By default, the DataNode uses the round-robin-based policy to write new blocks. However, in a long-running cluster, it is still possible for the DataNode to have created significantly imbalanced volumes due to events like massive file deletion in HDFS or the addition of new DataNode disks via the disk hot-swap feature. Even if you use the available-space-based volume-choosing policy instead, volume imbalance can still lead to less efficient disk I/O: For example, every new write will go to the newly-added empty disk while the other disks are idle during the period, creating a bottleneck on the new disk.
Recently, the Apache Hadoop community developed server offline scripts (as discussed inHDFS-1312, the dev@ mailing list, and GitHub) to alleviate the data imbalance issue. However, due to being outside the HDFS codebase, these scripts require that the DataNode be offline before moving data between disks. As a result, HDFS-1312 also introduces an online disk balancer that is designed to re-balance the volumes on a running DataNode based on various metrics. Similar to the HDFS Balancer, the HDFS disk balancer runs as a thread in the DataNode to move the block files across volumes with the same storage types.
This is a good read and sounds like a very useful feature.