Hadoop tips

• Change logging level
• For each daemon, there is a service at http://daemon_address:port/logLevel through which you can get and set logging level.
• Use command line
    hadoop daemonlog -getLevel daemon_address:port fullQualifiedClassName
  hadoop daemonlog -setLevel daemon_address:port fullQualifiedClassName logLevel
• Permanent change
  Change file log4j.properties. Example
      log4j.logger.org.apache.hadoop.mapred.JobTracker=DEBUG
      log4j.logger.org.apache.hadoop.mapred.TaskTracker=DEBUG
      log4j.logger.org.apache.hadoop.fs.FSNamesystem=DEBUG
  
• Commission and decommission nodes
  Following four config parameters are related:
  dfs.hosts
  dfs.hosts.exclude
  mapreduce.jobtracker.hosts.filename (mapred.hosts for old version)
  mapreduce.jobtracker.hosts.exclude.filename (mapred.hosts.exclude for old version)
  
  For HDFS, execute "hadoop dfsadmin -refreshNodes" after you change the include file or exclude file.
  From the mailing list, I know "mradmin -refreshNodes was added in 0.21".  So for MapReduce, you can use "hadoop mradmin -refreshNodes" after you change the include file or exclude file to commission or decommission a node respectively.
  To permanently add or remove a node, you also need to change slave file conf/slaves.
  
• Block scanner report
  http://datanode_address:50075/blockScannerReport
  
• If you want to check blocks and block locations of a specific file, use following command:
    hadoop fsck file_to_check -files -blocks -locations -racks
  Note: you should execute it on master node.
  Use "hadoop fsck /" to check health of the whole file system.