Object-based Storage Devices (OSDs) are the building block of Object-based Storage Systems. Object assignment in OSDs can largely affect the performance of OSDs. A real-time object assignment algorithm is proposed in the paper. The algorithm aims at minimizing the variance of Mean Response Time (MRT) across disks. To address the online problem with
unknown workload parameters, the algorithm employs an adaptive mechanism to estimate the parameters of workloads. The simulation results show that the algorithm can effectively balance the MRT in the disk I/O sub-systems.