NAND flash memory has gained widespread acceptance in storage systems because of its superior write/read performance, shock-resistance and low-power consumption. I/O scheduling for Solid State Drives (SSDs) has received much attention in recent years for its ability to take advantage of the rich parallelism within SSDs. However, most state-of-the-art I/O scheduling algorithms are oblivious to the increasingly significant inter-block variation introduced by the advanced technology scaling. This paper proposes a variation-aware I/O scheduler by exploiting the speed variation among blocks to minimize the access conflict latency of I/O requests. The proposed VIOS schedules I/O requests into a hierarchical-batch structured queue to preferentially exploit channel-level parallelism, followed by chip-level parallelism. Moreover, conflict write requests are allocated to faster blocks to reduce access conflict of waiting requests. Experimental results shows that VIOS reduces write latency significantly compared to state-of-the-art I/O schedulers while attaining high read efficiency.