-Re: Migration needed when updating within an Hadoop release
> has it ever happened that a migration of persistent data has been needed (or automatically executed) when updating a Hadoop installation within a release? If so, where could I find information regarding such needed migration?
Normally, when you change the minor release, you need to upgrade HDFS (http://hadoop.apache.org/docs/stable/hdfs_user_guide.html#Upgrade+and+Rollback). This will happen when you switch major branches.
> I would be interested because the runtime of such migration would probably depend on the amount of managed data and had to be planned.
Depends how much data you've stored. Michael has written a excellent blog post about:
German Hadoop LinkedIn Group: http://goo.gl/N8pCF