this, t, requiredSize, scheduledSize, remaining);
       return null;
     }
     return storage;