The HDFS Balancer only balances blocks between DataNodes. It does not to any balancing on individual DataNodes between drives.
You can set the DataNode Volume Choosing Policy (dfs.datanode.fsdataset.volume.choosing.policy) to Available Space (org.apache.hadoop.hdfs.server.datanode.fsdataset.AvailableSpaceVolumeChoosingPolicy). This will cause the DataNodes to write new blocks to the drive with the most space available. It does not affect blocks that have already been written.
For your question about wiping one datanode at a time, it would be better to decommission and then recommission a node. With a replication factor of 3 you may perform this action on 2 nodes at a time.
David Wilder, Community Manager
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