[Hadoop Wiki] Update of "Hbase/Troubleshooting" by JeanDanielCryans

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The following page has been changed by JeanDanielCryans:
http://wiki.apache.org/hadoop/Hbase/Troubleshooting

The comment on the change is:
Added: Scanners keep getting timeouts

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   1. [#7 Problem: DFS instability and/or regionserver lease timeouts]
   1. [#8 Problem: Instability on Amazon EC2]
   1. [#9 Problem: Zookeeper SessionExpired events]
+  1. [#10 Problem: Scanners keep getting timeouts]
  
  [[Anchor(1)]]
  == 1. Problem: Master initializes, but Region Servers do not ==
@@ -158, +159 @@

  }}}
   * For Java SE 6, some users have had success with {{{ -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:ParallelGCThreads=8 }}}
  
+ [[Anchor(10)]]
+ == 10. Problem: Scanners keep getting timeouts ==
+  * Client receives org.apache.hadoop.hbase.UnknownScannerException or timeouts even if the region server lease is really high. Fixed in HBase 0.20.0
+ === Causes ===
+  * The client, by default, fetches 30 rows when issuing the first next(). All the 29 other calls simply return rows from local memory. So if it takes 3 minutes to process a row and the timeout is set to 30 minutes, it is still not enough to cover 30 * 3 = 90 minutes.
+ === Resolution ===
+  * Set hbase.client.scanner.caching in hbase-site.xml at a very low value like 1 or use HTable.setScannerCaching(1).
+