Methods and apparatus for optimizing resource utilization in distributed storage systems. A data migration technique is described that may operate in the background in a distributed storage data center to migrate data among a fleet of storage units to achieve a substantially even and randomized data storage distribution among all storage units in the fleet. When new storage units are added to the fleet and coupled to the data center network, the new storage units are detected. Instead of processing and storing new data to the newly added storage units, as in conventional distributed storage systems, the new units are blocked from general client I/O to allow the data migration technique to migrate data from other, previously installed storage hardware in the data center onto the new storage hardware. Once the storage load on the new storage units is balanced with the rest of the fleet, the new storage units are released for general client I/O.