Before You Begin
If you are upgrading a cluster that is part of a production system, be sure to plan ahead. As with any operational work, be sure to reserve a maintenance window with enough extra time allotted in case of complications. The Hadoop upgrade process is well understood, but it is best to be cautious. For production clusters, Cloudera recommends allocating up to a full day maintenance window to perform the upgrade, depending on the number of hosts, the amount of experience you have with Hadoop and Linux, and the particular hardware you are using.
Considerations for Secure Clusters
If you are upgrading a cluster that has Kerberos security enabled, you must do the following:
- Before starting the upgrade, read the CDH4 Security Guide.
- Before shutting down Hadoop services, put the NameNode into safe mode and perform a saveNamespace operation; see the instructions on backing up the metadata.
- Before re-starting HDFS, make sure you set the appropriate variables following these instructions; otherwise the DataNodes will not start.