[Hadoop Wiki] Update of "Hbase/HowToMigrate" by JimKellerman

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

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  == Version-Specific Migration Notes ==
  
- === From 0.1.x to 0.2.x ===
+ === From 0.1.x to 0.2.x or 0.18.x ===
  
- The following are step-by-step instructions for migrating from HBase 0.1 to 0.2.  This migration requires an upgrade form Hadoop 0.16 to 0.17, although the [http://wiki.apache.org/hadoop/Hadoop%20Upgrade Hadoop Upgrade Instructions] are slightly out-of-date (as of this writing, July 2008).  As such, the below instructions also clarify the necessary steps for upgrading Hadoop.
+ The following are step-by-step instructions for migrating from HBase 0.1 to 0.2 or 0.18.  Migration from 0.1 to 0.2 requires an upgrade from Hadoop 0.16 to 0.17, and migration from 0.1 to 0.18 requires an upgrade from Hadoop 0.16 to 0.18. The [http://wiki.apache.org/hadoop/Hadoop%20Upgrade Hadoop Upgrade Instructions] are slightly out-of-date (as of this writing, September 2008).  As such, the below instructions also clarify the necessary steps for upgrading Hadoop.
  
  Assume Hadoop 0.16 and HBase 0.1 are already running with data you wish migrate to HBase 0.2.
   * Stop HBase 0.1.
@@ -35, +35 @@

  
  == Other Migration-Related Concerns ==
  
+ Migration is only supported between the file system version of the previous release and the file system version of the current release. If the existing HBase installation has an older file system version, it will be necessary to install a HBase release which can perform the upgrade, run the migration tool and then install the desired release and run its migration script. (Note that if the existing installation is several versions old, it may be necessary to repeat this process).
+ 
  === Redo Logs ===
  
  It is possible that, when running the HBase migration command, the migration will fail because of "unrecovered redo logs."  Redo logs are generated every time HBase is started, and under normal circumstances they are removed when HBase is stopped cleanly.  However, if you have ever stopped HBase in some atypical way (for example, using {{{kill -9}}}), these redo logs will persist in Hadoop DFS.  To see if you have any unrecovered redo logs, stop any currently-running instances of HBase and enter: {{{{$HADOOP_HOME}/bin/hadoop dfs -ls /hbase}}}.  All existing redo logs will be in this directory.  Redo log directories can be removed using dfs {{{-rm}}} option.  WARNING: redo logs are the only way to recover any data entered before HBase was improperly stopped.  Removing redo logs with file size greater than zero may result in irreversible data loss.