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The "DiskSetup" page has been changed by RaviPhulari.
The comment on this change is: Adding guide about limiting data node's disk 
usage..
http://wiki.apache.org/hadoop/DiskSetup?action=diff&rev1=7&rev2=8

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  Pass a list of disks to the `dfs.data.dir` parameter, Hadoop will use all of 
the disks that are available. When one goes offline it is taken out of 
consideration. Hadoop does not check for the disk coming back -it assumes it is 
"gone". 
  
+ === How to limit Data node's disk usage? ===
+ 
+ Use dfs.datanode.du.reserved configuration value in 
$HADOOP_HOME/conf/hdfs-site.xml for limiting disk usage.
+ 
+ {{{
+   <property>
+     <name>dfs.datanode.du.reserved</name>
+     <!-- cluster variant -->
+     <value>182400</value>
+     <description>Reserved space in bytes per volume. Always leave this much 
space free for non dfs use.
+     </description>
+   </property>
+ }}}
  === Logging ===
  
   * The environment variable, `HADOOP_LOG_DIR` sets the directory Hadoop logs 
to.

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