Current trends in data-intensive applications increase the demand for larger physical memory, resulting in the memory subsystem consuming a significant portion of system's energy. Furthermore, data-intensive applications heavily rely on a large buffer cache that occupies a majority of physical memory. Subsequently, we are focusing on the power management for physical memory dedicated to the buffer cache. Several techniques have been proposed to reduce energy consumption by transitioning DRAM into low-power states. However, transitions between different power states incur delays and may affect whole system performance. We take advantage of the I/O handling routines in the OS kernel to hide the delay incurred by the memory state transition so that performance degradation is minimized while maintaining high memory energy savings. Our evaluation shows that the best of the proposed mechanisms hides almost all transition latencies while only consuming 3% more energy as compared to the existing on-demand mechanism, which can expose significant delays.