http://hadoop.apache.org/docs/r2.6.0/hadoop-project-dist/hadoop-hdfs/HdfsRollingUpgrade.html Rolling Upgrade, 2.4.1 to 2.6.0 Downgrade without downtime, 2.6.0 to 2.4.1 Downgrade without downtime is not possible? org.apache.hadoop.hdfs.server.datanode.DataNode: Reported NameNode version '2.6.0' does not match DataNode version '2.4.1' but is within acceptable limits. Note: This is normal during a rolling upgrade.
