[Hadoop Wiki] Update of "PerformanceTuning" by JeanDanielCryans

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The "PerformanceTuning" page has been changed by JeanDanielCryans.
The comment on this change is: Added a new tip for HBase 0.21.
http://wiki.apache.org/hadoop/PerformanceTuning?action=diff&rev1=6&rev2=7

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   * Are you swapping?  JVMs hate swapping.  Adjusting [[http://www.sollers.ca/blog/2008/swappiness/|swappiness]] may help.
   * By default, each regionserver puts up 10 listeners only.  Up it if you have measurable traffic (See hbase.regionserver.handler.count in hbase-default.xml).
   * To speed up the inserts in a non critical job (like an import job), you can use Put.writeToWAL(false) to bypass writing to the write ahead log.
+  * New in HBase 0.21 (not released yet), you can set a table-scope attribute to defer the write ahead log's flushes in order to improve write performance by exposing your table to less durability. Use HTableDescriptor.setDeferredLogFlush(true) when creating your table. Canonical use of this feature would be a table where you only keep counters and losing a few seconds or milliseconds worth of edits isn't critical (when a region server dies). By setting hbase.regionserver.optionallogflushinterval to a value like 1000 would mean that in the worst case you lose 1 second of edits. Since the WAL is shared on a region server, any other table not using this feature will flush your other table's edit.
  
  == HBase JVM and GC ==