@Peter Coates
A good strategy if you are able to is to add a few nodes at a time, for example two or three and then wait to have these nodes be allocated with new file data before adding others. If you add all ten nodes at once, then yes , the cluster would be moderate to severely imbalanced based on what your node count and utilization was before. You can also selectively put one or two existing data nodes into maintenance mode and shut it down, and wait for its blocks to replicate before bringing it back up again.
The rebalancer does not defer to normal processing dynamically. Changing dfs.network,bandwidth.persecond setting to be higher in off hours sounds reasonable to me.
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