HDFS-10620. StringBuilder created and appended even if logging is disabled. Contributed by Staffan Friberg.

(cherry picked from commit f7dabe3addf3f6eb32ea9b8ec1354fb442ce4222)
(cherry picked from commit 2685e16f381532d53aea413ce1c0a14181ee7c89)

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

@@ -1210,16 +1210,19 @@ public class BlockManager implements BlockStatsMXBean {
     if (!isPopulatingReplQueues()) {
       return;
     }
-    StringBuilder datanodes = new StringBuilder();
+    StringBuilder datanodes = blockLog.isDebugEnabled()
+        ? new StringBuilder() : null;
     for (DatanodeStorageInfo storage : blocksMap.getStorages(b)) {
       if (storage.getState() != State.NORMAL) {
         continue;
       }
       final DatanodeDescriptor node = storage.getDatanodeDescriptor();
       invalidateBlocks.add(b, node, false);
-      datanodes.append(node).append(" ");
+      if (datanodes != null) {
+        datanodes.append(node).append(" ");
+      }
     }
-    if (datanodes.length() != 0) {
+    if (datanodes != null & datanodes.length() != 0) {
       blockLog.debug("BLOCK* addToInvalidates: {} {}", b, datanodes);
     }
   }