Disk Array is adopted widely because of its high performance I/O. To adapt the need of applications’ changeable I/O performance, I/O storage subsystem should be highly scalable. So DDA (dynamic disk array), which can scale adaptively, is an ideal system. The key technology of DDA is its data placement algorithm and online data reorganization algorithm. The main contribution of the paper is: first, a detailed study on DDA data placement is conducted and a new placement method, D/H, is presented. In D/H placement, the space in DDA is balanced after scale, and the reorganization cost is minimized; then, an online data reorganization algorithm based on control feedback theory is provided. With this strategy, the reorganization in DDA does little impression to the system QoS, and under this condition, data reorganization can be accomplished as quickly as possible; finally, simulation results show that Online Data Reorganization based on Control Theory is useful.