HDFS-14154. [SBN read] Document dfs.ha.tail-edits.period in user guide. Contributed by Chao Sun.

hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/HDFSHighAvailabilityWithQJM.md

@@ -27,7 +27,12 @@ This document assumes that the reader has a general understanding of general com
 Note: Using the Quorum Journal Manager or Conventional Shared Storage
 ---------------------------------------------------------------------
 
-This guide discusses how to configure and use HDFS HA using the Quorum Journal Manager (QJM) to share edit logs between the Active and Standby NameNodes. For information on how to configure HDFS HA using NFS for shared storage instead of the QJM, please see [this alternative guide.](./HDFSHighAvailabilityWithNFS.html)
+This guide discusses how to configure and use HDFS HA using the Quorum
+Journal Manager (QJM) to share edit logs between the Active and Standby
+NameNodes. For information on how to configure HDFS HA using NFS for
+shared storage instead of the QJM, please see [this alternative
+guide.](./HDFSHighAvailabilityWithNFS.html). For information on how to
+configure HDFS HA with Observer NameNode, please see [this guide](./ObserverNameNode.html)
 
 Background
 ----------

hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/ObserverNameNode.md

@@ -23,13 +23,13 @@ Purpose
 This guide provides an overview of the HDFS Observer NameNode feature
 and how to configure/install it in a typical HA-enabled cluster. For a
 detailed technical design overview, please check the doc attached to
-HDFS-12943.
+[HDFS-12943](https://issues.apache.org/jira/browse/HDFS-12943).
 
 Background
 -----------
 
 In a HA-enabled HDFS cluster (for more information, check
-[HDFSHighAvailabilityWithQJM](./HDFSHighAvailabilityWithQJM.md)), there
+[HDFSHighAvailabilityWithQJM](./HDFSHighAvailabilityWithQJM.html)), there
 is a single Active NameNode and one or more Standby NameNode(s). The
 Active NameNode is responsible for serving all client requests, while
 Standby NameNode just keep the up-to-date information regarding the
@@ -104,6 +104,20 @@ few configurations to your **hdfs-site.xml**:
           <value>true</value>
         </property>
 
+*  **dfs.ha.tail-edits.period** - how often Standby/Observer NameNodes
+   should fetch edits from JournalNodes.
+
+   This determines the staleness of Observer NameNode w.r.t the Active.
+   If too large, RPC time will increase as client requests will wait
+   longer in the RPC queue before Observer tails edit logs and catches
+   up the latest state of Active. The default value is 1min. It is
+   **highly recommend** to configure this to a much lower value.
+
+        <property>
+          <name>dfs.ha.tail-edits.period</name>
+          <value>0ms</value>
+        </property>
+
 *  **dfs.journalnode.edit-cache-size.bytes** - the in-memory cache size,
    in bytes, on the JournalNodes.