In an HA deployment, DataNodes must register with both NameNodes and send periodic heartbeats and block reports to both. However, if NameNodes and DataNodes are restarted simultaneously, then this can trigger a race condition in registration. The end result is that the BPServiceActor for one NameNode terminates, but the BPServiceActor for the other NameNode remains alive. The DataNode process is then in a "half-alive" state where it only heartbeats and sends block reports to one of the NameNodes. This could cause a loss of storage capacity after an HA failover. The DataNode process would have to be restarted to resolve this.