HDFS Balancer can run in the background and there is a controllable bandwidth that it consumes. In general, on a large cluster it can run continuously, but it is a must after adding new nodes to have a healthy system. Note for large clusters a single convergence run can be a full day or more (that shouldn't scare you away though), let it run. Also, some customers reported that had more stable experience when adding nodes in small batches of a few instead of adding a full rack at once, for example.
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