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We present a novel reordering write buffer which improves the performance of log-structured file systems (LFS). While UFS has a good write performance, high garbage-collection overhead degrades its performance under high disk space utilization. Previous research concentrated on how to improve the efficiency of the garbage collector after data is written to disk. We propose a new method that reduces the amount of work the garbage collector would do before data reaches disk. By classifying active and inactive data in memory into different segment buffers and then writing them to different disk segments, we force the disk segments to form a bimodal distribution. Most data blocks in active segments are quickly invalidated, while inactive segments remain mostly intact. Simulation results based on a wide range of both real-world and synthetic traces show that our method significantly reduces the garbage collection overhead, slashing the overall write cost of LFS by up to 53 percent, improving the write performance of LFS by up to 26 percent, and the overall system performance by up to 21 percent.