Add FAQ about VNode on the UI. (#5768)

* Add FAQ about VNode on the UI.
Co-authored-by: Nkezhenxu94 <kezhenxu94@apache.org>

CHANGES.md

@@ -25,7 +25,8 @@ Release Notes.
 
 #### UI
 
-#### Document
+#### Documentation
+* Add VNode FAQ doc.
 
 All issues and pull requests are [here](https://github.com/apache/skywalking/milestone/62?closed=1)
 

README.md

@@ -45,7 +45,7 @@ including
 SkyWalking OAP is using the STAM(Streaming Topology Analysis Method) to analysis topology in the tracing based agent scenario 
 for better performance. Read [the paper of STAM](https://wu-sheng.github.io/STAM/) for more details.
 
-# Document
+# Documentation
 Version Series | Version | 
 ----------- | ---------- |
 8.x| [8.x dev](docs/README.md)|

changes/changes-5.x.md

@@ -22,7 +22,7 @@
   - Make buffer file handler close more safety.
   - Fix NPE in AlarmService
 
-#### Documents
+#### Documentation
   - Fix compiling doc link.
   - Update new live demo address.
 
@@ -57,7 +57,7 @@
   - Fix Topology self link error.
   - Fix stack color mismatch label color in gc time chart.
   
-#### Documents
+#### Documentation
   - Add users list.
   - Fix several document typo.
   - Sync the Chinese documents.
@@ -120,7 +120,7 @@
   - Add application selector in service view.
   - Add order and status in trace query.
   
-#### Documents
+#### Documentation
   - Add architecture design doc.
   - Reformat deploy document. 
   - Adjust Tomcat deploy document.
@@ -182,7 +182,7 @@
   - Fix wrong startTime in trace view.
   - Fix some icons internet requirements.
  
-#### Documents
+#### Documentation
    - Add TLS document.
    - Add namespace document.
    - Add direct link document.

docs/README.md

 # Welcome
-**Here are SkyWalking 8 official documents. You're welcome to join us.**
+**Here are SkyWalking 8 official documentation. You're welcome to join us.**
 
 From here you can learn all about **SkyWalking**’s architecture, how to deploy and use SkyWalking, and develop based on SkyWalking contributions guidelines.
 

docs/en/FAQ/README.md

@@ -25,4 +25,7 @@ These are known and common FAQs. We welcome you to contribute yours.
 * [Use the profile exporter tool if the profile analysis is not right](../guides/backend-profile-export.md)
 * [Compatible with other javaagent bytecode processing](Compatible-with-other-javaagent-bytecode-processing.md)
 * [**Java agent memory leak** when enhance `Worker thread` at use Thread Pool](Memory-leak-enhance-Worker-thread.md)
-* [Thrift plugin](thrift-plugin.md)
\ No newline at end of file
+* [Thrift plugin](thrift-plugin.md)
+
+## UI
+* [What is **VNode**? And why does SkyWalking have that?](vnode.md)
\ No newline at end of file

docs/en/FAQ/vnode.md

+# What is VNode?
+In the trace page, sometimes, people could find there are nodes named **VNode** as the span name, and there is no attribute 
+for this span.
+
+**VNode** is created by the UI itself, rather than reported from the agent or tracing SDK. It represents there are some
+span(s) missed from the trace data in this query.
+
+## How does the UI detect the missing span(s)?
+The UI real check the parent spans and reference segments of all spans, if a parent id(segment id + span id) can't be found,
+then it creates a VNode automatically.
+
+## How does this happen?
+The VNode was introduced, because there are some cases which could cause the trace data are not always completed.
+1. The agent fail-safe mechanism activated. The SkyWalking agent has the capability to abandon the trace data, if
+there is agent->OAP network issue(unconnected, slow network speed), or the performance of the OAP cluster is not enough
+to process all traces. 
+1. Some plugins could have bugs, then some segments in the trace never stop correctly, it is hold in the memory.
+
+In these cases, the trace would not exist in the query. Then VNode shows up. 

docs/en/setup/backend/backend-storage.md

@@ -74,9 +74,6 @@ storage:
     advanced: ${SW_STORAGE_ES_ADVANCED:""}
 ```
 
-In order to use ElasticSearch 7, comment/remove the section `storage/elasticsearch` and find the corresponding config section(`storage/elasticsearch7`),
-uncomment to enable it.
-
 ### ElasticSearch 6 With Https SSL Encrypting communications.
 
 example: