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From Apache Wiki <wikidi...@apache.org>
Subject [Hadoop Wiki] Update of "InvalidJiraIssues" by SteveLoughran
Date Tue, 20 Oct 2015 11:52:58 GMT
Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Hadoop Wiki" for change notification.

The "InvalidJiraIssues" page has been changed by SteveLoughran:
https://wiki.apache.org/hadoop/InvalidJiraIssues?action=diff&rev1=13&rev2=14

Comment:
+ loglevel

   * The log messages do try to be helpful. Sometimes they are only meaningful to people who
understand how Hadoop works -but you are going to have to learn that anyway.
   * Some of the log messages include URLs to pages in this wiki. Follow the links -they are
there for a reason.
   * Search engines are a surprisingly useful way of finding out what an error message machines.
Before panicking and sending out messages to all known Hadoop mailing lists as well as filing
critical-level JIRA issues, why not copy the string from the logs and paste into the search
dialog of your search engine of choice -and see what comes up. The discussions on the user
list archives are always worth reading, as are any references on stack overflow.
+  * Hadoop log levels can be dynamically changed by way of the {{{hadoop --loglevel}}} command
(Hadoop 2.7+). Any of the core Hadoop services can have their log levels viewed and changed,
via the LogLevel servlet -just add the path `/logLevel` to the base URL of the service.
  
  '''Finding an answer by searching for it on the web is the fastest way to get help -and
log messages are ideal for searching on'''
+ 
+ == Embrace Metrics ==
+ 
+ Hadoop applications, and the JVMs themselves, all publish metric information for management
tools to view. These help you see what is happening in a cluster, including which parts of
the cluster are not actually running. As well as commercial products, the [https://ambari.apache.org/|Apache
Ambari] lets you see what is going on. Oracle's jvisualvm tool lets you see what is happening
inside a JVM. This is invaluable for debugging memory consumption and CPU performance problems.
+ 
+ Note also that management tools provide ways to configure, reconfigure and restart your
cluster without you having to edit XML files by hand.
  
  == Ask on the User Mailing Lists ==
  

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