If your company’s policy says that 3-way redundancy is needed, you need at least 3 servers for your setup.
Now, as your storage demands grow, you will encounter the need for additional servers. Rather than having to buy 3 more servers at the same time, you can rebalance your data across a single additional node.
In the figure above you can see the before and after states: from 3 nodes with three 25TiB volumes each (for a net 75TiB), to 4 nodes, with net 100TiB.
DRBD 9 makes it possible to do an online, live migration of the data; please see Section 5.19, “Data rebalancing” for the exact steps needed.