The block was initially created on excessSSD(rack r1),
storages[4](rack r3) and storages[5](rack r3) with
ONESSD_STORAGE_POLICY_NAME storage policy. Replication factor = 2.
In this case, no replica can be chosen as the excessive replica by
chooseReplicasToDelete because if the SSD storage is deleted,
the remaining storages[4] and storages[5] are the same rack (r3),
violating block placement policy (i.e. the number of racks >= 2).
TODO BlockPlacementPolicyDefault should be able to rebalance the replicas
and then delete excessSSD.