HDFS-8821. Explain message "Operation category X is not supported in state standby". Contributed by Gautam Gopalakrishnan.

(cherry picked from commit c5caa25b8f)

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -14,6 +14,9 @@ Release 2.8.0 - UNRELEASED
 
   IMPROVEMENTS
 
+    HDFS-8821. Explain message "Operation category X is not supported
+    in state standby" (Gautam Gopalakrishnan via harsh)
+
     HDFS-3918. EditLogTailer shouldn't log WARN when other node
     is in standby mode (todd via harsh)
 

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/ha/StandbyState.java

@@ -82,8 +82,9 @@ public class StandbyState extends HAState {
         (op == OperationCategory.READ && context.allowStaleReads())) {
       return;
     }
+    String faq = ". Visit https://s.apache.org/sbnn-error";
     String msg = "Operation category " + op + " is not supported in state "
-        + context.getState();
+        + context.getState() + faq;
     throw new StandbyException(msg);
   }