Add comment on HDFS's strategy for updating access time by MetadataUpdateEvent (#2071)

Author: philo-he

smart-hadoop-support/smart-hadoop/src/main/java/org/smartdata/hdfs/metric/fetcher/InotifyEventApplier.java

@@ -146,6 +146,9 @@ private List<String> getSqlStatement(Event event) throws IOException, MetaStoreE
             ", dest path:" + ((Event.RenameEvent) event).getDstPath());
         return this.getRenameSql((Event.RenameEvent)event);
       case METADATA:
+        // The property dfs.namenode.accesstime.precision in HDFS's configuration controls
+        // the precision of access time. Its default value is 1h. To avoid missing a
+        // MetadataUpdateEvent for updating access time, a smaller value should be set.
         path = ((Event.MetadataUpdateEvent)event).getPath();
         if (shouldIgnore(path)) {
           return Arrays.asList();