decommissioning a node

[John Lilley]

Our cluster has a node that reboot randomly.  So I've gone to Ambari, 
decommissioned its HDFS service, stopped all services, and deleted the node 
from the cluster.  I expected and fsck to immediately show under-replicated 
blocks, but everything comes up fine.  How do I tell the cluster that this node 
is really gone, and it should start replicating the missing blocks?
Thanks
John