If a Data Collector is offline or added you can use the rebalance option in the Data Aggregator to spread the load on the available data collectors evenly.
Also you have the option to move individual devices to a specific Data Collector if required
Will moving a Monitored device from one Data Collector to an other cause a loss of historic data for this or other devices?
CA Performance Management 2.x
CA Performance Management 3.x
If you move a Device from one Data Collector to an other it will not affect the historic data. The maximum impact will be a Gap of 1-3 Poll cycles for the moved devices while the move happens. All historic Data is stored on the Data Repository, so all the Data Aggregator does is changing the poll config to reflect the new data collector and update the Data collector with this information.