Setting Up a Backup/Failover Namenode

Dear Hadoop Experts,

The opening comments of the NameNode class state:

"There is a single NameNode running in any DFS
deployment.  (Well,
except when there is a second backup/failover
NameNode.)"

Unfortunately, I haven't been able to find any
documentation on how to
configure this second NameNode.  Can anyone give me some
pointers?

Thanks in advance,
Albert Chern