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.
Right after balancer moves the block from storages[5] to
storages[3](rack r2), the application changes the storage policy from
ONESSD_STORAGE_POLICY_NAME to HOT_STORAGE_POLICY_ID. In this case,
no replica can be chosen as the excessive replica as
chooseReplicasToDelete only considers storages[4] and storages[5] that
are the same rack. But neither's storage type is SSD.
TODO BlockPlacementPolicyDefault should be able to delete excessSSD.