HDFS-12744. More logs when short-circuit read is failed and disabled. Contributed by Weiwei Yang.

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/client/impl/BlockReaderFactory.java

@@ -646,7 +646,8 @@ private ShortCircuitReplicaInfo requestFileDescriptors(DomainPeer peer,
     default:
       LOG.warn(this + ": unknown response code " + resp.getStatus() +
           " while attempting to set up short-circuit access. " +
-          resp.getMessage());
+          resp.getMessage() + ". Disabling short-circuit read for DataNode "
+          + datanode + " temporarily.");
       clientContext.getDomainSocketFactory()
           .disableShortCircuitForPath(pathInfo.getPath());
       return null;

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DataXceiver.java

@@ -387,6 +387,8 @@ public void requestShortCircuitFds(final ExtendedBlock blk,
       } catch (IOException e) {
         bld.setStatus(ERROR);
         bld.setMessage(e.getMessage());
+        LOG.error("Request short-circuit read file descriptor" +
+            " failed with unknown error.", e);
       }
       bld.build().writeDelimitedTo(socketOut);
       if (fis != null) {