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To expand the capacity of a RAID-5 array with additional disks, data have to be migrated between disks to leverage extra space and performance gain. Conventional methods for expanding RAID-5 are very slow because they have to migrate almost all existing data and recalculate all parity blocks. This paper proposes a new online expansion method for RAID-5, named parity-based migration (PBM). This method only migrates blocks that form a special parallelogram with one side consisting of only parity blocks. When adding m disks to a RAID-5 with n disks, PBM achieves the minimal data migration which only needs to move m/(n+m) of all data blocks. Furthermore, no parity blocks are recalculated during the expansion. After expansion, although the RAID is not a standard RAID-5 distribution, the parity blocks are distributed evenly. Experimental results based on extensive trace-driven show that, on average, PBM can reduce the time of expansion by 73.6 percent while only reduces the performance of the expanded RAID by 1.83 percent when compared with Multiple-Device (MD), a toolkit provided in Linux kernel.